The optimal ratio of fat-to-carbohydrate in the diet.

A major issue in human nutrition is the optimal relation of carbohydrate-to-fat in the diet. According to some investigators, a high proportion of fat energy to total energy favors the development of several chronic diseases. Among these are obesity, coronary heart disease, diabetes, and cancer. The theory that a high proportion of fat relative to other nutrients promotes the development of obesity is founded on research with experimental animals and in human population surveys. This theory has been difficult to prove in prospective feeding studies in humans; therefore it remains a contentious issue. Regarding coronary heart disease, little evidence supports a claim that a high proportion of dietary fat predisposes to disease. On the other hand, strong evidence bolsters the claim that certain fatty acids raise the risk for coronary heart disease. These include saturated fatty acids and trans fatty acids, both of which raise serum cholesterol levels. In contrast, neither monounsaturated nor polyunsaturated fatty acids raise serum cholesterol levels and seemingly pose little risk for coronary disease. The relationship between dietary fat and type 2 diabetes is tied largely to the issue of obesity, because obesity is a major cause of diabetes. Although animal studies and epidemiological studies have implicated dietary fat as a factor in cancer, recent prospective epidemiological data in humans have cast doubt on the possibility of a strong relationship. In summary, clear evidence points to the need to reduce intakes of saturated and trans fatty acids in the diet. Beyond this change, a balanced ratio of unsaturated fatty acids to carbohydrate leading to fat intake of approximately 30% of total energy seems appropriate for the American public.