The cholesterol theory of cardiovascular disease is problematic for a number of reasons. However, most importantly, evidence suggest that dietary cholesterol does not cause elevations in plasma cholesterol. Many feeding studies attest to the inconsistency of dietary cholesterol to cause detrimental changes to plasma levels of cholesterol. For example, in a study published in the American Journal of Clinical Nutrition in 19851, four different lipid containing diets were fed to 11 healthy subjects. The diets contained two ratios of polyunsaturated to saturated fatty acids (1.8 or 0.28) and two different levels of dietary cholesterol (1000 or 300 mg per day). When the authors analysed the effects of the diets they found that the diet with the higher polyunsaturated to saturated fatty acid ratio decreased plasma cholesterol levels regardless of the level of cholesterol in the diet. In contrast the diets with a low polyunsaturated to saturated fatty acid ratio increased plasma cholesterol levels regardless of the levels of cholesterol in the diet.
These results suggest that the dietary cholesterol intake has little effect on total plasma cholesterol levels and that the polyunsaturated to saturated fatty acid ratio is a more important determinant of plasma cholesterol level. Interestingly, both the high and low polyunsaturated fatty acid diets increased the protective high density lipoprotein (HDL) in the presence of high levels of dietary cholesterol. Whereas in the presence of low levels of dietary cholesterol saturated fat in the diet raised levels of HDL. High levels of polyunsaturated fats also decreased levels of low density lipoprotein by 12 % and 25 % in the presence of low and high levels of dietary cholesterol, respectively. This suggests that high intakes of cholesterol are actually beneficial to plasma cholesterol levels in the presence of polyunsaturated fatty acid. Taken as a whole these results are not what would be expected if saturated fat or cholesterol were able to raise plasma levels of cholesterol as suggested by the early work of Ancel Keys and other cholesterol proponents.
For a theory to become invalid, only one aspect of it needs to be discredited. Although methodological problems could always explain such contradictory findings, other studies attest to the inability of feeding studies such as this to provide evidence that dietary cholesterol causes detrimental changes to plasma cholesterol. That polyunsaturated fatty acids are able to lower plasma levels of cholesterol has been reported consistently in the research literature. This likely relates to the fact that polyunsaturated fatty acids include the essential fatty acids alpha linolenic acid and linoleic acid that are able to modify cellular levels of eicosanoids and may have physiological effect that can alter cholesterol metabolism. In addition, the study showed that the diets high in polyunsaturated fatty acids increased bile acid excretion which may explain their ability to lower plasma cholesterol levels. These two processes may be related because polyunsaturated fatty acids are known to be able to alter steroid regulation in the liver.
In another study2, a group of researchers assessed the response of 21 subjects to the feeding of eggs. Subjects were fed 3 eggs per day in addition to their normal diet which increased the mean plasma cholesterol levels from 188 to 199 mg per 100 mL. However, this rise was accounted for by the excessive responses of 8 of the subjects, while most of the subjects showed no changes. When egg consumption increased from 3 to 6 per day the responders showed an increase in mean plasma cholesterol from 170 to 199 mg per 100 mL, but the plasma cholesterol in the non-responders actually fell. Assessing results from studies like this is difficult because the subjects are described as healthy. However, it is known that individuals with familial hypercholesterolaemia when included in studies can bias the results. It is unclear whether such individuals were included in this study. However, that dietary cholesterol even at 6 eggs per day does not raise plasma cholesterol levels in some people is enough to dismiss the cholesterol theory of cardiovascular disease as erroneous.
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