Debate about the optimum balance of macronutrients for adult weight maintenance or
weight loss continues to expand. Often this debate centers on the relative merits or risks of
carbohydrates vs.
fats; however, there is increasing interest in the optimal level of
dietary protein for
weight loss. Diets with a reduced ratio of
carbohydrates/
protein are reported to be beneficial for
weight loss, although diet studies appear to lack a fundamental hypothesis to support higher
protein intakes. Presently, needs for
dietary proteins are established by the recommended daily allowance (RDA) as the minimum level of
protein necessary to maintain
nitrogen balance. The RDA define the primary use of
amino acids as substrates for synthesis of body
proteins. There is emerging evidence that additional metabolic roles for some
amino acids require plasma and intracellular levels above minimum needs for
protein synthesis. The
branched-chain amino acid leucine is an example of an
amino acid with numerous metabolic roles that function in proportion with cellular concentration. This review provides an overview of the current understanding of metabolic roles of
leucine and proposes a metabolic framework to evaluate the merits of a higher
protein diet for
weight loss.