The sensitivity of Nitrogen balance to changes in energy intake has been demonstrated in numerous studies. It is well known that when diet provides adequate amounts of protein the addition of energy-yielding nutrients (either carbohydrate or fat) results in a linear improvement in N balance in humans and animals (Munro, 1951, 1964, 1978; Inoue et al. 1973; Garza et al. 1976; Reeds et al. 1981). However, the underlying biochemical mechanism whereby energy intake above requirements affects N metabolism in long-term studies remains obscure.