**5.1 Apolipoprotein A-I (Apo AI)**

The Apo AI gene, codes for apolipoprotein A-I, is a major structural and functional component of HDL constituting about 70% to 80% of HDL protein mass, and is the main activator of the enzyme lecithin cholesterol acyl transferase (LCAT) (Ordovas & Corella., 2004; Lovegrove & Gitau., 2008; Much et al., 2005; DeBusk., 2008).

Plasma HDL-cholesterol plays a protective role for CVD (Lovegrove & Gitau., 2008; Much et al., 2005). Its gene product Apo A-I plays a central role in lipid metabolism and CVD risk. Pedigree studies have reported associations between genetic variation at the Apo A1 locus and plasma lipid and lipoprotein levels (Ordovas & Corella., 2004). One of the variants that has been identified to be diet-related is -75G>A, in which the typical guanine has been replaced with an adenine at position 75 within the regulatory region of the Apo *A-I* gene (DeBusk., 2008).

It was reported that this polymorphism was associated with Apo A-I and HDL-C concentrations, and individuals carrying the A-allele presented with the highest levels, compared with subjects homozygotes for the G allele (G/G) but many studies have had contradictory results (Lovegrove & Gitau., 2008; Ordovas & Corella., 2004).

In the context of the Framingham Heart Study, individuals with a polymorphism in the Apo A1 gene promoter region (–75 G/A) were found to respond differently to dietary PUFA (Much et al., 2005; Lovegrove & Gitau., 2008).

The inconsistencies in reported studies outcomes are not necessarily a result of inherent differences, but are a result of a nutrient–gene interaction, i. e. a classic example of where individualized dietary advice could be important in relation to exerting a positive influence on HDL-C levels and CVD risk (Lovegrove & Gitau., 2008; Much et al., 2005). In brief, that individuals with the A allele showed an increase in HDL levels following an increased consumption of PUFA. In contrast, those with the more common G allele showed an inverse relationship between HDL levels and PUFA consumption. This study revealed that differences in sex also mediate the response. Indeed, men did not show a relationship between HDL and PUFA consumption, irrespective of their Apo A1 polymorphism (Lovegrove & Gitau., 2008; Much et al., 2005).

A common practice in treating dyslipidemia is to reduce the saturated fat content of the diet and increase the polyunsaturated fat content. Typically, HDL levels fall in women with the more common G allele as the polyunsaturated content of the diet increases, an effect counter to the desired one. These women would benefit from a fat modified diet that keeps amounts of both saturated and polyunsaturated fat low and increases amounts of monounsaturated fat. Women with the A allele, increasing polyunsaturated dietary fat leads to increased HDL levels, and the effect is "dose-dependent; so in women with the more common G allele,

functionally important variations; genes that have an important hierarchical role in biological cascades; polymorphisms that are highly prevalent in the population (usually >10% for public health relevance); and/or genes with associated biomarkers, rendering

Many studies have investigated this possibility and have largely focused on genes whose products affect lipoprotein metabolism, eg, apolipoproteins, enzymes, and receptors. Although there have been several reviews of such studies, many of them may have led to

The Apo AI gene, codes for apolipoprotein A-I, is a major structural and functional component of HDL constituting about 70% to 80% of HDL protein mass, and is the main activator of the enzyme lecithin cholesterol acyl transferase (LCAT) (Ordovas & Corella.,

Plasma HDL-cholesterol plays a protective role for CVD (Lovegrove & Gitau., 2008; Much et al., 2005). Its gene product Apo A-I plays a central role in lipid metabolism and CVD risk. Pedigree studies have reported associations between genetic variation at the Apo A1 locus and plasma lipid and lipoprotein levels (Ordovas & Corella., 2004). One of the variants that has been identified to be diet-related is -75G>A, in which the typical guanine has been replaced with an adenine at position 75 within the regulatory region of the Apo *A-I* gene

It was reported that this polymorphism was associated with Apo A-I and HDL-C concentrations, and individuals carrying the A-allele presented with the highest levels, compared with subjects homozygotes for the G allele (G/G) but many studies have had

In the context of the Framingham Heart Study, individuals with a polymorphism in the Apo A1 gene promoter region (–75 G/A) were found to respond differently to dietary PUFA

The inconsistencies in reported studies outcomes are not necessarily a result of inherent differences, but are a result of a nutrient–gene interaction, i. e. a classic example of where individualized dietary advice could be important in relation to exerting a positive influence on HDL-C levels and CVD risk (Lovegrove & Gitau., 2008; Much et al., 2005). In brief, that individuals with the A allele showed an increase in HDL levels following an increased consumption of PUFA. In contrast, those with the more common G allele showed an inverse relationship between HDL levels and PUFA consumption. This study revealed that differences in sex also mediate the response. Indeed, men did not show a relationship between HDL and PUFA consumption, irrespective of their Apo A1 polymorphism

A common practice in treating dyslipidemia is to reduce the saturated fat content of the diet and increase the polyunsaturated fat content. Typically, HDL levels fall in women with the more common G allele as the polyunsaturated content of the diet increases, an effect counter to the desired one. These women would benefit from a fat modified diet that keeps amounts of both saturated and polyunsaturated fat low and increases amounts of monounsaturated fat. Women with the A allele, increasing polyunsaturated dietary fat leads to increased HDL levels, and the effect is "dose-dependent; so in women with the more common G allele,

contradictory results (Lovegrove & Gitau., 2008; Ordovas & Corella., 2004).

articles being omitted and introduced bias toward positive findings (Ordovas., 2006).

2004; Lovegrove & Gitau., 2008; Much et al., 2005; DeBusk., 2008).

clinical trials useful (Lovegrove & Gitau., 2008).

(Much et al., 2005; Lovegrove & Gitau., 2008).

(Lovegrove & Gitau., 2008; Much et al., 2005).

**5.1 Apolipoprotein A-I (Apo AI)** 

(DeBusk., 2008).

increasing dietary Polyunsaturated Fat (PUFA) levels from less than 4% of total energy to 4% to 8% to greater than 8% resulted in a corresponding decline in HDL levels as PUFA levels increase. However, in women with the A allele, increasing PUFA concentrations (>8% of energy derived from PUFA) increased HDL levels and the increase is more dramatic in the presence of two copies of the A alleles than it is with just one. For these women, a diet low in saturated fat, moderate in polyunsaturated fat (8% or greater of total calories), and supplying the rest in monounsaturated fat has the greatest benefit in raising HDL levels. Clearly, whether a person has the -75G>A *A*po *AI* variant, and how many copies are present, will affect any therapeutic intervention developed to correct dyslipidemia (DeBusk., 2009; Much et al., 2005; Debra., 2008).

Juo et al (Hank Juo., 1999) used a meta analysis approach to show the lack of consistency between the less common A-allele and higher HDL-cholesterol concentrations. In view of the significant gene-diet interaction observed for those intervention studies, they examined whether these results could be extrapolated to a free living population, consisting of about 1600 Framingham Offspring Study participants (Ordovas et al., 2002). The results from the straightforward association between genotype and phenotype were disappointing and suggested that the G/A polymorphism was not associated with HDLcholesterol, Apo A-I concentrations, nor with any other anthropometrical or plasma lipid variable examined. To examine the potential modifying effect of dietary fat on these associations, they fitted multivariate linear regression models, including interaction terms for fat intake [total, Saturated Fatty Acid (SFA), Monounsaturated Fat (MUFA), and PUFA fat]. No significant interactions were observed between the G/A polymorphism, total, SFA, and MUFA fat intakes. However, in women, HDL-cholesterol concentrations were associated with a significant interaction between PUFA intake and Apo A1 genotype (p = 0. 005). Using PUFA as a dichotomous variable, their data show that G/G women consuming *<*6% PUFA/day had higher HDL-cholesterol (1. 48 ± 0. 40 m mol/L) than Acarriers (1. 43 ± 0. 40 m mol/L). Conversely, when consuming ≥6% PUFA/day, G/G had lower HDL-cholesterol concentrations (1. 44 ± 0. 39 m mol/L) than A-carriers (1. 49 ± 0. 39 m mol/L). In men, the situation was more complex because the effects were observed using three-way interactions, including smoking and alcohol consumption, in the analyzes (Ordovas & Corella., 2004).

The most evident application of these results may be to help us make more efficacious dietary recommendations based on genetic profile. It is clear that subjects with the A-allele at this Apo A1–75 (G/A) polymorphism will benefit from diets containing a high percentage (it is important to underscore that we are talking about percent in the diet and not about total amounts) of PUFA (i. e., vegetable oils, fish, nuts, and so on). According to their data, this should result in higher HDL-cholesterol concentrations, which in turn should lower CVD risk. These findings suggest that the expression of the Apo A1 gene may be regulated by PUFA (Ordovas & Corella., 2004).

On the other hand**,** of 13 reports, 5 found that the presence of the Apo A-I–75 (*G/A*) A allele instead of the common *G* allele resulted in greater LDL-cholesterol responses to changes in dietary. In addition, significant interactions between the *G/A* genotype and diet were found for changes in total and LDL cholesterol when subjects changed from a low-fat diet to a diet high in MUFAs. No significant interactions between diet and other polymorphisms in the Apo A-I gene were shown (Ordovas & Corella., 2004).

Nutrigenetics and Dyslipidemia 51

interpretation of results from studies involving gene-diet interactions (Ordovas & Corella.,

The presence of serine instead of threonine at position 347 in the Apo A-IV gene was associated with increased total and LDL-cholesterol responsiveness when subjects switched from a high-SFA diet to a National Cholesterol Education Program Step I diet. When the same subjects changed from the National Cholesterol Education Program Step I diet to a high-MUFA diet, subjects with the *Thr /Thr* genotype had a 1% decrease in total cholesterol concentrations, whereas subjects with the *Ser* allele had a 5% increase in total cholesterol concentrations. When the Thr347Ser and the Apo A-I–75 (*G/A*) genotypes were combined, carriers of the *A* and *Ser* alleles showed greater LDL-cholesterol responses to changes in dietary fat. However, Carmena-Ramon et al (Carmena-Ramon et al., 1998) investigated both the Gln360His and Thr347Ser polymorphisms and found no gene-diet or haplotype-diet interactions (Masson et al., 2003). The evidence that exists for an interaction between diet and the Apo A-IV glutamine-histidine mutation at position 360 (Gln360His) suggests that *Gln / Gln* subjects show significantly greater total and LDL-cholesterol responses and that *Gln /His* subjects show greater HDL-cholesterol responses to changes in dietary fat, cholesterol, or both. Although Wallace et al found no significant differences in LDLcholesterol responses between genotypes, dense LDL cholesterol decreased more in subjects carrying the *His* allele when polyunsaturated fatty acids (PUFAs) replaced SFAs in the diet (Wallace et al, 2000). In the same study, there was a significant difference in HDL-cholesterol responses between genotype groups such that concentrations decreased in *Gln /Gln* subjects

Apolipoprotein B is the main protein component of low-density-lipoprotein (LDL) and contains several domains. The human Apo B is 43 kb in length with 81 bp signal sequence.

The evidence for an interaction between the *Xba*I polymorphism and diet is inconsistent. In 2 studies, *X*-*X*- subjects showed greater LDL-cholesterol responses, whereas Tikkanen et al, found that subjects carrying the *X*+ allele had greater total, LDL-, and HDL-cholesterol responses. However in, analysis of these data, the *Xba*I polymorphism only explained a significant proportion of variance of the change in HDL cholesterol (Tikkanen et al., 1995). In one research they found no significant effect on LDL-cholesterol responsiveness, although *X*-*X*-subjects showed the greatest HDL2- and VLDL-cholesterol responses. Finally, in another study researchers studied the effect of the *Xba*I polymorphism in subjects with the common Apo *E3/3* genotype and found that *X*-*X*- subjects showed the greatest triacylglycerol response. Rantala et al conducted a meta-analysis of all published dietary trials. In their analysis of 8 studies, *X*-*X*+ subjects had greater LDL responses than did *X*+*X*+ subjects and no significant differences in the responses of total or HDL cholesterol or triacylglycerol were found between genotypes (Masson et al., 2003; Rantala., 2000). Two of 7 intervention studies found that the *Eco*RI *R*- allele was associated with significantly greater total and LDL-cholesterol responses to changes in dietary fat and cholesterol. Only one study found an interaction between the *Msp*I polymorphism and response to diet. Ten intervention studies found no significant effects of the Apo B signal peptide insertion/deletion (*I/D*) polymorphism on dietary responsiveness; however, 2 studies reported a significantly greater responsiveness in subjects homozygous for the *I* allele. In a

Numerous polymorphisms have been identified on this gene (Heilbronn et al., 2000).

and increased in *Gln /His* subjects (Masson et al., 2003).

**5.3 Apolipoprotein B (Apo B)** 

2004).

#### **5.2 Apolipoprotein A-IV (APOA4)**

Apo A-IV is a 46-Kd plasma glycoprotein that is synthesized by intestinal enterocytes during lipid absorption and is incorporated into nascent chylomicrons. Apo A-IV enters circulation on lymph chylomicrons, but then dissociates from their surface and circulates primarily as a lipid-free protein. Several genetically determined isoforms of Apo A-IV have been detected; amino acid positions 360 and 347 of the mature protein are the most common. The polymorphism at position 360 is due to a CAG → CAT substitution at codon 360 in the Apo A4 gene and encodes a Q360H (Gln → His) substitution in the carboxyl terminus, and produces an isoform, originally known as Apo A-IV-2, one charge unit more basic than the common isoform, Apo A-IV-. In some population studies the Apo A-IV-2 allele is associated with higher levels of HDL-cholesterol and or Apo A-I and/or lower triglyceride (TG) levels, as well as lower LDL-cholesterol, lower Lp (a), and higher fasting glucose and insulin levels, but no associations have been observed in other studies (Ordovas & Corella., 2004).

The other common mutation (Thr347→Ser) is due to an ACT→TCT substitution at codon 347 in the human Apo A-IV gene, it is found within subjects with the apoA-IV-1 isoform. Several population studies note that carriers of the 347S allele have lower plasma, total, LDL-cholesterol, Apo B levels, and Lp (a) levels, than 347T/T homozygotes. The results of many reports showing that male carriers of the less common allele at the Gln360His polymorphism were less responsive to changes in dietary fat and cholesterol or cholesterol alone (Ordovas & Corella., 2004).

Several studies have focused on the interaction between the Apo *A4* locus and dietary factors, both in the fasting and postprandial states. Similar to the findings for other genes, the data are conflicting when it comes to the effect of Apo A-IV polymorphisms on the LDL response to dietary cholesterol. However, according to Weinberg (Weinberg., 2002), the results from different studies can be partially reconciled if one assumes that the dietary fatty acid effects dominate over the allele effects. Therefore, if dietary cholesterol intake is the principal variable, and total fat intake is moderate and constant, Q/H subjects display an attenuated response of LDL-cholesterol. However, when dietary cholesterol intake is changed in the setting of a higher baseline dietary fat intake or with a change in fat saturation, the fatty acid effects on LDL levels predominate and overrule the allele effect. The impact of the Q360H polymorphism on cholesterol absorption may be greater on a high PUFAs intake. However, dietary PUFA counteract the effect of dietary cholesterol on the expression of hepatic LDL receptors. Thus, the final effect of Apo AIV alleles on the LDL response to dietary cholesterol may be determined by the relative amounts of cholesterol, saturated fatty acids (SAFAs), and PUFAs in the diet (winberg et al., 2000; Weggemans., 2000; Lopez-Miranda., 1998; Ordovas & Corella., 2004; Hockey., 2001).

There is more consistency and probably less complexity regarding the impact of Apo A-IV polymorphisms on HDL-cholesterol: When total fat intake is raised or lowered, Q/H subjects have an exaggerated, and Threonine /Serine (T/S) subjects an attenuated, response in plasma HDL levels. It has been suggested, and Weinberg demonstrated, that a high-PUFA intake may amplify this effect (Ordovas & Corella., 2004; Winberg et al., 2000).

Given the relationships between plasma TG and plasma HDL-cholesterol levels, it is possible that the response of plasma HDL-cholesterol levels to changes in dietary fat is mediated by Apo A-IV allele effects on postprandial triglyceride-rich lipoprotein metabolism. These studies clearly illustrate the extreme complexity associated with the

Apo A-IV is a 46-Kd plasma glycoprotein that is synthesized by intestinal enterocytes during lipid absorption and is incorporated into nascent chylomicrons. Apo A-IV enters circulation on lymph chylomicrons, but then dissociates from their surface and circulates primarily as a lipid-free protein. Several genetically determined isoforms of Apo A-IV have been detected; amino acid positions 360 and 347 of the mature protein are the most common. The polymorphism at position 360 is due to a CAG → CAT substitution at codon 360 in the Apo A4 gene and encodes a Q360H (Gln → His) substitution in the carboxyl terminus, and produces an isoform, originally known as Apo A-IV-2, one charge unit more basic than the common isoform, Apo A-IV-. In some population studies the Apo A-IV-2 allele is associated with higher levels of HDL-cholesterol and or Apo A-I and/or lower triglyceride (TG) levels, as well as lower LDL-cholesterol, lower Lp (a), and higher fasting glucose and insulin levels, but no associations have been observed in other studies (Ordovas

The other common mutation (Thr347→Ser) is due to an ACT→TCT substitution at codon 347 in the human Apo A-IV gene, it is found within subjects with the apoA-IV-1 isoform. Several population studies note that carriers of the 347S allele have lower plasma, total, LDL-cholesterol, Apo B levels, and Lp (a) levels, than 347T/T homozygotes. The results of many reports showing that male carriers of the less common allele at the Gln360His polymorphism were less responsive to changes in dietary fat and cholesterol or cholesterol

Several studies have focused on the interaction between the Apo *A4* locus and dietary factors, both in the fasting and postprandial states. Similar to the findings for other genes, the data are conflicting when it comes to the effect of Apo A-IV polymorphisms on the LDL response to dietary cholesterol. However, according to Weinberg (Weinberg., 2002), the results from different studies can be partially reconciled if one assumes that the dietary fatty acid effects dominate over the allele effects. Therefore, if dietary cholesterol intake is the principal variable, and total fat intake is moderate and constant, Q/H subjects display an attenuated response of LDL-cholesterol. However, when dietary cholesterol intake is changed in the setting of a higher baseline dietary fat intake or with a change in fat saturation, the fatty acid effects on LDL levels predominate and overrule the allele effect. The impact of the Q360H polymorphism on cholesterol absorption may be greater on a high PUFAs intake. However, dietary PUFA counteract the effect of dietary cholesterol on the expression of hepatic LDL receptors. Thus, the final effect of Apo AIV alleles on the LDL response to dietary cholesterol may be determined by the relative amounts of cholesterol, saturated fatty acids (SAFAs), and PUFAs in the diet (winberg et al., 2000; Weggemans.,

There is more consistency and probably less complexity regarding the impact of Apo A-IV polymorphisms on HDL-cholesterol: When total fat intake is raised or lowered, Q/H subjects have an exaggerated, and Threonine /Serine (T/S) subjects an attenuated, response in plasma HDL levels. It has been suggested, and Weinberg demonstrated, that a high-PUFA intake may amplify this effect (Ordovas & Corella., 2004; Winberg et al., 2000).

Given the relationships between plasma TG and plasma HDL-cholesterol levels, it is possible that the response of plasma HDL-cholesterol levels to changes in dietary fat is mediated by Apo A-IV allele effects on postprandial triglyceride-rich lipoprotein metabolism. These studies clearly illustrate the extreme complexity associated with the

2000; Lopez-Miranda., 1998; Ordovas & Corella., 2004; Hockey., 2001).

**5.2 Apolipoprotein A-IV (APOA4)** 

& Corella., 2004).

alone (Ordovas & Corella., 2004).

interpretation of results from studies involving gene-diet interactions (Ordovas & Corella., 2004).

The presence of serine instead of threonine at position 347 in the Apo A-IV gene was associated with increased total and LDL-cholesterol responsiveness when subjects switched from a high-SFA diet to a National Cholesterol Education Program Step I diet. When the same subjects changed from the National Cholesterol Education Program Step I diet to a high-MUFA diet, subjects with the *Thr /Thr* genotype had a 1% decrease in total cholesterol concentrations, whereas subjects with the *Ser* allele had a 5% increase in total cholesterol concentrations. When the Thr347Ser and the Apo A-I–75 (*G/A*) genotypes were combined, carriers of the *A* and *Ser* alleles showed greater LDL-cholesterol responses to changes in dietary fat. However, Carmena-Ramon et al (Carmena-Ramon et al., 1998) investigated both the Gln360His and Thr347Ser polymorphisms and found no gene-diet or haplotype-diet interactions (Masson et al., 2003). The evidence that exists for an interaction between diet and the Apo A-IV glutamine-histidine mutation at position 360 (Gln360His) suggests that *Gln / Gln* subjects show significantly greater total and LDL-cholesterol responses and that *Gln /His* subjects show greater HDL-cholesterol responses to changes in dietary fat, cholesterol, or both. Although Wallace et al found no significant differences in LDLcholesterol responses between genotypes, dense LDL cholesterol decreased more in subjects carrying the *His* allele when polyunsaturated fatty acids (PUFAs) replaced SFAs in the diet (Wallace et al, 2000). In the same study, there was a significant difference in HDL-cholesterol responses between genotype groups such that concentrations decreased in *Gln /Gln* subjects and increased in *Gln /His* subjects (Masson et al., 2003).

#### **5.3 Apolipoprotein B (Apo B)**

Apolipoprotein B is the main protein component of low-density-lipoprotein (LDL) and contains several domains. The human Apo B is 43 kb in length with 81 bp signal sequence. Numerous polymorphisms have been identified on this gene (Heilbronn et al., 2000).

The evidence for an interaction between the *Xba*I polymorphism and diet is inconsistent. In 2 studies, *X*-*X*- subjects showed greater LDL-cholesterol responses, whereas Tikkanen et al, found that subjects carrying the *X*+ allele had greater total, LDL-, and HDL-cholesterol responses. However in, analysis of these data, the *Xba*I polymorphism only explained a significant proportion of variance of the change in HDL cholesterol (Tikkanen et al., 1995). In one research they found no significant effect on LDL-cholesterol responsiveness, although *X*-*X*-subjects showed the greatest HDL2- and VLDL-cholesterol responses. Finally, in another study researchers studied the effect of the *Xba*I polymorphism in subjects with the common Apo *E3/3* genotype and found that *X*-*X*- subjects showed the greatest triacylglycerol response. Rantala et al conducted a meta-analysis of all published dietary trials. In their analysis of 8 studies, *X*-*X*+ subjects had greater LDL responses than did *X*+*X*+ subjects and no significant differences in the responses of total or HDL cholesterol or triacylglycerol were found between genotypes (Masson et al., 2003; Rantala., 2000). Two of 7 intervention studies found that the *Eco*RI *R*- allele was associated with significantly greater total and LDL-cholesterol responses to changes in dietary fat and cholesterol. Only one study found an interaction between the *Msp*I polymorphism and response to diet. Ten intervention studies found no significant effects of the Apo B signal peptide insertion/deletion (*I/D*) polymorphism on dietary responsiveness; however, 2 studies reported a significantly greater responsiveness in subjects homozygous for the *I* allele. In a

Nutrigenetics and Dyslipidemia 53

of various lipids and show the greatest lipid-lowering response to a low-fat diet. Taking into account which Apo E alleles a person has is helpful in developing diet and lifestyle

In 46 studies that examined the Apo E locus and alterations in dietary fat content, significantly different responses in total and LDL cholesterol by Apo E genotype were reported in 8 and 11 studies, respectively, with the Apo E4 individuals generally being the

Note that despite the numerous studies examining the relation between Apo E genetic variability and LDL-cholesterol response to diet intervention, there is considerable inconsistency regarding the magnitude and significance of the reported associations, and

In a study, there are 29 intervention studies that examine Apo E-diet interactions. A total of 3224 subjects participated in these studies, ranging from 16 to 420 subjects per study. Of the 29 studies, 12 demonstrated no significant Apo E-diet interactions, 15 reported significant interactions (E4 was usually associated with increased dietary response), and 2 were undefined. Using the same available literature, but different selection criteria, Masson, reviewed 62 dietary intervention periods, including 3223 subject-by-diet interventions (Masson et al., 2003). Again, the range of the studies varied between 8 and 210 subjects per dietary intervention. According to this review, 42 of the diet interventions did not demonstrate significant Apo E-diet interactions, and only 19 provided evidence for significant interactions, clearly demonstrating the diversity of the results presented in the original papers as well as those obtained from review papers (Ordovas & Corella., 2004;

The heterogeneous response to changes in dietary fat may be attributed to a number of factors including age, gender, baseline LDL-C levels, disease status and drug use

One difference between the negative studies and those reporting significant Apo E gene–diet interactions relates to the baseline lipid levels of the subjects. Studies reporting significant associations often included subjects who were moderately hypercholesterolemic and/or had significant differences in base total cholesterol and LDL-cholesterol among the Apo E genotype groups. This suggests that the significant gene-diet interaction is apparent only in subjects susceptible to hypercholesterolemia. Concerning differences in dietary interventions, there were significant interactions in studies in which total dietary fat and cholesterol were modified. Several mechanisms are proposed to explain these Apo E-related differences in individual response to dietary therapy. Some studies show that intestinal cholesterol absorption is related to Apo E phenotype, with Apo E4 carriers absorbing more cholesterol than non-Apo E4 carriers. Other mechanisms such as different distribution of Apo E on the lipoprotein fractions, LDL Apo B production, bile acid, and cholesterol synthesis, and postprandial lipoprotein

On the other hand although the obvious dietary factors implicated in gene-diet interactions affecting plasma lipid levels are dietary fats and cholesterol, other dietary components have

Although the raising effect of alcohol consumption on high-density lipoprotein (HDL) cholesterol levels is well established, the effect on LDL-cholesterol is still unclear. It is possible that the reported variability will be due to interactions between genetic factors and

this locus continues to be the subject of intense research (Ordovas & Corella., 2004).

interventions for improving serum lipid levels (DeBusk., 2009).

most responsive (Lovegrove & Gitau., 2008; Masson et al., 2003).

clearance may also be involved (Ordovas & Corella., 2004).

revealed significant interactions. This is the case for alcohol intake.

Masson et al., 2003).

(Lovegrove & Gitau., 2008).

study, 43 men and women were observed to compare the effects of insoluble and soluble fiber on plasma lipids. Their statistical model identified gene-diet interactions. However, they did not look specifically at differences between genotype groups. It was found that *D/D* subjects had similar decreases in HDL cholesterol after consumption of the insoluble- and soluble-fiber diets. However, *I/I* subjects had larger HDL-cholesterol decreases with the soluble-fiber diet, whereas *I/D* subjects had larger HDL-cholesterol decreases with the insoluble-fiber diet. The gene-diet interaction was significant (*P* = 0. 021) (Masson et al., 2003; Rantala., 2000).

In response, low-fat, low-cholesterol diet, *I/I* subjects showed the greatest decrease in HDL2. In addition, *I/I* and *I/D* subjects showed increased VLDL-cholesterol and decreased LDLcholesterol concentrations, whereas *D/D* subjects showed decreased VLDL-cholesterol and increased LDL-cholesterol concentrations. The *I/D* polymorphism showed no significant effect on the responsiveness of total, LDL, or HDL cholesterol or triacylglycerol in a metaanalysis of 7 studies (Masson et al., 2003).

#### **5.4 Apolipoprotein E (APO E)**

Apo E gene variants have implications for nutrition therapy related to preventing and treating CVD and the responses to dietary fat, soluble fiber, and alcohol. The impact of Apo E genotype on individual variability in its LDL cholesterol response to diet interventions and CVD risk has been extensively investigated over the past 30 years. Apo E contains 299 amino acids, considering Apo E's key role in lipoprotein metabolism, being involved in chylomicron metabolism, very low-density lipoprotein synthesis and secretion, and in the cellular removal of lipoprotein remnants from the circulation. Apo E serves as a ligand for multiple lipoprotein receptors. This gene locus is polymorphic, with 84 gene variants being characterized to date. The prevalence of this SNP varies in different populations (Lovegrove & Gitau., 2008; Rubin & Berglund., 2002; Ordovas & Corella., 2004; DeBusk., 2009).

Apo E is present in a subfraction of lipoprotein (a). The receptor-binding properties reside in the N-terminal part of Apo E, whereas the lipid-binding domain resides in the C-terminal portion. It was recognized that Apo E was present as three different Apo E isoforms (E2, E3, and E4), coded by three different alleles (e2, e3, and e4), resulting in six homo and heterozygous genotypes (e2/e2, e2/e3, e2/e4, e3/e3, e3/e4, e4/e4). Apo E2 differs from the wild type, Apo E3, by a substitution of arginine for cysteine at amino acid 158, and Apo E4 differs from Apo E3 by a substitution of cysteine for arginine at amino acid 112. In addition, several other genetic variants have been described at the Apo E locus (Rubin & Berglund., 2002; Lovegrove & Gitau., 2008; Masson et al., 2003; Farhud et al., 2010).

Persons with E4 variant respond to a high-fat diet negatively with an increased risk for coronary heart disease (CHD). In these individuals, low-fat diet should be useful (Farhud et al., 2010; Sheweta et al., 2011).

Population studies show that plasma cholesterol, LDL cholesterol, and Apo B levels are highest in subjects carrying the Apo E4, intermediate in those with the Apo E3, and lowest in those with the Apo E2 isoform. An initial observation was that the association of the Apo E4 isoform with elevated serum cholesterol levels was greater in populations consuming diets rich in saturated fat and cholesterol than in other populations (Ordovas & Corella., 2004).

Corella and Ordovas reviewed the numerous studies that have investigated the diet-gene interaction for Apo E variants. People with at least one E4 allele have the highest basal levels

study, 43 men and women were observed to compare the effects of insoluble and soluble fiber on plasma lipids. Their statistical model identified gene-diet interactions. However, they did not look specifically at differences between genotype groups. It was found that *D/D* subjects had similar decreases in HDL cholesterol after consumption of the insoluble- and soluble-fiber diets. However, *I/I* subjects had larger HDL-cholesterol decreases with the soluble-fiber diet, whereas *I/D* subjects had larger HDL-cholesterol decreases with the insoluble-fiber diet. The gene-diet interaction was significant (*P* = 0. 021) (Masson et al.,

In response, low-fat, low-cholesterol diet, *I/I* subjects showed the greatest decrease in HDL2. In addition, *I/I* and *I/D* subjects showed increased VLDL-cholesterol and decreased LDLcholesterol concentrations, whereas *D/D* subjects showed decreased VLDL-cholesterol and increased LDL-cholesterol concentrations. The *I/D* polymorphism showed no significant effect on the responsiveness of total, LDL, or HDL cholesterol or triacylglycerol in a meta-

Apo E gene variants have implications for nutrition therapy related to preventing and treating CVD and the responses to dietary fat, soluble fiber, and alcohol. The impact of Apo E genotype on individual variability in its LDL cholesterol response to diet interventions and CVD risk has been extensively investigated over the past 30 years. Apo E contains 299 amino acids, considering Apo E's key role in lipoprotein metabolism, being involved in chylomicron metabolism, very low-density lipoprotein synthesis and secretion, and in the cellular removal of lipoprotein remnants from the circulation. Apo E serves as a ligand for multiple lipoprotein receptors. This gene locus is polymorphic, with 84 gene variants being characterized to date. The prevalence of this SNP varies in different populations (Lovegrove

& Gitau., 2008; Rubin & Berglund., 2002; Ordovas & Corella., 2004; DeBusk., 2009).

2002; Lovegrove & Gitau., 2008; Masson et al., 2003; Farhud et al., 2010).

Apo E is present in a subfraction of lipoprotein (a). The receptor-binding properties reside in the N-terminal part of Apo E, whereas the lipid-binding domain resides in the C-terminal portion. It was recognized that Apo E was present as three different Apo E isoforms (E2, E3, and E4), coded by three different alleles (e2, e3, and e4), resulting in six homo and heterozygous genotypes (e2/e2, e2/e3, e2/e4, e3/e3, e3/e4, e4/e4). Apo E2 differs from the wild type, Apo E3, by a substitution of arginine for cysteine at amino acid 158, and Apo E4 differs from Apo E3 by a substitution of cysteine for arginine at amino acid 112. In addition, several other genetic variants have been described at the Apo E locus (Rubin & Berglund.,

Persons with E4 variant respond to a high-fat diet negatively with an increased risk for coronary heart disease (CHD). In these individuals, low-fat diet should be useful (Farhud et

Population studies show that plasma cholesterol, LDL cholesterol, and Apo B levels are highest in subjects carrying the Apo E4, intermediate in those with the Apo E3, and lowest in those with the Apo E2 isoform. An initial observation was that the association of the Apo E4 isoform with elevated serum cholesterol levels was greater in populations consuming diets rich in saturated fat and cholesterol than in other populations (Ordovas &

Corella and Ordovas reviewed the numerous studies that have investigated the diet-gene interaction for Apo E variants. People with at least one E4 allele have the highest basal levels

2003; Rantala., 2000).

analysis of 7 studies (Masson et al., 2003).

**5.4 Apolipoprotein E (APO E)** 

al., 2010; Sheweta et al., 2011).

Corella., 2004).

of various lipids and show the greatest lipid-lowering response to a low-fat diet. Taking into account which Apo E alleles a person has is helpful in developing diet and lifestyle interventions for improving serum lipid levels (DeBusk., 2009).

In 46 studies that examined the Apo E locus and alterations in dietary fat content, significantly different responses in total and LDL cholesterol by Apo E genotype were reported in 8 and 11 studies, respectively, with the Apo E4 individuals generally being the most responsive (Lovegrove & Gitau., 2008; Masson et al., 2003).

Note that despite the numerous studies examining the relation between Apo E genetic variability and LDL-cholesterol response to diet intervention, there is considerable inconsistency regarding the magnitude and significance of the reported associations, and this locus continues to be the subject of intense research (Ordovas & Corella., 2004).

In a study, there are 29 intervention studies that examine Apo E-diet interactions. A total of 3224 subjects participated in these studies, ranging from 16 to 420 subjects per study. Of the 29 studies, 12 demonstrated no significant Apo E-diet interactions, 15 reported significant interactions (E4 was usually associated with increased dietary response), and 2 were undefined. Using the same available literature, but different selection criteria, Masson, reviewed 62 dietary intervention periods, including 3223 subject-by-diet interventions (Masson et al., 2003). Again, the range of the studies varied between 8 and 210 subjects per dietary intervention. According to this review, 42 of the diet interventions did not demonstrate significant Apo E-diet interactions, and only 19 provided evidence for significant interactions, clearly demonstrating the diversity of the results presented in the original papers as well as those obtained from review papers (Ordovas & Corella., 2004; Masson et al., 2003).

The heterogeneous response to changes in dietary fat may be attributed to a number of factors including age, gender, baseline LDL-C levels, disease status and drug use (Lovegrove & Gitau., 2008).

One difference between the negative studies and those reporting significant Apo E gene–diet interactions relates to the baseline lipid levels of the subjects. Studies reporting significant associations often included subjects who were moderately hypercholesterolemic and/or had significant differences in base total cholesterol and LDL-cholesterol among the Apo E genotype groups. This suggests that the significant gene-diet interaction is apparent only in subjects susceptible to hypercholesterolemia. Concerning differences in dietary interventions, there were significant interactions in studies in which total dietary fat and cholesterol were modified. Several mechanisms are proposed to explain these Apo E-related differences in individual response to dietary therapy. Some studies show that intestinal cholesterol absorption is related to Apo E phenotype, with Apo E4 carriers absorbing more cholesterol than non-Apo E4 carriers. Other mechanisms such as different distribution of Apo E on the lipoprotein fractions, LDL Apo B production, bile acid, and cholesterol synthesis, and postprandial lipoprotein clearance may also be involved (Ordovas & Corella., 2004).

On the other hand although the obvious dietary factors implicated in gene-diet interactions affecting plasma lipid levels are dietary fats and cholesterol, other dietary components have revealed significant interactions. This is the case for alcohol intake.

Although the raising effect of alcohol consumption on high-density lipoprotein (HDL) cholesterol levels is well established, the effect on LDL-cholesterol is still unclear. It is possible that the reported variability will be due to interactions between genetic factors and

Nutrigenetics and Dyslipidemia 55

women, a lower hepatic lipase activity might allow the detection of the modulating effect of Apo E genotypes, whereas this effect might be overwhelmed by a higher enzyme activity in men (Mahley et al., 2000). This is an analogous situation to the suggestion above that differences in susceptibility might be uncovered by a metabolic challenge (Rubin &

Other causes for the observed differences between studies may be the presence of confounders, the type of dietary intervention used, the population studied and, importantly, the number of subjects in the respective studies. A small number of subjects limits the possibility of detecting differences, or could alternatively lead to spurious associations. Although the number of studies addressing the gene nutrient interaction for Apo E is growing, in most studies so far this has been a secondary endpoint, usually analyzed post hoc. Perhaps the most likely possibility is that a number of dietary interventions will elicit variable responses across Apo E genotypes, but that the ability to detect such differences will depend on the strength and type of intervention as well as on specific recipient factors (type of population, presence of hyperlipidemia, etc. ). In the end, however, our ability to confirm or refute the presence of Apo E gene nutrient interactions as well as to understand their metabolic basis fully will require larger and more detailed studies (Rubin

Inconsistency in nutrient–gene interactions in relation to Apo E polymorphisms may be a result, in part, of retrospective genotyping of small study cohorts, for which the genotype– diet–LDL-C interactions were not the primary outcome. This factor has resulted in the under-representation of the less-frequent genotypes and, although trends may have been evident, many of the studies were clearly under-powered to detect significant genotype– treatment effects. The prospective genotyping of larger study cohorts has been used as an

 However, recent evidence strongly suggests that variations in a number of key genes may also be important, including common variants of the Apo E gene. The most convincing evidence to date for genotypic effects on dietary response comes from the extensively

A metaanalysis has been published recently that summarizes the overall findings from studies using a variety of end-point measures. A mean 40–50% increase in CHD risk was observed in E4 carriers (overall OR 1. 42) relative to the wild-type E3/E3 genotype, with no apparent differences for either the E2 and E3 subgroups (OR 0. 98). Although a causal mechanism to link E4 with increased CHD risk has not been fully elucidated, the association has been ascribed to a higher concentration of LDL-C. This higher LDL-C is believed to arise from the Apo E4 isoform having a relatively higher affinity for its membrane (LDL/chylomicron remnant) receptor and feedback inhibition of receptor activity in E4 carriers Other mechanisms relating to reduced antioxidant status may also be operative

There is the large variation that is observed in the concentration of serum LDL-cholesterol (LDL-C) in response to fish oil supplementation. The cardioprotective effects of the fatty acids in fish oil include eicosapantanoiec acid (EPA) and docosahesanoiec acid (DHA) are well recognized. However, a potentially deleterious increase in LDL-C (5–10%) has been consistently reported after moderate to high doses of fish oil (>2 g EPA+ DHA/d). These data showed the DHA rather than the EPA in fish oils that is responsible for the LDL-C

alternative approach to increase statistical power (Lovegrove & Gitau., 2008).

studied Apo E gene variant (Lovegrove & Gitau., 2008).

raising effects in E4 individuals (Lovegrove & Gitau., 2008).

Berglund., 2002).

& Berglund., 2002).

(Lovegrove & Gitau., 2008).

alcohol consumption. Using cross-sectional analysis, researcher examined whether variation at the Apo E locus modulates the association between alcohol consumption and LDLcholesterol levels in a healthy population based sample of 1014 male and 1133 female participants in the Framingham Offspring Study (Corella et al., 2001). In male nondrinkers, LDL-cholesterol levels were not different across Apo *E* groups; however, in male drinkers, there were differences in LDL-cholesterol, with Apo E2 subjects displaying the lowest levels. When LDL cholesterol levels were compared among the Apo E subgroups by drinking status, LDL-cholesterol levels in Apo E2 male drinkers were lower than in Apo E2 non drinkers. Conversely, in Apo E4 males, LDL-cholesterol was higher in drinkers than in nondrinkers. This Apo E-alcohol interaction remained significant after controlling for age, BMI, smoking, fat, and energy intake. In women, the expected effect of Apo E alleles on LDL-cholesterol levels was present in both drinkers and nondrinkers. Multiple linear regression models showed a negative association between alcohol and LDL-cholesterol levels in Apo E2 men, with alcohol intake a continuous variable. Conversely, in Apo E4 men, this association was positive. There were no statistically significant associations in either Apo E3 men or in women. These data suggest that in men, variability at the Apo E locus partially modulates the effects of consuming alcoholic beverages on LDL-cholesterol levels (Ordovas & Corella., 2004).

The effect of alcohol was also investigated in the Copenhagen City Heart Study (Frikke-Schmidt., 2000). In that study, there was an interaction between alcohol and Apo E among women, in which higher triglyceride levels were associated with both the E2 and E4 alleles among women who regularly consumed alcohol. For men, increased triglyceride levels among E2 and E4 carriers were seen across the entire alcohol distribution spectrum, perhaps because of some degree of alcohol consumption among all men. Overall, the results suggested that metabolic stresses, such as the postprandial situation or alcohol consumption, might contribute to uncover underlying differences between Apo E genotypes in cholesterol, triglyceride or lipoprotein metabolism (Frikke-Schmidt., 2000).

The effect of the Apo E gene on lipoproteins may differ with age. In elderly individuals as well as in children, there is less difference in LDL cholesterol levels in individuals carrying the E4 allele versus non-E4 carriers. Interestingly, in both of these age groups, the presence of the Apo E2 allele was associated with lower LDL cholesterol levels. An age-dependent variation between Apo E and plasma lipids was also seen by Jarvik et al (Jarvik et al, 1997). By longitudinally following male Caucasian twins, the authors demonstrated that whereas E4 carriers initially had higher triglyceride and cholesterol levels compared with E3 homozygotes, this difference disappeared over an 18-year period (Rubin & Berglund., 2002). A sex-specific association between Apo E2 and HDL cholesterol levels has been described in Turkish individuals. In Turkish women, but not men, the frequency of the Apo E2 allele increased almost six fold from the lowest to the highest HDL cholesterol tertiary (Rubin & Berglund., 2002). The available information show, significant diet–Apo E gene interactions occurred in male-only studies. In studies including men and women, significant effects were noted only in men, suggesting a significant gene-sex interaction (Ordovas & Corella., 2004).

As pointed out above, in studies in which an Apo E gene nutrient interaction was found, it was generally more common among men than women, suggesting a modulation by sex. Interestingly, in the study by Mahley et al, on HDL levels in Turkish individuals, the authors suggested that the association of Apo E2 with higher HDL cholesterol levels found in women but not in men may be caused by a sex difference in hepatic lipase. Among

alcohol consumption. Using cross-sectional analysis, researcher examined whether variation at the Apo E locus modulates the association between alcohol consumption and LDLcholesterol levels in a healthy population based sample of 1014 male and 1133 female participants in the Framingham Offspring Study (Corella et al., 2001). In male nondrinkers, LDL-cholesterol levels were not different across Apo *E* groups; however, in male drinkers, there were differences in LDL-cholesterol, with Apo E2 subjects displaying the lowest levels. When LDL cholesterol levels were compared among the Apo E subgroups by drinking status, LDL-cholesterol levels in Apo E2 male drinkers were lower than in Apo E2 non drinkers. Conversely, in Apo E4 males, LDL-cholesterol was higher in drinkers than in nondrinkers. This Apo E-alcohol interaction remained significant after controlling for age, BMI, smoking, fat, and energy intake. In women, the expected effect of Apo E alleles on LDL-cholesterol levels was present in both drinkers and nondrinkers. Multiple linear regression models showed a negative association between alcohol and LDL-cholesterol levels in Apo E2 men, with alcohol intake a continuous variable. Conversely, in Apo E4 men, this association was positive. There were no statistically significant associations in either Apo E3 men or in women. These data suggest that in men, variability at the Apo E locus partially modulates the effects of consuming alcoholic beverages on LDL-cholesterol

The effect of alcohol was also investigated in the Copenhagen City Heart Study (Frikke-Schmidt., 2000). In that study, there was an interaction between alcohol and Apo E among women, in which higher triglyceride levels were associated with both the E2 and E4 alleles among women who regularly consumed alcohol. For men, increased triglyceride levels among E2 and E4 carriers were seen across the entire alcohol distribution spectrum, perhaps because of some degree of alcohol consumption among all men. Overall, the results suggested that metabolic stresses, such as the postprandial situation or alcohol consumption, might contribute to uncover underlying differences between Apo E genotypes

The effect of the Apo E gene on lipoproteins may differ with age. In elderly individuals as well as in children, there is less difference in LDL cholesterol levels in individuals carrying the E4 allele versus non-E4 carriers. Interestingly, in both of these age groups, the presence of the Apo E2 allele was associated with lower LDL cholesterol levels. An age-dependent variation between Apo E and plasma lipids was also seen by Jarvik et al (Jarvik et al, 1997). By longitudinally following male Caucasian twins, the authors demonstrated that whereas E4 carriers initially had higher triglyceride and cholesterol levels compared with E3 homozygotes, this difference disappeared over an 18-year period (Rubin & Berglund., 2002). A sex-specific association between Apo E2 and HDL cholesterol levels has been described in Turkish individuals. In Turkish women, but not men, the frequency of the Apo E2 allele increased almost six fold from the lowest to the highest HDL cholesterol tertiary (Rubin & Berglund., 2002). The available information show, significant diet–Apo E gene interactions occurred in male-only studies. In studies including men and women, significant effects were noted only in men, suggesting a significant gene-sex interaction (Ordovas & Corella., 2004). As pointed out above, in studies in which an Apo E gene nutrient interaction was found, it was generally more common among men than women, suggesting a modulation by sex. Interestingly, in the study by Mahley et al, on HDL levels in Turkish individuals, the authors suggested that the association of Apo E2 with higher HDL cholesterol levels found in women but not in men may be caused by a sex difference in hepatic lipase. Among

in cholesterol, triglyceride or lipoprotein metabolism (Frikke-Schmidt., 2000).

levels (Ordovas & Corella., 2004).

women, a lower hepatic lipase activity might allow the detection of the modulating effect of Apo E genotypes, whereas this effect might be overwhelmed by a higher enzyme activity in men (Mahley et al., 2000). This is an analogous situation to the suggestion above that differences in susceptibility might be uncovered by a metabolic challenge (Rubin & Berglund., 2002).

Other causes for the observed differences between studies may be the presence of confounders, the type of dietary intervention used, the population studied and, importantly, the number of subjects in the respective studies. A small number of subjects limits the possibility of detecting differences, or could alternatively lead to spurious associations. Although the number of studies addressing the gene nutrient interaction for Apo E is growing, in most studies so far this has been a secondary endpoint, usually analyzed post hoc. Perhaps the most likely possibility is that a number of dietary interventions will elicit variable responses across Apo E genotypes, but that the ability to detect such differences will depend on the strength and type of intervention as well as on specific recipient factors (type of population, presence of hyperlipidemia, etc. ). In the end, however, our ability to confirm or refute the presence of Apo E gene nutrient interactions as well as to understand their metabolic basis fully will require larger and more detailed studies (Rubin & Berglund., 2002).

Inconsistency in nutrient–gene interactions in relation to Apo E polymorphisms may be a result, in part, of retrospective genotyping of small study cohorts, for which the genotype– diet–LDL-C interactions were not the primary outcome. This factor has resulted in the under-representation of the less-frequent genotypes and, although trends may have been evident, many of the studies were clearly under-powered to detect significant genotype– treatment effects. The prospective genotyping of larger study cohorts has been used as an alternative approach to increase statistical power (Lovegrove & Gitau., 2008).

 However, recent evidence strongly suggests that variations in a number of key genes may also be important, including common variants of the Apo E gene. The most convincing evidence to date for genotypic effects on dietary response comes from the extensively studied Apo E gene variant (Lovegrove & Gitau., 2008).

A metaanalysis has been published recently that summarizes the overall findings from studies using a variety of end-point measures. A mean 40–50% increase in CHD risk was observed in E4 carriers (overall OR 1. 42) relative to the wild-type E3/E3 genotype, with no apparent differences for either the E2 and E3 subgroups (OR 0. 98). Although a causal mechanism to link E4 with increased CHD risk has not been fully elucidated, the association has been ascribed to a higher concentration of LDL-C. This higher LDL-C is believed to arise from the Apo E4 isoform having a relatively higher affinity for its membrane (LDL/chylomicron remnant) receptor and feedback inhibition of receptor activity in E4 carriers Other mechanisms relating to reduced antioxidant status may also be operative (Lovegrove & Gitau., 2008).

There is the large variation that is observed in the concentration of serum LDL-cholesterol (LDL-C) in response to fish oil supplementation. The cardioprotective effects of the fatty acids in fish oil include eicosapantanoiec acid (EPA) and docosahesanoiec acid (DHA) are well recognized. However, a potentially deleterious increase in LDL-C (5–10%) has been consistently reported after moderate to high doses of fish oil (>2 g EPA+ DHA/d). These data showed the DHA rather than the EPA in fish oils that is responsible for the LDL-C raising effects in E4 individuals (Lovegrove & Gitau., 2008).

Nutrigenetics and Dyslipidemia 57

with Apo E3 or E4, leading to an accumulation of HDL in plasma. In addition, there might be a difference in the clearing mechanisms between HDL containing Apo E2 compared with

This mixed pattern was recently addressed by Weggemans et al, who performed a metaanalysis of 26 controlled clinical diet trials conducted. The effect of Apo E genotypes on response to dietary change in 395 healthy subjects, well balanced for sex, was evaluated. The authors pooled data in the response of LDL and HDL cholesterol from four types of trials; replacement of cis-unsaturated fat for saturated fat (n = 7 studies), replacement of cisunsaturated fat for trans unsaturated fat (n = 2), changes in dietary cholesterol (n = 8) and changes in coffee diterpenes (n = 9). Overall, there were small, non-significant differences between Apo E genotypes in the response of LDL cholesterol, and results were unchanged after adjusting for age, sex and body mass index. For HDL cholesterol, a sex difference was noted, as the response to trans fat and cholesterol differed across Apo E genotypes in men

Appropriately, the authors caution against the over interpretation of this result because of

Friedlander et al, compared plasma levels across Apo E genotypes in response to two diets, a high saturated fat/high cholesterol and a low saturated fat/low cholesterol diet, in 214 free-living individuals in two kibbutz settlements in Israel. Although the baseline total and LDL cholesterol levels were higher among E4 carriers and lower among E2 carriers compared with E3 homozygotes, the plasma lipid response to the diet intervention did not

Loktionov et al, investigated 132 free-living healthy individuals participating in the European Prospective Investigation of Cancer study, a cohort study with approximately 25 000 subjects. The reported subgroup was part of a quality control study on the dietary methods used. In the 132 subjects, serum cholesterol levels correlated with the intake of total and saturated fat. For LDL cholesterol, a significant correlation with relative saturated fat intake was seen only for Apo E 4/3, and not for Apo E3/3 or 3/2 (Loktionov et al., 2000). In another recent study, researchers analyzed lipid levels in relation to Apo E genotypes in 420 randomly selected free-living Costa Rican individuals consuming a low fat intake (53% of energy). In accordance with most previous studies, E2 carriers had lower, and E4 carriers higher LDL cholesterol and Apo B levels compared with E3 homozygotes. The population was dichotomized in two groups depending on the intake of saturated fat. High saturated fat intake (mean intake 13. 5% of energy) was associated with increased VLDL cholesterol, decreased HDL cholesterol and smaller LDL sizes in Apo E2 carriers, whereas the opposite was found for Apo E4 carriers. Effects on LDL size had previously been noted by Dreon et al, in which a more pronounced decrease in large, buoyant LDL particles during reduced fat intake was seen for Apo E4 carriers (Dreon et al, 1995). The findings of Campos et al suggested, as pointed out by the authors, that in E2 carriers, a high saturated fat intake may result in increased VLDL production and delayed clearance. Such a metabolic challenge

might thus unmask a relative susceptibility in E2 carriers (Campos et al, 2001).

triglyceride levels among E2 carriers (Rubin & Berglund., 2002).

Finally, the study on plasma lipid response to dietary fat and carbohydrate in men and women with coronary heart disease provided further support for the association of triglyceride metabolism with Apo E2. Overall, E2 carriers had lower LDL cholesterol as well as a tendency to higher triglyceride levels than E3 and E4 carriers. In addition, there was a positive association between dietary sucrose (6±7% of the total energy intake) and plasma

Apo E3 or E4 (Mahley et al., 2000).

but not in women (Weggemans et al., 2001).

chance associations (Rubin & Berglund., 2002).

differ across Apo E genotypes (Friedlander et al., 2000).

In contrast, those with one or more E4 alleles have the highest serum total cholesterol, LDL-, and Apo B levels, the lowest HDL-C levels, and have elevated fasting and postprandial triglyceride levels. They respond best to a low-fat diet but are the least responsive to soluble fiber for lowering serum lipids or to exercise for increasing HDL levels. Fish oil supplementation in these people increases total cholesterol and reduces HDL. Whether a person has the *U* allele or the E4 allele appears to make a difference in the diet and lifestyle recommendations that would be appropriate for improving vascular health (DeBusk., 2009). Two prospectively genotyped studies designed to test the hypothesis that Apo E polymorphism has a significant effect on the LDL-C response to EPA and DHA have recently been completed (Lovegrove & Gitau., 2008).

Overall, the triglyceride response to the fat load was lower during fish oil supplementation, and interestingly the decrease in the incremental area under the curve for triglyceride levels was significantly higher for E2 carriers compared with E3 homozygotes and E4 carriers (Rubin & Berglund., 2002).

Although a number of previous studies have observed effects of Apo E genotype in response to dietary total fat and saturated fatty acid (SFA) manipulation, only one study to date has examined the Apo E genotype–dietary fat-LDL-C association using prospective recruitment by genotype. A study reported a significant effect of Apo E genotype on the plasma lipid response to a low fat diet, with a 5%, 13% and 16% reduction in LDL-C in E3/E3, E3/E4 and E4/E4 males, respectively. Other studies have examined the association between Apo E genotype and fish oil (EPA/DHA) on LDL-C responses. In a retrospectively genotyped study it was observed that a mean increase of 7. 1% in LDL-C for the group as a whole was solely attributable to a 16% rise in LDL-C in the Apo E4 participants, and it was speculated that Apo E genotype may, in part, predict the blood lipid response to fish oil intervention. Variable effects of EPA and DHA on LDL-C have been reported previously (Kobayashi et al., 2001; Lovegrove & Gitau., 2008).

The ApoE gene locus accounts for approximately 7% of the population variance in total and LDL cholesterol levels; in general, E4 carriers have higher and E2 carriers have lower LDL cholesterol levels. It has also been suggested that Apo E variations impact triglyceride levels, as higher triglyceride levels have been reported for both E4 and E2 carriers compared with E3 homozygotes (Rubin & Berglund, 2002).

In a recent study of more than 9000 individuals from the Copenhagen City Heart Study, Frikke-Schmidt and colleagues demonstrated that the association between the Apo E locus and cholesterol or plasma Apo B levels was invariant, i. e. present in most contexts (e. g. present in both men and women), whereas associations between Apo E and other lipoproteins such as triglycerides, Apo A-I, HDL cholesterol and lipoprotein (a) were found to be context dependent (Frikke-Schmidt., 2000). As the associations of Apo E with Apo B remained significant when adjusting for cholesterol but not the other way around, this suggested that Apo B is the factor primarily associated with Apo E genotype. It should be pointed out, however, that in their study triglyceride levels represented nonfasting conditions, and LDL cholesterol was not included in the analysis (Rubin & Berglund., 2002).

Furthermore, the Apo E2 allele was more common in individuals with high LpA-I levels, i. e. HDL with Apo AI but not Apo A-II. This HDL subfraction generally corresponds to the larger HDL2 subpopulation, which interestingly, in a study by Isasi et al, was associated with Apo E2 in children (Isasi et al., 2000). In view of their results, Mahley et al, suggest that HDL containing Apo E2 might be a poorer substrate for hepatic lipase compared with HDL

In contrast, those with one or more E4 alleles have the highest serum total cholesterol, LDL-, and Apo B levels, the lowest HDL-C levels, and have elevated fasting and postprandial triglyceride levels. They respond best to a low-fat diet but are the least responsive to soluble fiber for lowering serum lipids or to exercise for increasing HDL levels. Fish oil supplementation in these people increases total cholesterol and reduces HDL. Whether a person has the *U* allele or the E4 allele appears to make a difference in the diet and lifestyle recommendations that would be appropriate for improving vascular health (DeBusk., 2009). Two prospectively genotyped studies designed to test the hypothesis that Apo E polymorphism has a significant effect on the LDL-C response to EPA and DHA have

Overall, the triglyceride response to the fat load was lower during fish oil supplementation, and interestingly the decrease in the incremental area under the curve for triglyceride levels was significantly higher for E2 carriers compared with E3 homozygotes and E4 carriers

Although a number of previous studies have observed effects of Apo E genotype in response to dietary total fat and saturated fatty acid (SFA) manipulation, only one study to date has examined the Apo E genotype–dietary fat-LDL-C association using prospective recruitment by genotype. A study reported a significant effect of Apo E genotype on the plasma lipid response to a low fat diet, with a 5%, 13% and 16% reduction in LDL-C in E3/E3, E3/E4 and E4/E4 males, respectively. Other studies have examined the association between Apo E genotype and fish oil (EPA/DHA) on LDL-C responses. In a retrospectively genotyped study it was observed that a mean increase of 7. 1% in LDL-C for the group as a whole was solely attributable to a 16% rise in LDL-C in the Apo E4 participants, and it was speculated that Apo E genotype may, in part, predict the blood lipid response to fish oil intervention. Variable effects of EPA and DHA on LDL-C have been reported previously

The ApoE gene locus accounts for approximately 7% of the population variance in total and LDL cholesterol levels; in general, E4 carriers have higher and E2 carriers have lower LDL cholesterol levels. It has also been suggested that Apo E variations impact triglyceride levels, as higher triglyceride levels have been reported for both E4 and E2 carriers compared with

In a recent study of more than 9000 individuals from the Copenhagen City Heart Study, Frikke-Schmidt and colleagues demonstrated that the association between the Apo E locus and cholesterol or plasma Apo B levels was invariant, i. e. present in most contexts (e. g. present in both men and women), whereas associations between Apo E and other lipoproteins such as triglycerides, Apo A-I, HDL cholesterol and lipoprotein (a) were found to be context dependent (Frikke-Schmidt., 2000). As the associations of Apo E with Apo B remained significant when adjusting for cholesterol but not the other way around, this suggested that Apo B is the factor primarily associated with Apo E genotype. It should be pointed out, however, that in their study triglyceride levels represented nonfasting conditions, and LDL cholesterol was not included in the analysis (Rubin & Berglund., 2002). Furthermore, the Apo E2 allele was more common in individuals with high LpA-I levels, i. e. HDL with Apo AI but not Apo A-II. This HDL subfraction generally corresponds to the larger HDL2 subpopulation, which interestingly, in a study by Isasi et al, was associated with Apo E2 in children (Isasi et al., 2000). In view of their results, Mahley et al, suggest that HDL containing Apo E2 might be a poorer substrate for hepatic lipase compared with HDL

recently been completed (Lovegrove & Gitau., 2008).

(Kobayashi et al., 2001; Lovegrove & Gitau., 2008).

E3 homozygotes (Rubin & Berglund, 2002).

(Rubin & Berglund., 2002).

with Apo E3 or E4, leading to an accumulation of HDL in plasma. In addition, there might be a difference in the clearing mechanisms between HDL containing Apo E2 compared with Apo E3 or E4 (Mahley et al., 2000).

This mixed pattern was recently addressed by Weggemans et al, who performed a metaanalysis of 26 controlled clinical diet trials conducted. The effect of Apo E genotypes on response to dietary change in 395 healthy subjects, well balanced for sex, was evaluated. The authors pooled data in the response of LDL and HDL cholesterol from four types of trials; replacement of cis-unsaturated fat for saturated fat (n = 7 studies), replacement of cisunsaturated fat for trans unsaturated fat (n = 2), changes in dietary cholesterol (n = 8) and changes in coffee diterpenes (n = 9). Overall, there were small, non-significant differences between Apo E genotypes in the response of LDL cholesterol, and results were unchanged after adjusting for age, sex and body mass index. For HDL cholesterol, a sex difference was noted, as the response to trans fat and cholesterol differed across Apo E genotypes in men but not in women (Weggemans et al., 2001).

Appropriately, the authors caution against the over interpretation of this result because of chance associations (Rubin & Berglund., 2002).

Friedlander et al, compared plasma levels across Apo E genotypes in response to two diets, a high saturated fat/high cholesterol and a low saturated fat/low cholesterol diet, in 214 free-living individuals in two kibbutz settlements in Israel. Although the baseline total and LDL cholesterol levels were higher among E4 carriers and lower among E2 carriers compared with E3 homozygotes, the plasma lipid response to the diet intervention did not differ across Apo E genotypes (Friedlander et al., 2000).

Loktionov et al, investigated 132 free-living healthy individuals participating in the European Prospective Investigation of Cancer study, a cohort study with approximately 25 000 subjects. The reported subgroup was part of a quality control study on the dietary methods used. In the 132 subjects, serum cholesterol levels correlated with the intake of total and saturated fat. For LDL cholesterol, a significant correlation with relative saturated fat intake was seen only for Apo E 4/3, and not for Apo E3/3 or 3/2 (Loktionov et al., 2000).

In another recent study, researchers analyzed lipid levels in relation to Apo E genotypes in 420 randomly selected free-living Costa Rican individuals consuming a low fat intake (53% of energy). In accordance with most previous studies, E2 carriers had lower, and E4 carriers higher LDL cholesterol and Apo B levels compared with E3 homozygotes. The population was dichotomized in two groups depending on the intake of saturated fat. High saturated fat intake (mean intake 13. 5% of energy) was associated with increased VLDL cholesterol, decreased HDL cholesterol and smaller LDL sizes in Apo E2 carriers, whereas the opposite was found for Apo E4 carriers. Effects on LDL size had previously been noted by Dreon et al, in which a more pronounced decrease in large, buoyant LDL particles during reduced fat intake was seen for Apo E4 carriers (Dreon et al, 1995). The findings of Campos et al suggested, as pointed out by the authors, that in E2 carriers, a high saturated fat intake may result in increased VLDL production and delayed clearance. Such a metabolic challenge might thus unmask a relative susceptibility in E2 carriers (Campos et al, 2001).

Finally, the study on plasma lipid response to dietary fat and carbohydrate in men and women with coronary heart disease provided further support for the association of triglyceride metabolism with Apo E2. Overall, E2 carriers had lower LDL cholesterol as well as a tendency to higher triglyceride levels than E3 and E4 carriers. In addition, there was a positive association between dietary sucrose (6±7% of the total energy intake) and plasma triglyceride levels among E2 carriers (Rubin & Berglund., 2002).

Nutrigenetics and Dyslipidemia 59

explain the reduced postprandial clearance in E4 carriers. However, it is possible that a differential distribution of the varying Apo E isoforms over different lipoprotein fractions, as well as variations in Apo E levels, could play a role. In addition, a lower LDL receptor activity in E4 carriers may contribute to a decreased postprandial clearance (Rubin &

How could we reconcile these varying results? Even if most studies have established associations between Apo E and baseline lipoprotein levels, the absolute differences between the Apo E genotypes are relatively modest. It might thus be expected that intergenotype differences in response to nutrient variations may generally be even smaller in magnitude, and thus more difficult to detect, although they might be enhanced by metabolic challenges affecting the synthetic or clearance systems in lipoprotein metabolism described above. Examples of such metabolic stresses in which Apo E gene nutrient interactions may be more readily detectable may be hyperlipidemia, an increased intake of saturated fat or cholesterol, the postprandial state, or alcohol intake. In agreement with this, studies indicating Apo E gene nutrient interactions have been more common in hyperlipidemic settings, whereas it has been more difficult to detect differences across Apo E genotypes in normolipidemic individuals or populations. However, Apo E gene± nutrient interaction has not been seen in all hyperlipidemic states. In familial heperlipidemia heterozygotes, no difference in plasma lipid response to a step 1 diet was seen across Apo E genotypes, indicating that the modulating effects of Apo E may be overwhelmed by other genetic

In conclusion, Apo E has important functions in lipoprotein metabolism and the Apo E polymorphism is associated with plasma lipoprotein levels. Although a large number of studies have addressed whether there is an interaction between Apo E genotypes and diet in affecting plasma lipid levels, this issue is presently unresolved. Most studies to date have involved a small number of subjects, analyzed the Apo E polymorphism post hoc, or included populations in which the effects might be modest, making discrepancies difficult to detect. Studies conducted with conditions representing a metabolic challenge have generally been more successful in finding differential effects across Apo E genotypes, and such studies may be helpful in the future to clarify Apo E gene nutrient relationships. The mixed results obtained indicate that, at present, it is premature to suggest the use of genotyping of Apo E in the design of therapeutic diet interventions (Rubin & Berglund.,

All studies have demonstrated a strong association between plasma cholesterol and Apo E phenotypes in the following order: E4/E4 > E4/E3 > E3/E3 > E3/E2. It has been thought possible that the Apo E gene might be involved in the modulation of dietary plasma cholesterol responses, perhaps explaining the differences in cholesterol concentrations. Some dietary intervention studies have suggested that Apo E4 individuals react to dietary change with exaggerated cholesterol responses. In one study, Apo E4/E4 individuals responded by increased cholesterol reductions during low fat intake, and by increased cholesterol elevations during a switchback to high fat diet. Plausible mechanisms have been postulated which could explain such differences. However, other studies have reported no differences in plasma lipid responses among Apo E phenotypes. The studies cannot be directly compared because of different designs and study populations with differing Apo E

Although Tikkanen et al, found that subjects with the E4/4 phenotype showed the greatest total and LDL-cholesterol responses to dietary change (Tikkanen et al., 1995) Xu et al

defects, such as LDL receptor deficiency (Rubin & Berglund., 2002).

Berglund., 2002).

2002).

allele frequencies (Tikkanen., 1995).

Postprandial studies as Apo E have important functions in chylomicron remnant metabolism, there has been substantial interest in the role of Apo E genotypes in the postprandial setting. Furthermore, a postprandial challenge could serve as a tool to uncover more precisely the differences between different Apo E alleles (Rubin & Berglund., 2002).

In a study of normolipidemic adults by Rubin & Berglund, the Apo E2 allele was associated with an increased postprandial triglyceride response. A similar response has also been demonstrated in other studies. Regarding the Apo E4 allele, more controversial results have been obtained (Rubin & Berglund., 2002).

However, although such studies are compatible with a faster clearance of VLDL and chylomicron remnants in E4 compared with E3 carriers, the meta-analysis showed higher triglyceride and lower HDL cholesterol levels among E4/3 individuals compared with E3 homozygotes. This would perhaps suggest an impaired postprandial clearance among E4 carriers (Rubin & Berglund., 2002). In support of this, another study found an impaired clearance of chylomicron and VLDL remnants in normolipidemic male E4 carriers compared with E3/3. Furthermore, several recent studies have also reported an increased postprandial triglyceride excursion in E4 carriers. In children, they did not observe any difference in triglyceride or retinyl palmitate response between E3/3 and E4 carriers, although a nonsignificant trend towards higher baseline triglyceride levels as well as higher triglyceride and retinyl palmitate levels 3 h postprandially among E4 carriers was seen (Couch et al., 2000). Another research found no significant effects of the Apo E4 allele on the postprandial triglyceride response after adjusting for baseline triglyceride levels, although a delayed retinyl palmitate clearance in E2 carriers was observed (Rubin & Berglund., 2002). In a recent study by Kobayashi et al, individuals with the E3/3 and E3/4 genotypes were matched for intra-abdominal visceral fat accumulation. Postprandial triglyceride levels did not differ between the two genotypes when adjusting for baseline levels, whereas retinyl palmitate levels among lipoproteins with Sf 5400 were higher among male E3/4 subjects, indicating a slower remnant clearance. As pointed out by the authors, there were fewer women in the study, which might contribute to the non-significant finding in this sex group (Kobayashi et al., 2001).

In a study, the researchers investigated postprandial fat load tolerance in 55 healthy volunteers with an atherogenic lipid profile, defined as triglyceride levels of 1. 5±4 m M, cholesterol 5±8 m M and HDL cholesterol less than 1. 1 m M, as part of a double-blind placebo-controlled crossover study with the consumption of either 6 g of fish oil or 6 g of olive oil supplements for 6 weeks. At the end of each period, a postprandial study was carried out. The difference in LDL cholesterol levels among Apo E genotypes is associated with differences in LDL receptor activity, with Apo E2 carriers having higher and Apo E4 carriers lower activity compared with Apo E3 homozygotes. Conditions with increased stress of this system, such as the increased intake of cholesterol and saturated fat, could therefore result in a variable response in LDL cholesterol levels across Apo E genotypes. In addition, E2 carriers may have decreased lipolytic function with an inhibition of the conversion of VLDL to LDL, as well as a compromised clearance system for triglyceride-rich lipoproteins. Therefore, even a modestly increased VLDL production in response to increased precursor availability might result in differences in plasma triglyceride levels across Apo E genotypes (Rubin & Berglund., 2002).

Although the results from postprandial studies are generally in agreement with the established metabolic differences between Apo E2 and E3, it is currently more difficult to

Postprandial studies as Apo E have important functions in chylomicron remnant metabolism, there has been substantial interest in the role of Apo E genotypes in the postprandial setting. Furthermore, a postprandial challenge could serve as a tool to uncover more precisely the differences between different Apo E alleles (Rubin & Berglund., 2002). In a study of normolipidemic adults by Rubin & Berglund, the Apo E2 allele was associated with an increased postprandial triglyceride response. A similar response has also been demonstrated in other studies. Regarding the Apo E4 allele, more controversial results have

However, although such studies are compatible with a faster clearance of VLDL and chylomicron remnants in E4 compared with E3 carriers, the meta-analysis showed higher triglyceride and lower HDL cholesterol levels among E4/3 individuals compared with E3 homozygotes. This would perhaps suggest an impaired postprandial clearance among E4 carriers (Rubin & Berglund., 2002). In support of this, another study found an impaired clearance of chylomicron and VLDL remnants in normolipidemic male E4 carriers compared with E3/3. Furthermore, several recent studies have also reported an increased postprandial triglyceride excursion in E4 carriers. In children, they did not observe any difference in triglyceride or retinyl palmitate response between E3/3 and E4 carriers, although a nonsignificant trend towards higher baseline triglyceride levels as well as higher triglyceride and retinyl palmitate levels 3 h postprandially among E4 carriers was seen (Couch et al., 2000). Another research found no significant effects of the Apo E4 allele on the postprandial triglyceride response after adjusting for baseline triglyceride levels, although a delayed retinyl palmitate clearance in E2 carriers was observed (Rubin & Berglund., 2002). In a recent study by Kobayashi et al, individuals with the E3/3 and E3/4 genotypes were matched for intra-abdominal visceral fat accumulation. Postprandial triglyceride levels did not differ between the two genotypes when adjusting for baseline levels, whereas retinyl palmitate levels among lipoproteins with Sf 5400 were higher among male E3/4 subjects, indicating a slower remnant clearance. As pointed out by the authors, there were fewer women in the study, which might contribute to the non-significant finding in this sex group

In a study, the researchers investigated postprandial fat load tolerance in 55 healthy volunteers with an atherogenic lipid profile, defined as triglyceride levels of 1. 5±4 m M, cholesterol 5±8 m M and HDL cholesterol less than 1. 1 m M, as part of a double-blind placebo-controlled crossover study with the consumption of either 6 g of fish oil or 6 g of olive oil supplements for 6 weeks. At the end of each period, a postprandial study was carried out. The difference in LDL cholesterol levels among Apo E genotypes is associated with differences in LDL receptor activity, with Apo E2 carriers having higher and Apo E4 carriers lower activity compared with Apo E3 homozygotes. Conditions with increased stress of this system, such as the increased intake of cholesterol and saturated fat, could therefore result in a variable response in LDL cholesterol levels across Apo E genotypes. In addition, E2 carriers may have decreased lipolytic function with an inhibition of the conversion of VLDL to LDL, as well as a compromised clearance system for triglyceride-rich lipoproteins. Therefore, even a modestly increased VLDL production in response to increased precursor availability might result in differences in plasma triglyceride levels

Although the results from postprandial studies are generally in agreement with the established metabolic differences between Apo E2 and E3, it is currently more difficult to

been obtained (Rubin & Berglund., 2002).

(Kobayashi et al., 2001).

across Apo E genotypes (Rubin & Berglund., 2002).

explain the reduced postprandial clearance in E4 carriers. However, it is possible that a differential distribution of the varying Apo E isoforms over different lipoprotein fractions, as well as variations in Apo E levels, could play a role. In addition, a lower LDL receptor activity in E4 carriers may contribute to a decreased postprandial clearance (Rubin & Berglund., 2002).

How could we reconcile these varying results? Even if most studies have established associations between Apo E and baseline lipoprotein levels, the absolute differences between the Apo E genotypes are relatively modest. It might thus be expected that intergenotype differences in response to nutrient variations may generally be even smaller in magnitude, and thus more difficult to detect, although they might be enhanced by metabolic challenges affecting the synthetic or clearance systems in lipoprotein metabolism described above. Examples of such metabolic stresses in which Apo E gene nutrient interactions may be more readily detectable may be hyperlipidemia, an increased intake of saturated fat or cholesterol, the postprandial state, or alcohol intake. In agreement with this, studies indicating Apo E gene nutrient interactions have been more common in hyperlipidemic settings, whereas it has been more difficult to detect differences across Apo E genotypes in normolipidemic individuals or populations. However, Apo E gene± nutrient interaction has not been seen in all hyperlipidemic states. In familial heperlipidemia heterozygotes, no difference in plasma lipid response to a step 1 diet was seen across Apo E genotypes, indicating that the modulating effects of Apo E may be overwhelmed by other genetic defects, such as LDL receptor deficiency (Rubin & Berglund., 2002).

In conclusion, Apo E has important functions in lipoprotein metabolism and the Apo E polymorphism is associated with plasma lipoprotein levels. Although a large number of studies have addressed whether there is an interaction between Apo E genotypes and diet in affecting plasma lipid levels, this issue is presently unresolved. Most studies to date have involved a small number of subjects, analyzed the Apo E polymorphism post hoc, or included populations in which the effects might be modest, making discrepancies difficult to detect. Studies conducted with conditions representing a metabolic challenge have generally been more successful in finding differential effects across Apo E genotypes, and such studies may be helpful in the future to clarify Apo E gene nutrient relationships. The mixed results obtained indicate that, at present, it is premature to suggest the use of genotyping of Apo E in the design of therapeutic diet interventions (Rubin & Berglund., 2002).

All studies have demonstrated a strong association between plasma cholesterol and Apo E phenotypes in the following order: E4/E4 > E4/E3 > E3/E3 > E3/E2. It has been thought possible that the Apo E gene might be involved in the modulation of dietary plasma cholesterol responses, perhaps explaining the differences in cholesterol concentrations. Some dietary intervention studies have suggested that Apo E4 individuals react to dietary change with exaggerated cholesterol responses. In one study, Apo E4/E4 individuals responded by increased cholesterol reductions during low fat intake, and by increased cholesterol elevations during a switchback to high fat diet. Plausible mechanisms have been postulated which could explain such differences. However, other studies have reported no differences in plasma lipid responses among Apo E phenotypes. The studies cannot be directly compared because of different designs and study populations with differing Apo E allele frequencies (Tikkanen., 1995).

Although Tikkanen et al, found that subjects with the E4/4 phenotype showed the greatest total and LDL-cholesterol responses to dietary change (Tikkanen et al., 1995) Xu et al

Nutrigenetics and Dyslipidemia 61

nucleotide polymorphisms (SNPs) on the 5\_-flanking region of the HL gene (*LIPC*) [−763 (A/G), −710 (T/C), −514 (C/T), and –250 (G/A)] are in total linkage disequilibrium and define a unique haplotype that is associated with variation in HL activity and HDL-

The less common A-allele of the SNP at position−250 is associated with lower HL activity and buoyant LDL particles. Normal and CAD subjects heterozygous for the A-allele have lower HL activity and significantly more buoyant LDL particles. Homozygosity for this allele (AA) is associated with an even lower HL activity. The A-allele is associated with

An early intervention study with a low-saturated-fat, low-cholesterol diet found that, although significant improvements in fasting lipids occurred, there was no difference in response between genotypes at the hepatic lipase gene (LIPC) polymorphism measured. However, the study of 83 subjects may not have had adequate power to detect a modest effect of genotype (Fisler & Warden., 2005; Masson & McNeill., 2005; Ordovas., 2006). Dietary information collected from Framingham Heart Study participants shows that subjects carrying the CC genotype react to higher contents of fat in their diets by increasing the concentrations of HDL-cholesterol, which could be interpreted as a "defense mechanism" to maintain the homeostasis of lipoprotein metabolism. Conversely, carriers of the TT genotype cannot compensate, and experience decreases on the HDL-cholesterol levels. These data could identify a segment of the population especially susceptible to dietinduced atherosclerosis. Considering the higher frequency of the T allele among certain ethnic groups (i. e., African-Americans), these data could shed some light on the impaired ability of certain ethnic groups to adapt to new nutritional environments, as clearly seen for Native Americans and Asian Indians. In this regard, they replicated the significant gene-diet interaction demonstrated in the Caucasian population of Framingham in another multiethnic cohort that consisted of Chinese, Malays, and Indians representing the population of Singapore. In addition to the significant gene-diet interactions reported in these papers, these data provides clues about the reasons why genotype-phenotype association studies fail to show consistent results. In theory, this polymorphism at the hepatic lipase gene will show dramatically different outcomes in association studies depending on the dietary environment of the population studies. The impact of these interactions will be magnified in populations with a high prevalence of the T-allele, as it is

Three larger observational studies on the effect of a common polymorphism in the LIPC promoter gene –514C→T on the response of HDL cholesterol to dietary fat intake have been published. In examining the effects of the –514C→T LIPC polymorphism x dietary fat interaction on HDL in 2130 men and women participating in the Framingham Study, Ordovas et al found that the rarer *TT* genotype was associated with significantly higher HDL-cholesterol concentrations only in subjects consuming <30% of energy from fat. This same interaction was found for saturated and monounsaturated fats but not for polyunsaturated fat. A second association study, in an Asian population of 2170 subjects, found that Asian Indian subjects with a total fat intake of <30% of energy and with *TT* genotype at the –514C→T polymorphism had the highest HDL-cholesterol concentrations. This interaction, however, did not apply to the Chinese or Malay subjects in that study, and the significant interactions found for saturated or monounsaturated fats found by Ordovas et al were not found in the study by Tai et al. However, these 2 studies are consistent with

cholesterol levels (Fisler & Warden., 2005; Ordovas & Corella., 2004).

with Asians and African-Americans (Ordovas & Corella., 2004).

higher HDL2-cholesterol (Ordovas & Corella., 2004).

analyzed the same data and concluded that the Apo E polymorphism did not explain a significant proportion of the variation of the response (Xu et al., 1990). In a meta-analysis of 9 studies involving 612 subjects and found that the presence of the E4 allele was associated with a significantly greater LDL response to dietary intervention (Masson et al, 2003).

Four studies found significantly different HDL-cholesterol responses between genotype groups: one study found that carriers of the E4 allele had the smallest HDL-cholesterol response, whereas the other 3 studies found that carriers of the E4 allele had the largest response (Masson et al., 2003).

However, recent evidence strongly suggests that variations in a number of key genes may also be important, including common variants of the Apo E gene. The most convincing evidence to date for genotypic effects on dietary response comes from the extensively studied Apo E gene variant (Lovegrove & Gitau., 2008).
