**9. Possible reasons for conflicting results**

There are many possible reasons why studies have been unable to show statistically significant gene-diet interactions. First, it is highly probable that lipid responses to dietary change are under polygenic control, with each gene contributing a relatively small effect. However, most studies have attempted to find only single-gene effects (Masson et al, 2003). Most of the studies summarized in this review lacked sufficient statistical power to detect any but a very strong effect because the sample sizes were too small, particularly for genotypes with low frequencies of the rare allele. However, many of the studies were retrospective and were not designed to examine gene-diet interactions, but data were

attributable to a single polymorphism is not likely to be > 10% (Xu et sal., 1990). Therefore, individual genes contribute only a small part to the variation in the lipid response; however, when several genes are considered, the proportion of variance explained could be larger

Evidence suggests that variation in the genes for apolipoproteins A-I, A-IV, B, and E may contribute to the heterogeneity in the lipid response to dietary intervention. Many studies were unable to show significantly different responses between these genotype groups, and the genotypes showing the greatest response are not necessarily consistent between studies. There was insufficient evidence to assess whether lipid responsiveness is affected by variation in the genes for Apo C-III, lipoprotein lipase, hepatic lipase, the cholesteryl ester transfer protein. Although each of these gene products is essential in lipid metabolism, only a handful of studies have investigated variation in these genes, and most of these studies

Publication bias is a problem with any review because "studies with results that are significant, interesting, from large well-funded studies, or of higher quality are more likely to be submitted, published, or published more rapidly than work without such characteristics" (Sutton, 2000). Therefore, it is possible that other relevant dietary intervention studies with genotype information exist but were not included in this review because they have not been published. It is possible that the literature strategy for this review missed studies because the genotype analyses were not mentioned in their title,

In the search for explanations for the heterogeneity in lipid responses, reviewers may tend to highlight studies showing significant effects of genetic variation while ignoring a large proportion of studies that found no such results. Studies showing nonsignificant or conflicting results cannot be ignored, especially because they outnumber the studies showing significant effects, notwithstanding the unpublished studies that could have nonsignificant and uninteresting results. Therefore, one has to ask the question "If genetic variability plays a role in the heterogeneity of lipid and lipoprotein responses to dietary change, why have so many studies been unable to demonstrate this with statistical

There are many possible reasons why studies have been unable to show statistically significant gene-diet interactions. First, it is highly probable that lipid responses to dietary change are under polygenic control, with each gene contributing a relatively small effect. However, most studies have attempted to find only single-gene effects (Masson et al, 2003). Most of the studies summarized in this review lacked sufficient statistical power to detect any but a very strong effect because the sample sizes were too small, particularly for genotypes with low frequencies of the rare allele. However, many of the studies were retrospective and were not designed to examine gene-diet interactions, but data were

were unable to show significant gene-diet interactions (Masson et al, 2003).

(Masson et al, 2003).

**8. Publication bias** 

**7. Evidence for a gene-diet interaction** 

abstract, or subject headings (Masson et al, 2003).

**9. Possible reasons for conflicting results** 

significance?" (Masson et al, 2003).

reexamined after the availability of new information from genotype analyses. Therefore, it is perhaps not surprising that significant effects were not found in many studies because the numbers of individuals in each genotype group were so small. In many studies there were too few subjects homozygous for the rare allele to allow an analysis that would take into account differences in the response between heterozygote's and homozygote's. For Apo E, where there are 6 possible genotypes, differences in the grouping of these could also lead to differences in results between studies. This illustrates that meta-analyses are important because they can detect effects with greater power and greater precision because of their inflated sample size (Rantala et al., 2000; Lopez-Miranda et al., 1994). In addition, in studies with small sample sizes, genotype misclassification of one individual may significantly affect the interpretation and validity of the results. Conflicting results may also occur because of the different dietary protocols that were followed. The studies reviewed varied widely in the composition and length of the baseline and experimental diets. The dietary factors responsible for the changes seen in each genotype group are not clear because many studies modified several dietary factors, and so the dietary content in future studies should be tightly controlled and compliance must be strictly measured not only for cholesterol and the amount and type of fatty acids but also for other influential dietary components such as fiber and plant sterols. In addition, these studies investigated fasting lipid and lipoprotein concentrations; however, the effect of genetic variation may be more evident in the postprandial state than in the less-common fasting state). Differences in the age, sex, body mass index, menopausal status, dietary backgrounds, and baseline lipid values of the participants could also have contributed to the discrepancies between the results. For instance, subjects with the E4 allele tend to have higher baseline total and LDL-cholesterol concentrations, and so greater responses in these subjects could reflect the regression to the mean phenomenon. It is also possible that weight change could account for differences in lipid and lipoprotein changes. In addition, a significant effect may not reflect a causal relation but the allele may be in linkage disequilibrium with another one that does. For example, the base change that results in the *Xba* I site in the gene for Apo B does not alter the amino acid, and so it may be in linkage disequilibrium with another functional mutation (Masson et al., 2003).

The studies varied widely in terms of the number and type of study participants, the composition and duration of the dietary interventions, the nutrients studied and dietary assessment methods used in the observational studies, and the polymorphisms analyzed some of which had not been studied before with regard to the lipid response to diet (Masson et al., 2005).

#### **10. Conclusion**

Evidence suggests that genetic variation may contribute to the heterogeneity in lipid responsiveness. At present, the evidence is limited but suggestive and justifies the need for future studies with much larger sample sizes based on power calculations, with carefully controlled dietary interventions, and that investigate the effects of polymorphisms in multiple genes rather than in single genes. Investigating gene-diet interactions will increase our knowledge of the mechanisms involved in lipid metabolism and improve our understanding of the role of diet in reducing cardiovascular disease risk (Masson et al., 2003).

Nutrigenetics and Dyslipidemia 67

whether the genetic tests and personalized food products would be affordable, cost-effective and socially acceptable. It is also of concern that only the well educated and affluent would benefit. The open accessibility of genetic information to third parties has major implications for the availability of health insurance and increased premiums (Lovegrove, Gitau, 2008). Moreover, it is still unknown whether people will want to undertake genetic tests or even understand the concept of such technology. A survey was conducted by Cogent Research in 2003 on 1000 Americans in which 62% of respondents reported they had never heard of 'nutrigenomics'. However, if specific products did arise from nutrigenomic research, those interviewed did express interest in an in-depth well-being assessment and also a strong interest in vitamins, fortified foods and natural foods. More research is required to determine whether individuals would want to undergo such tests, and for understanding the value to the individual of an increased awareness of personalized nutrition regimens. There is already a large gap between the existing dietary guidelines and what people actually eat. Knowledge of being at higher than average risk of CVD may motivate people to actually make positive changes to their diets. However, genetic testing could undermine current healthy eating messages, by implying that only those with the 'risky gene' need to eat a healthy diet. These are important unanswered questions that must be addressed if personalized nutritional advice is ever to become part of mainstream disease prevention and treatment. It may be that the interactions between genotype–phenotype and the environment are just too complex to be properly understood from human dietary

There is resistance to the use and perceived effectiveness of personalized nutrition that is based on genomics, and whether this can offer a solution to diseases caused by a diet that is inappropriate for health (Canon & Leitzmann, 2005). It has been suggested that it may be more beneficial to use current risk factors as a basis for population screening and the management of CVD (McCluskey et al., 2007). There has also been dialogue on the social, economic and environmental causes of CVD, shifting the emphasis away from dietary intake to food manufacturing as being more effective in disease management (Canon &

Progression of knowledge in the fields of nutrient–gene interactions promises a future revolution in preventative health care. However, although there is increasing evidence for interactions between diets/nutrients, genes and environmental factors, there are inconsistencies in the evidence that will limit the application of nutrigenetics in diet-related disease in the immediate future. In addition to the need for adequately powered intervention studies, greater attention should be given to ethical issues, such as the public's acceptance of genetic testing and the economics of this relatively new science (Lovegrove &

Ethical issues fall into a number of categories: (1) why nutrigenomics? Will it have important public health benefits? (2) questions about research, e. g. concerning the acquisition of information about individual genetic variation; (3) questions about who has access to this information, and its possible misuse; (4) the applications of this information in terms of public health policy, and the negotiation of the potential tension between the interests of the individual in relation to, for example, prevention of conditions such as obesity and allergy; (5) the appropriate ethical approach to the issues, e. g. the moral difference, if any, between therapy and enhancement in relation to individualised diets; whether the 'technological fix' is always appropriate, especially in the wider context of the

intervention studies (Lovegrove, Gitau, 2008).

Leitzmann, 2005; Lovegrove & Gitau., 2008).

Gitau., 2008).

Future studies will have to be large in order to assess the effects of multiple polymorphisms, and will have to control for many factors other than diet. At present, it is premature to recommend the use of genotyping in the design of therapeutic diets. However, such studies may be useful in identifying the mechanisms by which dietary components influence lipid levels (Masson & McNeill., 2005).

However, current knowledge is still very limited and so is the potential benefit of its application to clinical practice. Thinking needs to evolve from simple scenarios (e. g., one single dietary component, a single nucleotide polymorphism and risk factor) to more realistic situations involving multiple interactions. One of the first situations where personalized nutrition is likely to be beneficial is in patients with dyslipidemia who require special intervention with dietary treatment. This process could be more efficient if the recommendations were carried out based on genetic and molecular knowledge. Moreover, adherence to dietary advice may increase when it is supported with information based on nutritional genomics, and a patient believes the advice is personalized. However, a number of important changes in the provision of health care are needed to achieve the potential benefits associated with this concept, including a teamwork

approach with greater integration among physicians, food and nutrition professionals, and genetic counselors (Ordovas., 2006).

The ultimate goal of nutritional genomics is to provide sufficient knowledge to allow diagnosis and nutritional treatment recommendations based on an individual's genotype. Defining the interaction effects of nutrients and genes on complex phenotypes will be the challenge of this field of nutrition research for some time to come (Fisler & Warden, 2005; Gregori et al, 2011; Ordovas & Corella., 2004).
