Protein requirements: from nitrogen balance to functional impact.

Today because of tools capable of probing the details of metabolism, we are able to study the utilization of amino acids in specific, physiologically important processes. Such information may lead to recommended dietary allowances for each amino acid specific for subpopulations and perhaps individuals. These new findings identify the metabolic end-products of some amino acids as crucial intermediates in the maintenance of a variety of physiological functions that bear no direct relationship to protein metabolism. The precursor amino acids for these intermediate compounds are either non-essential or conditionally essential and thus may play a specific, non-protein, role in amino acid requirements in individuals close to nitrogen equilibrium. Another exciting area arises from the marked dissimilarity between the amino acid composition of milk proteins and other body proteins, suggesting that some milk proteins may have evolved to serve unique functions through specific digestion products. Such products may be important in micronutrient absorption in the neonate. Thus, amino acid requirements appear to reach beyond the tradition areas of nitrogen and protein metabolism.