Proponents of the cholesterol theory of cardiovascular disease claim that high intakes of saturated fat cause elevations in plasma cholesterol in normal healthy people and this is a cause of atherosclerosis. However, the theory is in tatters because evidence in the literature does not support this contention. A number of poorly designed and possibly deliberately biased studies have been published that claim to support the contention that saturated fat causes elevations in plasma cholesterol. For example, one study published in the American Journal of Clinical Nutrition in 19881 collated data from a number of studies to show that increasing intakes of saturated fatty acids cause elevation in levels of total and low density lipoprotein cholesterol. However, a closer look at the data shows that this is not the case, and the conclusion does not match the empirical evidence.
The first problem with the study was that the fatty acids were part of a liquid meal formula. Food in not liquid, but a complex mixture of solid and semi liquid foods. The fact that the foods were supplied in liquid form also suggests that fibre was absent or in low concentrations in the food. Because fibre is known to regulate cholesterol metabolism in humans, the data from this study should be treated with caution. In addition, the subjects employed in the study were selected because they have borderline abnormal lipoprotein concentrations. This suggests that the study population may have included a number of individuals with metabolic abnormalities, perhaps some with metabolic syndrome. Therefore the results of this study are not representative of normal healthy people and the data can in no way be extrapolated to the population as a whole.
However, more importantly, the study had a fundamental flaw in that the subjects were consuming high polyunsaturated fatty acid diets prior to consuming the high saturated fatty acid diets. In particular, the unsaturated oils contained safflower oil, a polyunsaturated fatty acid high in γ-linolenic acid (GLA, C18:3 (n-6)) as well as the essential fatty acid linoleic acid (LA, C18:2 (n-6)). Both GLA and LA may have modulatory effects on plasma lipoprotein levels although this is controversial. However, because the effects of the essential fatty acids on blood lipids are not fully understood, using subjects consuming a product that may have beneficial effects on plasma cholesterol calls into question the design of the study. The fact that the authors also used lard as the source of saturated fat in one experiment, but lard in over 50 % unsaturated, also calls into question their nutritional knowledge.
RdB
