For weight maintenance, the fuel mix oxidized must match the nutrient distribution in the diet. The composition of the fuel mix oxidized and the respiratory quotient are influenced by circulating substrate and hormone concentrations, which, among other things, reflect the degree of replenishment of the body's fuel reserves. Carbohydrate and protein balances are maintained without obvious changes in the body's glycogen stores and protein pools. One of the most readily quantifiable measures by which to judge the effect of various inherited and circumstantial factors on energy metabolism and body composition is the percentage body fat for which weight maintenance occurs. The amount of body fat for which the steady state of weight maintenance tends to become established is therefore a key fact on which an understanding of body weight regulation should be based, rather than rates of energy turnover or past circumstances that may have led to fat accumulation, as often believed.