Title : The lipid-heart hypothesis and the dietary guidelines: Does the evidence support low dietary fat and saturated fat?
Abstract:
In response to a perceived epidemic of coronary heart disease, the lipid–heart hypothesis was introduced in 1953 by Ancel Keys, who asserted that high intakes of total fat, saturated fat, and cholesterol lead to atherosclerosis and that consuming less fat and cholesterol, and replacing saturated fat with polyunsaturated fat, would reduce serum cholesterol and consequently the risk of heart disease. Keys proposed an equation to predict changes in total serum cholesterol based on the relative consumption of saturated fat and polyunsaturated fat which was used for decades in dietary fat research. However, the Keys equation conflated natural saturated fat and industrial trans-fat into a single parameter and considered only linoleic acid as the polyunsaturated fat. Natural sources of saturated fat such as lard and butter were not studied separately from hydrogenated shortenings and margarines which often contained up to 35-45% trans-fat and were widely consumed in the US and other countries. By the mid-1940s consumption of shortening and margarines exceeded consumption of lard and butter in the US. Replacement of saturated fat with polyunsaturated fat due to its cholesterol lowering effect has also promoted an imbalance of omega-6 to omega-3 fatty acids in the diet. Studies have failed to show that reducing dietary cholesterol or the percentage of total calories as fat has a significant effect of lowering serum cholesterol or cardiovascular disease. While many studies have found that replacing saturated fat with polyunsaturated fat reduces serum cholesterol levels, numerous observational, epidemiological, interventional, and autopsy studies of the lipid–heart hypothesis have failed to show that lowering serum cholesterol in this way has any effect of reducing cardiovascular disease mortality and most studies found no reduction in serious cardiac events. Raw data recovered from two major studies completed prior to 1980, but unpublished at the time, found that a high intake of linoleic acid in place of saturated fat was associated with higher risk of all-cause and cardiovascular mortality than the typical American diet at that time which was higher in saturated fat. Nevertheless, guidelines to reduce total fat intake and replace saturated fat with polyunsaturated fat have been the cornerstone of national and international dietary guidelines which have focused disproportionately on heart disease and much less so on cancer and metabolic disorders, such as obesity and diabetes, which affect people of all ages and have steadily increased since the adoption of the lipid-heart hypothesis.
