What are the least amounts of protein or essential amino acids a normal person may consume without danger of nutritional deficiency? The answer to this question is unclear, despite more than a century of investigation and debate. 1 - 5 In part, this is because research in this area is hampered by methodologic and conceptual problems, and in part because our understanding of the mechanisms that control adaptation to variations in dietary protein and amino acid intake is still rudimentary. Thus, Hegsted 6 argues that all current methods for estimating the adult protein requirement are inadequate, for they lack an appropriate measure of health. Waterlow 7 notes that 84even at the simple metabolic level, we still do not understand how the body maintains nitrogen balance in response to varying nitrogen intakes.