Food quotient, respiratory quotient, and energy balance

This paper reviews evidence that the macro-nutrient composition of the diet and the maintenance of energy balance are correlated. Intervention studies show that subjects lose weight on low-fat diets and gain weight on high-fat diets. Descriptive studies show that overweight subjects eat relatively more fat but have the same total energy intake as nonoverweight subjects. The body has a limited ability to oxidize fat compared with its ability to oxidize carbohydrate and protein. The conclusion is that becoming overweight can be prevented by reducing the fat content of the diet. Studies on nutrient utilization show a ready increase in carbohydrate oxidation whereas fat oxidation does not change after meals enriched with, respectively, carbohydrate or fat. However, in the long term, the respiratory quotient (RQ) is closer to the food quotient (FQ) for subjects eating high-fat diets than it is for subjects eating high-carbohydrate diets. For high-carbohydrate diets, the RQ is lower than is the FQ, indicating that subjects must mobilize body fat. This is supported by data on body weight loss in subjects changing from a standard maintenance diet to a low-fat diet, even while energy intake was increased with nearly 20%. Direct evidence for a higher energy expenditure for low-fat diets is not yet available.