A low-fat diet is one that restricts fat and often saturated fat and cholesterol as well. Low-fat diets are intended to reduce the occurrence of conditions such as heart disease and obesity. For weight loss, they perform similarly to a low-carbohydrate diet, since macronutrient composition does not determine weight loss success. Reducing fat in the diet can make it easier to cut calories. Fat provides nine calories per gram while carbohydrates and protein each provide four calories per gram, so choosing low-fat foods makes it possible to eat a larger volume of food for the same number of calories. This effect is countered by the rapidity of digestion of carbohydrates compared to fats. The Institute of Medicine recommends limiting fat intake to 35% of total calories to help prevent obesity and to help control saturated fat intake. A low-fat diet is not well defined, but a very low fat diet is one that gets less than 15% of daily calories from fat.
According to the National Academies Press, a high-fat diet can contain "unacceptably high" amounts of saturated fat, even if saturated fats from animal products and tropical oils are avoided. This is because all fats contain some saturated fatty acids. For example, if a person chose fats with only 20% saturated fatty acids, setting fat intake at 35% of total calories would mean that 7% of calories would come from saturated fat. For this reason, the Institute of Medicine recommends consuming no more than 35% of calories from fat.
Studies have shown that the effectiveness of low-fat diets for weight loss is broadly similar to that of low-carbohydrate diets in the long-term. The Endocrine Society state that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet."
Low-fat diets have been promoted for the prevention of heart disease. Lowering fat intake from 35-40% of total calories to 15-20% of total calories has been shown to decrease total and LDL cholesterol by 10 to 20%; however, most of this decrease is due to a reduction in saturated fat intake. Saturated fat has been shown to raise total and LDL cholesterol in a large number of studies and has also been correlated with a higher risk of heart disease.
A 2013 meta-analysis of randomized controlled trials of low- and high-fat diets showed low-fat diets decreased total cholesterol and LDL, but these decreases were not found when only considering low-calorie diets. It also showed HDL increases and triglyceride decreases in high-fat diets. Furthermore, lower total cholesterol was associated with lower intake of saturated fat and higher intake of polyunsaturated fat, HDL increases were associated with high monounsaturated fat intake and triglycerides associated with high carbohydrate intake. Decrease in saturated fat intake was only marginally related to decrease in LDL cholesterol. The meta-analysis concluded that neither high-fat nor low-fat diets could be unequivocally recommended.
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