Coronary heart disease and lipoproteins

Coronary Heart Disease (CHD), characterized by a limited supply of oxygen to the heart muscle, has clinical manifestations ranging from angina pectoris to myocardial infarction (MI) and sudden death. The primary cause of CHD is coronary atherosclerosis (ATS), due to lipid-rich lesions in the intima of the coronary arteries. These begin in early life as "fatty streaks" and later form fibrous, often calcified and ulcerated, lesions that narrow the arterial lumen. A thrombus, superimposed on the lesions, may precipitate MI and sudden death. These events depend upon the atherosclerotic lesion and a complex interplay of haemostatic factors. While the mechanisms are only beginning to be elucidated, it is apparent that the processes leading to CHD involve the development of ATS, thrombosis and vascular reactivity as well as their interrelationships.

Epidemiology and experimental evidence

Since the 1940s and 1950s, studies within populations as well as cross- cultural comparisons produced abundant evidence that higher serum cholesterol levels are associated with increased risk of CHD (Levy et al., 1979; Anderson, Castelli and Levy, 1987; Committee on Diet and Health, 1989; Pooling Project, 1978). Recent studies, such as the Multiple Risk Factor Intervention Trial (Stamler, Wentworth and Neston, 1986) in the USA showed a continuous, graded, statistically significant association between baseline serum cholesterol and six-year, age-adjusted CHD death rates for both hypertensive and normotensive individuals and for both smokers and non-smokers. A threshold level, below which an increase in serum cholesterol has no effect on risk, could not be identified. Similarly, in a Chinese population with generally low serum cholesterol levels, a statistically significant relationship between serum cholesterol and CHD mortality was found (Chen, et al., 1991) indicating that any increase in serum cholesterol increases risk of CHD. Cross-country studies, such as the Seven Countries Study (Keys et al., 1986; Keys, 1970) also showed a graded increase in risk of CHD as serum cholesterol levels rise. These associations between serum cholesterol levels are primarily related to levels of low density lipoprotein (LDL), the primary carrier of cholesterol in the serum.

Associations between diet, serum cholesterol levels and risk of CHD have been welldocumented in cross-country comparisons (Keys et al., 1986; Levy et al., 1979; Committee on Diet and Health, 1989; Lewis et al., 1978). Populations with relatively high intakes of fat, especially animal fat and cholesterol, have relatively high serum cholesterol levels and high mortality rates from CHD when compared with populations consuming low-fat diets.

Numerous studies on people who immigrated from low-risk to high-risk populations (Nichaman et al., 1975; Halfon et al., 1982; Kato et al., 1973) demonstrated that environmental factors rather than genetic susceptibility led to these differences and that diet consistently played a primary role. Generally, these high-risk populations consume diets of affluent societies. It is recognized that these populations differ from low-risk populations in many ways. For example, diets of low risk populations are not only low in fat, they are commonly higher in dietary fibre and other components of plant origin. Also, individuals in such societies are less sedentary. Abundant experimental data, however, in both animals and humans confirm that there is a primary role for dietary fat and cholesterol in the control of serum lipid levels.

Comparisons of diet and serum cholesterol levels within population groups where diet is relatively homogeneous, may or may not show a correlation between dietary fat consumption and serum cholesterol levels (Morris, Marr and Clayton, 1977; Garcia-Palmieri et al., 1980; Jacobs, Anderson and Blackburn, 1979). It is well-known that individuals vary greatly in serum cholesterol level, even when diet is constant (Keys, Anderson and Grande, 1959) and there are "hyper- and hyporesponders" when dietary fat and cholesterol are modified (Katan and Beynen, 1987; Katan et al., 1988). In addition, the methods for estimating the intake and composition of food consumed by individuals are severely limited and often unreliable (Jacobs, Anderson and Blackburn, 1979; Livingstone et al., 1990; Schoeller, 1990; Black et al., 1993). Hence, it is not surprising that some studies within a population show little association between serum lipids and estimates of the fat in the diet. Intervention studies in which diet can be controlled demonstrate that marked changes in serum lipids can be induced by changes in fat and cholesterol intake, thus, they are consistent with the epidemiological findings.

Dietary fat and serum lipids

Many studies have demonstrated that the amount and composition of dietary fat are primary determinants of serum cholesterol levels and LDL. It has been concluded that, relative to carbohydrates, the saturated fatty acids elevate serum cholesterol, while the polyunsaturated fatty acids (linoleic acid) lower serum cholesterol, and the monounsaturated fatty acid (oleic acid) has no statistically significant effect (Keys, Anderson and Grande, 1957; Hegsted et al., 1965). While some studies over the past 30 years have yielded various results, on balance the current data (Hegsted et al., 1993) confirm these general conclusions as to the relative effects of saturated, polyunsaturated and monounsaturated fatty acids upon serum cholesterol levels. The specific potency of the saturated and polyunsaturated fatty acids to modify serum lipid levels has not been, and probably cannot be, clearly defined for all conditions. Originally, Keys and his colleagues (1957) thought the combined saturated fatty acids were twice as potent in elevating serum cholesterol as the polyunsaturated fatty acids were in lowering it; later studies led by Hegsted (1965) attributed greater potency to the polyunsaturates. However, individual studies conducted with few subjects and diets may show substantial differences in the effects observed. Combined analyses (Mensink and Katan, 1992; Hegsted et al., 1993) agree that the cholesterol-lowering effect of linoleic acid is 2 to 3 times less than the cholesterol-raising effect of saturates. However, they indicate substantial differences in relative potency depending upon the data selected for analysis. Whether this is due to the traits of individuals, diet characteristics or fats studied, the quality of specific studies, and so forth, is uncertain. It is clear that the amount of dietary fat and its composition are major determinants of serum cholesterol levels; that saturated fatty acids and dietary cholesterol elevate serum cholesterol and that polyunsaturated fatty acids (linoleic acid) have a modest cholesterollowering effect relative to carbohydrates.

Specific fatty acids and cholesterol

Saturated fatty acids. Early studies (Keys, Anderson and Grande, 1965; Hegsted et al., 1965) suggested that various saturated fatty acids have different effects upon serum cholesterol levels. Particular fats with high levels of stearic acid did not appear to be as hypercholesterolemic as expected from their content of saturated fatty acids. These differences were investigated by the use of transesterified fats, incorporating lauric, myristic, palmitic and stearic acid into olive and safflower oil (McGandy, Hegsted and Myers, 1970). Many of these preparations appeared to be approximately equal in their cholesterol-raising ability, thus suggesting that the position of the fatty acids in the triacylglyceride may be important as well. More recent data (Bonanome and Grundy, 1988) also indicated that stearic acid may not elevate serum cholesterol levels appreciably. Overall, the data indicate that stearic acid in most natural fats has minimal effects upon serum cholesterol levels. It should be specifically noted, however, that the effects of stearic acid and other saturated fatty acids on susceptibility to hypertension, cancer, obesity, and other illnesses, are unknown. Also, data on the activity of saturated fatty acids with regard to thrombotic activity are inadequate. Hence, whether it is desirable to substitute stearate for other fatty acids in the diet is still uncertain.

There is still considerable disagreement about the relative activity of the other saturated fatty acids - lauric, myristic and palmitic. The early data (Hegsted et al., 1965) suggesting that myristic acid was the most hypercholesterolemic of the saturated fatty acids may be due to the designs of the studies. Butterfat and coconut oil, both sources of myristic acid, were the primary saturated fats studied and the intake of total saturated fatty acids correlated with myristic acid intake. A direct comparison of myristic and palmitic acid showed that both raise LDL cholesterol relative to oleic acid, but that myristic acid was slightly more powerful (Zock, 1994). Most other metabolic and epidemiologic studies have not reported the intake of the specific saturated fatty acids (Denke and Grundy, 1992; Doherty and Iacono, 1992; Sundram, Hayes and Siru, 1994). Some animal and human studies (Hayes et al., 1991; Ng et al., 1991) have reported minimal effects of palmitic acid but this may be due to the specific diets utilized (Hayes et al., 1991; Pronczuk, Khosla and Hayes, 1994). In studies of cebus monkeys, it was reported that palmitic acid is only hypercholesterolemic when the cholesterol intake is high (Khosla and Hayes, 1993), however, metabolic studies in normolipidemic volunteers showed that palmitic acid strongly raised total cholesterol (Bonanome, 1988; Denke, 1992; Zock, 1994). Palmitic acid is the major saturated fatty acid in most diets and lauric, myristic and palmitic acid are considered to be the principal hypercholesterolemic fatty acids although they may differ in potency. Recent findings on individual fatty acids are summarized in Figure 9.1.

Polyunsaturated fatty acids. Replacing saturated fatty acids by either oleic or linoleic acid lowers serum cholesterol levels. In many specific studies, it was not possible to determine whether it was the addition of oleic and/or linoleic acid or the lowering of intake of saturated fatty acids which caused the reduction in serum cholesterol. Predictive equations attribute the changes in serum cholesterol to changes in saturated and polyunsaturated fatty acids while the monounsaturated fatty acid (oleic) was shown to be neutral (Keys, Anderson and Grande, 1957; Hegsted et al., 1965; Mensink and Katan, 1992; Hegsted et al., 1993) or to have a small cholesterol-lowering effect (Mensink and Katan, 1992).

FIGURE 9.1 : Effects of individual dietary fatty acids on serum total and lipoprotein cholesterol

Interestingly, in several studies the linoleic acid content of adipose tissue which is probably a better indicator of usual, long-term, linoleic acid intake than dietary data (van Staveren et al., 1986), as well as the linoleic acid content of serum phospholipids or cholesterol esters, showed an inverse relationship between linoleic acid levels and the incidence of myocardial infarction (Wood et al., 1984; Logan et al., 1978; Valek et al., 1985; Riemersma et al., 1986). This may indicate a protective role of the polyunsaturated fatty acids other than their effect on serum lipid levels (Renaud et al., 1986). Studies with animals (Charnock et al., 1985; Charnock, Abeywardena and McLennon, 1986) indicate that the polyunsaturated fatty acids, especially the n-3 fatty acids, may protect against cardiac arrythmias.

EPA and DHA. In recent years, evidence that fish-consuming populations have a low prevalence of CHD has generated a great deal of interest in fish oils, which are major sources of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Dyerberg et al., 1978).

Although the evidence is somewhat controversial, the consumption of such oils appears to have relatively little effect upon LDL and HDL levels but does appear to be effective in lowering serum triacylglycerols and very low-density lipoproteins (Leaf and Weber, 1988). The epidemiological data indicate that there is an apparent protective effect which is probably due to the effects on the thrombotic or immunologic mechanisms rather than effects upon serum lipoproteins. Much of the experimental data has dealt with diets in which fish oils were the major source of dietary fat and it is unlikely that results can be extrapolated to ordinary levels of consumption. This, of course, is not true of the epidemiological findings.

Dietary cholesterol. While it has long been apparent that dietary cholesterol elevated serum cholesterol, the shape of the dose-response curve has been debated. It is clear that the observed responses to changes in cholesterol intake are highly variable and may depend, in part, upon the nature of the dietary fat, cholesterol intake, and perhaps, other dietary constituents (Hopkins, 1992). Hence, as with changes in dietary fat, a good quantitative definition of the response expected from changes in cholesterol intake under all conditions is probably not possible.

It is relevant to note that in the hamster model (Spady and Dietschy, 1985; Woollett et al., 1992) the cholesterol-lowering activity of linoleic acid was clearly evident only when the LDL receptor was adequately suppressed by dietary cholesterol and/or saturated fat. This may explain, in part, the limited potency of linoleic acid which was reported when human subjects were fed formula diets which were low in cholesterol (Hegsted and Nicolosi, 1990).

Intervention teals. Intervention trials generally confirm the effects of dietary fat and cholesterol although the changes observed are often less than in metabolic trials in which better dietary control is possible. For example, a significant drop in serum cholesterol was found when a diet which was low in animal fat and high in polyunsaturated fat was substituted for the usual Finnish diet (Ehnholm et al., 1982) which is high in saturated fat and cholesterol. In southern Italy, the reverse type of study, in which animal fat was substituted for olive oil and carbohydrate, resulted in a substantial rise in serum cholesterol and LDL (Ferro-Luzzi et al., 1984). Relatively small changes in serum lipids have been observed in some field trials (Hunninghake e, al., 1993) undoubtedly due to the difficulties in obtaining adequate food consumption data and good dietary compliance in large trials. The Finnish Mental Hospital Study (Turpeinen et al., 1979) reported a significant decrease in CHD in individuals who were fed a diet higher in polyunsaturated fatty acids. When the results of a number of trials, many of which included drugs to lower cholesterol, were summarized (Committee on Diet and Health, 1989; Smith, Song and Sheldon, 1993) the findings were varied but consistent: diet can have a desirable effect in lowering serum cholesterol. Several studies (Blankenhorn et al., 1987; Ornish et al., 1990; Schuler et al., 1992) demonstrated that a severely restricted diet can limit or actually reverse the development of atherosclerotic lesions.

Such immediate effects of dietary modification upon lesion development appear to be consistent with change in the diets of populations, such as those observed in some European countries during World War II (Schettler, 1979). Large decreases in mortality from CHD in some countries have been observed in recent years. They were greater in countries such as the USA and Australia which emphasized dietary modification in the general population (Dwyer and Hetzel, 1980). Overall, the data from field trials support the epidemiologic and metabolic conclusions about the desirability of dietary modifications to lower serum cholesterol levels (Stamler et al., 1993).

Dietary effects on lipoproteins

Low density lipoproteins (LDL) carry most cholesterol and are identified as the most important cause of atherosclerosis. The dietary effects of the various fatty acids and cholesterol upon LDL levels generally parallel those described above for total serum cholesterol (Mensink and Katan, 1992; Hegsted et al., 1993). The saturated fatty acids, presumably lauric, myristic and palmitic acids, elevate LDL levels; linoleic acid lowers LDL levels; and oleic acid appears to be neutral or slightly lowers it relative to carbohydrates. It has recently been suggested that oxidized LDL is the major, perhaps the cause, of atherosclerosis. Oxidized LDL is more readily taken up by the monocytes which leads to the atherosclerotic plaque.

Some studies suggest that various antioxidants limit the development of atherosclerosis in animals and humans (Steinberg et al., 1989; Frei, England and Ames, 1989; Jialal, Vega and Grundy, 1990; Riemersma et al., 1991). Vitamin E, carotenoids and vitamin C have received the most attention but other antioxidants may be effective as well (Bjorkhem et al., 1991). High intakes of vitamin E have been associated with reduced risk of coronary heart disease in both men (Rimm et al., 1993) and women (Stampfer et al., 1993). Hence, the intake and circulating levels of these and perhaps other antioxidants must be considered in assessing the relative risk of disease. Presumably, this will become more important as this field develops. Insufficient evidence is available to indicate the relative importance of the circulating levels of LDL compared to the levels of antioxidants. This does not detract from the convincing evidence that high levels of LDL constitute a major risk of CHD.

Within populations, high levels of high density lipoproteins (HDL) are strongly associated with reduced risk of CHD (Wilson, Abbott and Castelli, 1988; Gordon and Rifkind, 1989; Gordon et al., 1989; Knuiman et al., 1987). HDL is thought to actively protect against CHD (NIH Consensus, 1993) presumably by reverse cholesterol transport, that is, transport from the periphery to the liver although this is uncertain in humans. Within populations, the level of HDL is determined partly by genetic factors and partly by environmental conditions. HDL levels are lowered by smoking, obesity and male hormones, and are raised by physical activity as well as by consumption of alcohol, saturated fats and cholesterol. However, the increase in HDL which is attributable to saturated fats and cholesterol is outweighed by greater increases in LDL. Evidence that manipulation of HDL levels, by dietary or other means, modifies susceptibility to CHD is unavailable and the significance of dietary-induced changes in HDL levels needs to be clarified. Summaries of the current data (Mensink and Katan, 1992; Hegsted et al., 1993) indicate that all three classes of fatty acids tend to elevate HDL levels, with saturated fatty acids being the most potent and linoleic acids being the least influential. On the other hand, low-fat diets which are known to protect against CHD also lower HDL levels (Denke and Breslow, 1988). After HDL is reduced by a low-fat diet, its metabolism is modified (Brinton, Eisenberg and Breslow, 1990). However, there is little reason to believe that dietary modifications which reduce LDL levels are undesirable although they may also reduce HDL levels to some extent.

Trans fatty acids

The effects of trans fatty acids have been discussed elsewhere in this report. Available data indicate that the serum lipoprotein responses to monounsaturated trans fatty acids approximate the effects of saturated fatty acids. Whether they have specific effects on HDL, as indicated by the studies of Mensink and Katan (1990), or not (Judd et al., 1994) is a question which remains to be clarified.

Conclusion

Abundant evidence supports the conclusion that elevated levels of serum cholesterol and LDL constitute a major risk of atherosclerosis and coronary heart disease. The degree of risk may be modified by various antioxidants and complex interactions between the degree of atherosclerosis, thrombotic and fibrolytic, and vascular reactivity.

When various fats are fed to human beings under controlled conditions, differences in chain length and the number and geometry of the double bonds of the fatty acids induce marked differences in the concentration of lipids and lipoproteins in fasting blood serum. Relative to carbohydrate, the saturated fatty acids - lauric, myristic and palmitic - raise both LDL and HDL cholesterol and lower VLDL cholesterol and triglycerides. In most fats, stearic acid appears to have minor effects. Linoleic acid lowers LDL while oleic acid appears to be neutral. Oleic and linoleic acids may elevate HDL levels modestly relative to carbohydrates with linoleic acid having the least effect. Trans isomers of oleic acid elevate LDL levels and may depress HDL levels while the effects on other lipoproteins are, as yet, uncertain. The fatty acids of fish oils lower serum triglycerides markedly but appear to have little effect upon LDL and HDL levels. Dietary cholesterol also elevates LDL levels and probably HDL.

In general, the metabolic studies on the dietary effects of fats and cholesterol upon serum lipids and lipoproteins are consistent with epidemiologic and intervention studies and the trends over time observed in various populations. Each type of study concludes that dietary modifications that lower serum cholesterol and LDL levels decrease the risk of CHD.