Importance of nutrient balance in body weight regulation.

Most individuals maintain a stable body weight over long periods of their lives. This indicates that individual average energy intakes usually vary around the same mean as average energy expenditures. But energy balance is likely to be maintained only if body composition remains constant, since accumulation or depletion of protein, carbohydrate, or fat would in time lead to alterations in food intake. It has long been recognized that the body spontaneously tends to maintain nitrogen balance even though substantial daily deviations occur. Changes in the level of protein intake lead to limited gains or losses in the body's protein mass, but soon the nitrogen balance is achieved again. The mechanisms involved in the maintenance of a constant protein content, although not well understood, operate effectively on diets of very diverse composition, provided that they supply adequate amounts of protein. Since protein balance tends to maintain itself spontaneously, and because protein contributes only a minor fraction of energy intake, it is the metabolism of carbohydrates and of fats that primarily influences the regulation of body weight. The following discussion will consider the manner in which carbohydrate disposal adjusts itself to carbohydrate intake and fat oxidation to fat intake.