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Requirements and Role in Diet
D J Millward, University of Surrey, Guildford, UK
© 2005 Elsevier Ltd. All rights reserved.
Defining minimum amino acid and protein requirements is inherently difficult. Humans are exposed to a wide range of protein intakes, which enable full expression of their genotypical lean body mass throughout the range, and identifying the lower limits of this range has proved intractable. Without unequivocal symptoms of deficiency, the adequacy of an intake can only be assessed in terms of nitrogen or amino acid balance, which is unsatisfactory for several reasons. In particular, adaptation causes major difficulties in designing balance studies and interpreting results. Furthermore, balance methods are inherently imprecise and logistically extremely difficult. It is therefore not surprising that there is much debate about both the nature and the extent of protein requirements.
Protein requirements are best discussed in terms of metabolic demand, dietary requirement, and dietary allowances. Metabolic demand concerns amino acids and is determined by the nature and extent of those metabolic pathways (e.g., net protein synthesis) that consume amino acids and that vary with the phenotype and the developmental and physiological state of the individual. The dietary requirement is the amount of protein and/or its constituent amino acids that must be supplied in the diet in order to satisfy the metabolic demand. The requirement will usually be greater than the metabolic demand. Thus, factors associated with digestion and absorption may limit digestibility (i.e., dietary nitrogen lost in the feces) and biological value (i.e., the availability of the absorbed amino acid pattern in relation to cellular needs, which influences urinary nitrogen excretion). Dietary allowances are a range of intakes derived from estimates of individual requirements taking into account variability between individuals. They are designed to meet the dietary requirements of the population. In the United Kingdom, these allowances are described in terms of Dietary Reference Values (DRVs) and in the United States as Dietary Reference Intakes (DRIs).
Current evidence supports the representation of the metabolic demands as in Figure 1. The metabolic demand for amino acids is to maintain tissue protein at appropriate levels and to provide for all amino acid-derived metabolites and any additional needs during growth, rehabilitation, pregnancy, and lactation. Tissue proteins are diverse, including structural or fibrous insoluble types and soluble globular
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