Diet and serum lipids: controlled studies in the United States.

The influence of dietary fat on serum lipids is well established. The general principles of a fat-modified diet--calorie control and the amount and composition of dietary fat--are demonstrated in studies in which nutrient composition was systematically altered. Weight loss, even when caloric deficit involves no other changes in nutrients, reduces serum lipids. With isocaloric diets, the fatty acid composition rather than the amount of fat influences the serum cholesterol concentration. Saturated fatty acids are twice as effective in raising serum cholesterol levels as polyunsaturated fatty acids are in reducing them. The amount of cholesterol in the diet alters the amount in the serum by approximately 5 mg/dl for every 100 mg of change in diets with less than 300 mg/1000 kcal. Cholesterol in excess of this amount has no additional effect. With no dietary cholesterol, serum lipids are unaffected by the fat composition. In the presence of dietary cholesterol, serum lipids are affected by saturated and polyunsaturated fatty acids through an interaction with cholesterol. Polyunsaturates counteract the influence of both saturated fatty acids and cholesterol. The more cholesterol there is in the diet, the more polyunsaturated fatty acids are required to counteract its effect. Both the fatty acid composition of the lipoproteins and lipid metabolism are affected by the fat and cholesterol composition of the diet.