Proponents of the lipid hypothesis of cardiovascular disease claim that dietary cholesterol and saturated fat are able cause increases in plasma cholesterol levels. Such proponents also claim that these increases in plasma cholesterol are a cause of cardiovascular disease. However, the nutritional and medical literature does not support this contention. Firstly, increasing dietary cholesterol and saturated fat does not increase blood cholesterol levels unless fibre intake falls concomitantly. Many papers that have observed increases in blood cholesterol levels following increases in dietary cholesterol or saturated fatty acids have failed to adequately control fibre intakes. Secondly, the literature shows that there is an association between blood cholesterol levels and cardiovascular disease, but the cause and effect is not proven. In fact available evidence suggests that it is other variables, most likely the insulin resistance that results from a poor quality diet, that is the causative factor in blood lipid changes and cardiovascular disease.
It is easy to disprove the lipid hypothesis of cardiovascular disease, because one of the most effective ways to lower blood cholesterol levels is to lose weight. For example, in one study1, researchers gave a number of communities (1 control and 2 treatment groups) based in the United States an education programme that aimed to reduce their saturated fat and cholesterol intakes. It was hoped that by reducing intakes their blood cholesterol levels would fall. The study progressed for 2 years and the blood cholesterol of the subjects was taken annually to monitor any reductions. A dietary questionnaire was used to assess and monitor changes in the dietary lipid intakes of the subjects. In the two communities receiving the education programmed there was a 23 to 34 % reduction in cholesterol and a 25 to 30 % reduction in saturated fat intake when compared the control community that received no educational advice about nutrition. These reductions in lipid intake were accompanied by reductions in blood cholesterol.
However, there was only a very weak correlation between the reduction in dietary lipids and the reduction in cholesterol suggesting that the reductions in the former was not the cause of reductions in the latter. The factor that predicted the lowering of blood cholesterol levels most closely was weight loss. Using multiple regression statistics, the authors showed that when weight loss was used as the primary factor of analysis the dietary lipid intake made no significant contribution to the change in blood cholesterol levels. In other words, it was likely the weight loss that was causing the lowering of blood cholesterol levels. Blood lipid changes likely result from the development of insulin resistance, the latter also being a primary driver of weight gain. It is likely that the subjects in this study experienced improved insulin sensitivity due to weight loss, and it was this that caused reductions in cholesterol levels. In addition, as the lipid content of their diet decreased, the subjects would have unknowingly increased their fibre intakes.
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