It is believed by many that saturated fatty acids can detrimentally modify lipoprotein levels, and that this is a cause of cardiovascular disease. However, care should always be exercised when analysing data from studies that manipulate the fat content of the diet, because confounding variables are common. In particular polyunsaturated fatty acids are thought to be able to lower plasma cholesterol levels, and this can lead to ambiguity in studies that do not carefully control experimental variables. If subjects are for example eating a diet with reasonably high levels of polyunsaturated fatty acids, and then switched to a diet high in saturated fatty acids, it can appear that the saturated fatty acids are raising levels of plasma cholesterol. However, in reality, the plasma lipoprotein levels are simply returning to their normal, pre-polyunsaturated fatty acid levels. Studies that alter dietary fat intakes must therefore be carefully analysed in order to fully understand the effects of the manipulations.
The ability of polyunsaturated fatty acids to lower plasma cholesterol levels has been reported in a number of studies and is a reasonably well known phenomenon. Different polyunsaturated fatty acids appear to have similar effects. Some well designed studies have used carefully controlled variables to isolate the effects of polyunsaturated fatty acids. For example, in ones study1, subjects were fed diets comprising 40 % of calories as carbohydrate, 40 % as fat and 20 % as protein. However, the fat content of the diet was altered such that it contained polyunsaturated fatty acid to saturated fatty acid ratios of 0.4, 1.0 or 2.0. Each diet was consumed by healthy individuals for 2 weeks in a random order and plasma cholesterol levels were measured during this time. The mean plasma total cholesterol levels fell 6 and 12 % in the medium (1.0) and high (2.0) polyunsaturated diets. In addition the plasma low density lipoprotein (LDL) cholesterol and high density lipoprotein (HDL) cholesterol also fell on the higher polyunsaturated fatty acid diets.
Therefore increasing the concentration of polyunsaturated fatty acids in the diet lowers the plasma concentrations of cholesterol. However, because the LDL and HDL cholesterol levels fall to a similar degree, there are no change in the LDL to HDL ratio. As the LDL to HDL ratio is a better indicator of cardiovascular risk than total cholesterol, the actual ability of polyunsaturated fatty acids to protect from cardiovascular disease is therefore controversial. Because the cholesterol intake in the subjects was maintained at 400 mg per day throughout the study, it can be seen that dietary cholesterol is not the driver of plasma cholesterol levels as we are led to believe by the medical establishment. Studies such at this that control for other variables and alter only the fat content of the diet are actually quite rare in the nutritional literature. This is because when the studies are well designed, they do not support the contention that saturated fat or cholesterol is able to alter plasma levels of lipoproteins or alter the risk of cardiovascular disease.
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