**2.1 Total and LDL cholesterol**

Two decades after World War II, large population studies had been performed in different countries in order to determine risk factors of heart disease. The most famous studies include the Framingham Study, Chicago and Tecumseh in USA (Butler et al., 1985; Dawber et al., 1951; Dyer et al., 1981; Keys, 1970) and Seven Country Studies including studies in England, Sweden and Norway (Fager et al., 1981; Keys et al., 1984; Miller et al., 1977) in European countries. The major finding of these cohort studies was that in addition to serum cholesterol levels, other factors also are involved in development of coronary heart disease. Among the main risk factors, dyslipidemia, especially increase in LDL levels and decrease in HDL concentrations were considered as the important factors. Table-1 demonstrates the Population Attributable Factors (PARs) with its 99 percent confidence interval (CI) associated with lipids by sex and geographic region (Labarthe, 2011; Yusuf et al., 2004). In some countries, PAR estimation in women is based on small numbers which makes them less reliable.


Legend: CI: Confidence Interval.

Table 1. Population Attributable Factors (PARs) associated with lipids in men & women by geographic region.

Dyslipidemia and Cardiovascular Disease 307

The WHO MONICA project showed (WHO MONICA project, 1989) that the average of total cholesterol in 30 studied areas varied from 158 mg/dl (in the Beijing, China) to 246 mg/dl (Loczamburk, Germany) for men and from 162 mg/dl (Beijing, China) to 246 mg/dl (Glasgow, UK) in women. In addition, there was a difference in prevalence of hypercholesterolemia in different regions, from 2 percent in Beijing, China to nearly 50 percent in Lille, France (WHO MONICA project, 1989). An intermediate reduction in cholesterol level of MONICA project study populations during 5-6 year follow-up was observed. The mean annual decrease in total serum cholesterol was 0.4-3 mg/dl (Dobsn et

The highest incidence of hyperlipidemia is shown in patients with premature coronary artery disease, which occurs before age 55 years in men and 65 years in women. Prevalence of dyslipidemia in these patients is equal to 80-88 percent, compared to 40-48 percent in agematched controls without CHD (Genest et al., 1992; Roncaglioni et al., 1992). In these conditions, 12.5 percent of patients with a prior history of premature coronary disease and 58.5 percent of age-matched controls without prior history of coronary disease have normal

MRFIT6 study performed in more than 350,000 middle-aged men demonstrated (Stamler et al., 1986) that a sigmoid relationship (curvilinear) between total serum cholesterol level and prevalence of coronary artery disease especially in total cholesterol more than 240 mg/dl is

The strongest association was found in population from United States and Finland, the intermediate association was observed in European population, and the least correlation was related to Japanese men and rural area of Greece. The relationship between serum cholesterol and incidence of CVD become stronger when the number of risk factors was

Fig. 1. Association between plasma cholesterol and coronary risk among MRFIT study

al., 1996).

lipid profiles.

presented (Figure-1).

increased (Kannel, 1983).

<sup>6</sup> Multiple Risk Factor Intervention Trial

In parallel to these large population studies, a series of case studies were also performed. In one study, serum lipid levels were evaluated in 500 men with a prior history of myocardial infarction. Overall 30 percent of study population had abnormal blood lipid levels (Goldstein et al., 1973). High levels of cholesterol in 8 percent, triglycerides in7 percent and concomitant high cholesterol and triglycerides in15 percent were reported by this study.

In normal individuals from different communities, plasma levels of lipids vary due to differences in genetic background and diet. For example, the average cholesterol levels, according to age, in western and Chinese men are 202 mg/dl and165 mg/dl, respectively (Caroll et al., 2005; Wu et al., 2004). Based on results of the National Health and Nutrition Examination Surveys (NHANES) from 1999 to 2004, the percentage of adults with triglyceride levels above 150 and 200 mg/dl in the United States, were 33 and 18 percent, respectively (Ford et al., 2009). In the United States, the NHANES from 2005 to 2008 found that 98.8 million adults have total cholesterol levels ≥ 200 mg/dl, 33.6% of them having a total cholesterol level ≥ 240 mg/dl (American Heart Association [AHA], 2011).

Table-2 shows the prevalence of high levels of total cholesterol (cholesterol ≥ 200 mg/dl), LDL (LDL cholesterol ≥130 mg/dl), and HDL (HDL cholesterol≤ 40 mg/dl) in adults aged ≥20 years, according to NHANES (American Heart Association [AHA], 2011).


Legend: M: Male; F: Female; LDL: Low Density Lipoprotein; HDL: High Density Lipoprotein.

Table 2. Proportion of USA adults aged ≥ 20 years with dyslipidemia by ethnicity and gender

In MONICA5 project designed for more than 30 countries in different regions of WHO coverage except the US, the percentage of hypercholesterolemia for individuals aged between 35-64 years and total cholesterol levels between 5.2-7.8 mmol/l (approximately 200- 300 mg/dl) was found to be lowest (20%) among the men in China-Beijing and highest (76%) in France-Strasbourg. The lowest percent of women with hypercholesterolemia (5%) was in Australia-Perth population and the highest percent (76%) was observed in Germany-Bremen (WHO MONICA project, 2008). However, these figures were different when the total cholesterol level >7.8 mmol/l was considered as hypercholesterolemia. None of the China-Beijing's men had the serum cholesterol levels >7.8 mmol/l (0%) while 15% of Switzerland-Ticino men had hypercholesterolemia (highest percent). for women these figures were 0% in China-Beijing and 14% in Lithuania-Kaunas (WHO MONICA project, 2008).

<sup>5</sup> Multinational MONItoring of trends and determinants in CArdiovascular disease

In parallel to these large population studies, a series of case studies were also performed. In one study, serum lipid levels were evaluated in 500 men with a prior history of myocardial infarction. Overall 30 percent of study population had abnormal blood lipid levels (Goldstein et al., 1973). High levels of cholesterol in 8 percent, triglycerides in7 percent and concomitant high cholesterol and triglycerides in15 percent were reported by this study. In normal individuals from different communities, plasma levels of lipids vary due to differences in genetic background and diet. For example, the average cholesterol levels, according to age, in western and Chinese men are 202 mg/dl and165 mg/dl, respectively (Caroll et al., 2005; Wu et al., 2004). Based on results of the National Health and Nutrition Examination Surveys (NHANES) from 1999 to 2004, the percentage of adults with triglyceride levels above 150 and 200 mg/dl in the United States, were 33 and 18 percent, respectively (Ford et al., 2009). In the United States, the NHANES from 2005 to 2008 found that 98.8 million adults have total cholesterol levels ≥ 200 mg/dl, 33.6% of them having a

total cholesterol level ≥ 240 mg/dl (American Heart Association [AHA], 2011).

≥20 years, according to NHANES (American Heart Association [AHA], 2011).

47.0 16.9

<sup>5</sup> Multinational MONItoring of trends and determinants in CArdiovascular disease

**Non-Hispanic** 

**White** 

41.2 13.7

**Total cholesterol**  200-239 mg/dl ≥ 240 mg/dl

**LDL cholesterol** 

**HDL cholesterol** 

gender

2008). 

Table-2 shows the prevalence of high levels of total cholesterol (cholesterol ≥ 200 mg/dl), LDL (LDL cholesterol ≥130 mg/dl), and HDL (HDL cholesterol≤ 40 mg/dl) in adults aged

**Non-Hispanic** 

**M F M F M F** 

41.2 13.3 **Mexican-American** 

46.5 14.0

50.1 16.9

**Black** 

37.0 9.7

≥130 mg/dl 30.5 32.0 34.4 27.7 41.9 31.6

 ≤ 40 mg/dl 29.5 10.1 16.6 6.6 31.7 12.2 Legend: M: Male; F: Female; LDL: Low Density Lipoprotein; HDL: High Density Lipoprotein. Table 2. Proportion of USA adults aged ≥ 20 years with dyslipidemia by ethnicity and

In MONICA5 project designed for more than 30 countries in different regions of WHO coverage except the US, the percentage of hypercholesterolemia for individuals aged between 35-64 years and total cholesterol levels between 5.2-7.8 mmol/l (approximately 200- 300 mg/dl) was found to be lowest (20%) among the men in China-Beijing and highest (76%) in France-Strasbourg. The lowest percent of women with hypercholesterolemia (5%) was in Australia-Perth population and the highest percent (76%) was observed in Germany-Bremen (WHO MONICA project, 2008). However, these figures were different when the total cholesterol level >7.8 mmol/l was considered as hypercholesterolemia. None of the China-Beijing's men had the serum cholesterol levels >7.8 mmol/l (0%) while 15% of Switzerland-Ticino men had hypercholesterolemia (highest percent). for women these figures were 0% in China-Beijing and 14% in Lithuania-Kaunas (WHO MONICA project, The WHO MONICA project showed (WHO MONICA project, 1989) that the average of total cholesterol in 30 studied areas varied from 158 mg/dl (in the Beijing, China) to 246 mg/dl (Loczamburk, Germany) for men and from 162 mg/dl (Beijing, China) to 246 mg/dl (Glasgow, UK) in women. In addition, there was a difference in prevalence of hypercholesterolemia in different regions, from 2 percent in Beijing, China to nearly 50 percent in Lille, France (WHO MONICA project, 1989). An intermediate reduction in cholesterol level of MONICA project study populations during 5-6 year follow-up was observed. The mean annual decrease in total serum cholesterol was 0.4-3 mg/dl (Dobsn et al., 1996).

The highest incidence of hyperlipidemia is shown in patients with premature coronary artery disease, which occurs before age 55 years in men and 65 years in women. Prevalence of dyslipidemia in these patients is equal to 80-88 percent, compared to 40-48 percent in agematched controls without CHD (Genest et al., 1992; Roncaglioni et al., 1992). In these conditions, 12.5 percent of patients with a prior history of premature coronary disease and 58.5 percent of age-matched controls without prior history of coronary disease have normal lipid profiles.

MRFIT6 study performed in more than 350,000 middle-aged men demonstrated (Stamler et al., 1986) that a sigmoid relationship (curvilinear) between total serum cholesterol level and prevalence of coronary artery disease especially in total cholesterol more than 240 mg/dl is presented (Figure-1).

The strongest association was found in population from United States and Finland, the intermediate association was observed in European population, and the least correlation was related to Japanese men and rural area of Greece. The relationship between serum cholesterol and incidence of CVD become stronger when the number of risk factors was increased (Kannel, 1983).

Fig. 1. Association between plasma cholesterol and coronary risk among MRFIT study

<sup>6</sup> Multiple Risk Factor Intervention Trial

Dyslipidemia and Cardiovascular Disease 309

also seen in post hoc analysis of TNT7 study, in which 10000 known cases of CVD were

Legend: CHD: Coronary heart disease; L M H HDL: Low, middle, high, high density lipoprotein; CVD: Cardiovascular disease; FHS: Framingham Heart Study; LRCF: Lipid Research Clinics Prevalence Mortality Follow-up Study; CPPT: Lipid Research Clinics Coronary Primary Prevention Trial; MRFIT:

As mentioned previously, the cardioprotective effect of HDL was shown to be present at serum levels higher than 60 mg/dl (Castelli et al., 1983). These effects are more prominent

In assessment of 18 relatives with familial hyperalfa–lipoproteinemia, the life long of these men and women were found to be 5 and 7 years, respectively, more than general population

when the serum levels of HDL cholesterol reach 75 mg/dl and higher (Table-3).

Multiple Risk Factor Intervention Trial.

(Glueck et al., 1976).

<sup>7</sup> Treating to New Targets trial

Fig. 2. Inverse association between HDL and CVD events.

under-treatment with different doses of statins (Barter et al., 2007).

Similar results were obtained from Framingham and Migration studies (Kannel et al., 1971, 1979). The Migration study is one of the strong studies evaluating the relationship between increased serum cholesterol and risk of CVD. This study was done in 1960 and compared Japanese men residing in Japan with immigrated Japanese to Honolulu and San Francisco. In Japanese men living in their native country, the mean total cholesterol levels and CHD rate were lower compared to immigrated population. In immigrated Japanese, those who live in Hawaii had lower lipid levels than those in San Francisco. Considering race similarity in this study, the reason for observed differences in rate of CHD and cholesterol levels can be related to differences in dietary cholesterol and fat consumption (Kagen et al., 1974).

However the results of other studies on immigrants were not always similar to the Migration study. In one study (Kushi et al., 1985), diet produced no effect on cholesterol levels or heart disease mortality. In General, the importance of age, sex and race on levels of cholesterol has been shown in population-based studies.

Invention of ultracentrifuge has facilitated measurement of the various lipid parameters. LRCP (Lipid Research Clinics Program) was one of the first surveys during 1970 that was conducted to determine the total cholesterol, HDL cholesterol, LDL cholesterol and triglyceride levels in American adults (Heiss et al., 1980). In another study, difference in distribution of cholesterol and its components in the blood in accordance to age were described (Glueek & Stein, 1979). In both sexes, the slope of total cholesterol curve is increased by increase in age until the end of middle-age. After that, by increasing the age, slope of the curve is downward until reaching the old age. Mean total cholesterol in men and women aged between 20 -50 years is similar, however, the levels of HDL cholesterol in women after puberty is higher than men (Rifkind & Segal, 1983).

Among patients with a prior history of myocardial infarction, an elevated total cholesterol following recovery was a major independent risk factor for reinfarction, death from heart disease and total mortality. Cardiovascular mortality is varied in different populations. The highest and lowest mortality rate was found in Finland and Japan, respectively, with a direct relationship to serum cholesterol levels (Rosenson, 2011).

#### **2.2 HDL cholesterol**

The negative relationship between low HDL cholesterol and the risk of heart disease is well stablished in the general population (Abbott et al., 1988; Abbott et al., 1998; Castelli, 1983; Gordon et al, 1989; Harper & Jacobson, 1999; Rosenson, 2005) (figure-2). In the Framingham Heart study, the protective role of HDL has been well described (Kannel et al., 1971).

Based on results of this study, by each 5 mg /dl decrease in serum levels of HDL (compared to mean normal values for men and women), the risk of myocardial infarction was increased by 25 percent.

Predictive role of HDL against coronary events was also well documented in patients with known heart disease. The results of Lipid and Care clinical trial showed that low levels of HDL cholesterol is a stronger predictor of heart disease incidence in presence of serum LDL cholesterol < 125 mg/dl than LDL cholesterol ≥ 125 mg/dl (Sacks et al., 2002). They also found that in serum LDL<125 mg/dl, each 10 mg /dl increase in HDL level, will cause 29 percent reduction in the incidence of cardiovascular events , while with the serum LDL cholesterol ≥ 125 mg/dl, this attenuation will be lowered to 10 percent. This association was

Similar results were obtained from Framingham and Migration studies (Kannel et al., 1971, 1979). The Migration study is one of the strong studies evaluating the relationship between increased serum cholesterol and risk of CVD. This study was done in 1960 and compared Japanese men residing in Japan with immigrated Japanese to Honolulu and San Francisco. In Japanese men living in their native country, the mean total cholesterol levels and CHD rate were lower compared to immigrated population. In immigrated Japanese, those who live in Hawaii had lower lipid levels than those in San Francisco. Considering race similarity in this study, the reason for observed differences in rate of CHD and cholesterol levels can be related to differences in dietary cholesterol and fat consumption

However the results of other studies on immigrants were not always similar to the Migration study. In one study (Kushi et al., 1985), diet produced no effect on cholesterol levels or heart disease mortality. In General, the importance of age, sex and race on levels of

Invention of ultracentrifuge has facilitated measurement of the various lipid parameters. LRCP (Lipid Research Clinics Program) was one of the first surveys during 1970 that was conducted to determine the total cholesterol, HDL cholesterol, LDL cholesterol and triglyceride levels in American adults (Heiss et al., 1980). In another study, difference in distribution of cholesterol and its components in the blood in accordance to age were described (Glueek & Stein, 1979). In both sexes, the slope of total cholesterol curve is increased by increase in age until the end of middle-age. After that, by increasing the age, slope of the curve is downward until reaching the old age. Mean total cholesterol in men and women aged between 20 -50 years is similar, however, the levels of HDL cholesterol in

Among patients with a prior history of myocardial infarction, an elevated total cholesterol following recovery was a major independent risk factor for reinfarction, death from heart disease and total mortality. Cardiovascular mortality is varied in different populations. The highest and lowest mortality rate was found in Finland and Japan, respectively, with a

The negative relationship between low HDL cholesterol and the risk of heart disease is well stablished in the general population (Abbott et al., 1988; Abbott et al., 1998; Castelli, 1983; Gordon et al, 1989; Harper & Jacobson, 1999; Rosenson, 2005) (figure-2). In the Framingham

Based on results of this study, by each 5 mg /dl decrease in serum levels of HDL (compared to mean normal values for men and women), the risk of myocardial infarction was increased

Predictive role of HDL against coronary events was also well documented in patients with known heart disease. The results of Lipid and Care clinical trial showed that low levels of HDL cholesterol is a stronger predictor of heart disease incidence in presence of serum LDL cholesterol < 125 mg/dl than LDL cholesterol ≥ 125 mg/dl (Sacks et al., 2002). They also found that in serum LDL<125 mg/dl, each 10 mg /dl increase in HDL level, will cause 29 percent reduction in the incidence of cardiovascular events , while with the serum LDL cholesterol ≥ 125 mg/dl, this attenuation will be lowered to 10 percent. This association was

Heart study, the protective role of HDL has been well described (Kannel et al., 1971).

cholesterol has been shown in population-based studies.

women after puberty is higher than men (Rifkind & Segal, 1983).

direct relationship to serum cholesterol levels (Rosenson, 2011).

(Kagen et al., 1974).

**2.2 HDL cholesterol** 

by 25 percent.

also seen in post hoc analysis of TNT7 study, in which 10000 known cases of CVD were under-treatment with different doses of statins (Barter et al., 2007).

Legend: CHD: Coronary heart disease; L M H HDL: Low, middle, high, high density lipoprotein; CVD: Cardiovascular disease; FHS: Framingham Heart Study; LRCF: Lipid Research Clinics Prevalence Mortality Follow-up Study; CPPT: Lipid Research Clinics Coronary Primary Prevention Trial; MRFIT: Multiple Risk Factor Intervention Trial.

Fig. 2. Inverse association between HDL and CVD events.

As mentioned previously, the cardioprotective effect of HDL was shown to be present at serum levels higher than 60 mg/dl (Castelli et al., 1983). These effects are more prominent when the serum levels of HDL cholesterol reach 75 mg/dl and higher (Table-3).

In assessment of 18 relatives with familial hyperalfa–lipoproteinemia, the life long of these men and women were found to be 5 and 7 years, respectively, more than general population (Glueck et al., 1976).

<sup>7</sup> Treating to New Targets trial

Dyslipidemia and Cardiovascular Disease 311

event–free survival after coronary artery bypass graft surgery (CABG) (Haim et al., 1999;

Nevertheless, because hypertriglyceridemia is an independent risk factor for CVD, measurement of triglycerides as a part of routine cholesterol screening is recommended by NECP ATPIII guidelines (Haim et al., 1999). Fasting triglyceride measurement is important for evaluating the risk of heart disease especially in cases who are suffering from diabetes, glucose intolerance, insulin resistance syndrome, obesity and low HDL. Although, triglyceride measurement is commonly done after 8–12 hours fasting, an association between nonfasting triglyceride levels and CVD is also present (Nordestgaard et al., 2007;

Non–HDL cholesterol is defined as the difference between total and HDL cholesterols. Thus it includes LDL, Lp (a), IDL and VLDL (Ballantyne et al., 2000). In both LRCP study and the Women's Health Study non-HDL cholesterol has been suggested as a better tool for risk assessment of CVD than LDL levels (Cobbaert et al., 1997; Ridker et al., 2005). In the LRCP study in which the patients were followed for an average of 19 years, a 30 mg/dl difference in non–HDL and LDL concentrations, produced 19 and 15 percent, increase in mortality risk of CVD among men, respectively, and 11 and 8 percent, among women, respectively,

Lipoprotein (a), also called Lp (a), is established as an independent risk factor for CVD. Lp (a) is a modified form of LDL with a structure similar to plasminogen (Steyrer et al., 1994) that could interfere with fibrinolysis by competing with plasminogen for binding to cells (Loscalzo et al., 1990; Palabrica et al., 1995). Lp (a) also binds to macrophages to promote foam cell formation and deposition of cholesterol in atherosclerotic plaques (Zioncheck et al., 1991). Thus, Lp (a) accelerates atherosclerosis process by impairing fibrinolysis and increasing LDL oxidation (Stein & Rosenson, 1997). Evidences of association between Lp (a) excess [Lp (a) levels above the 95th percentile] and CVD mostly come from 2 large metaanalyses that found positive continuous correlation between Lp (a) and risk of CVD events (Bennet et al., 2008; Emerging et al., 2009). The 24 cohort studies in the meta-analysis (Bennet et al., 2008) found a risk ratio of 1.13 (95 percent CI, 1.09 to 1.18) between the top and third bottom baseline Lp (a) levels after adjustment for multiple traditional cardiovascular risk factors. Lp (a) excess concentration is usually detected in patients with premature CHD. In one study 18.6 percent of patients with premature CHD had excess levels of Lp (a), while

LP (a) increases the risk of cerebrovascular disease, peripheral vascular disease, myocardial infarction (MI), re–stenosis after angioplasty, and failure after CABG (Rosengren et al., 1990; Schaefer et al., 1994). 12 years and more follow–up of patients in the Framingham Heart study showed that Lp (a) can increase the risk of premature coronary heart disease by two-times (Bostom et al., 1996), and augment the risk of MI, intermittent claudication, cerebrovascular disease, and coronary artery stenosis. In the 4S9 study an association between increased Lp (a)

12.7 percent of them had no dyslipidemia (Genest et al., 1992).

levels and overall mortality rate was also observed (Bostom et al., 1994).

<sup>9</sup> Scandinavian Simvastatin Survival Study

Sprecher et al., 2000).

Bansal et al., 2007).

**2.4 Non-HDL cholesterol** 

(Cobbaert et al., 1997).

**2.5 Lipoprotein (a)** 

In the Lipid Research Clinics study, the Framingham heart Study and the HHS8 the ratio of LDL to HDL was shown to be the best predictor of cardiovascular events (Manninen et al., 1992; Kinosian et al., 1994). In HHS study, the risk of new coronary events such as myocardial infarction and sudden cardiac death in patients with LDL/HDL ≥ 5 and a concomitant serum triglycerides ≥ 200 mg /dl, was fourfold more than patients with lower LDL/HDL ratio and triglycerides levels. Overall, among men, an LDL/HDL ratio of ≥ 6.4 had 2–14 percent higher predictive value than serum total cholesterol or LDL levels. Among women the predictive value of LDL/HDL ≥ 5.6 was 25–45 percent greater than serum total cholesterol or LDL level (Kinosian et al., 1994).


Legend: HDL: High Density Lipoprotein.

Table 3. Inverse relation between plasma HDL-cholesterol levels and cardiovascular risk in men and women.
