CAN WE DEFINE INDISPENSABLE AMINO-ACID-REQUIREMENTS AND ASSESS PROTEIN-QUALITY IN ADULTS

Adult indispensable amino acid (IAA) requirements, as exemplified by l ysine, are examined in a metabolic framework in which dietary IAA are needed to replace both regulatory losses (extrinsic and adaptive IAA o xidation) and obligatory losses (intrinsic metabolic consumption). Thu s requirements need to be defined as minimal, operational and optimal. For minimal needs, animal studies show different patterns for mainten ance and growth, with lower lysine and leucine in the former compared with the latter pattern, which mirrors tissue protein. Reliable human studies also confirm a low minimum lysine requirement, 15 mg/kg or les s, compared with growth needs, supporting the 1985 FAO adult value and lower than the value for preschool children proposed as the basis of a scoring pattern for adults by FAO/WHO in 1991. This pattern derives from studies in children depositing considerable nitrogen and is unsui table for an adult scoring pattern. Stable isotope lysine balance stud ies do not provide unequivocal support for an increased minimum requir ement value for lysine. As for operational requirements, these are hig her because of increased regulatory losses due to the higher intakes o f IAA in habitual diets and because of the diurnal feeding pattern. De finition of optimal needs requires value judgements of the beneficial metabolic influence of intakes in excess of minimal. This influence (t he anabolic drive) is a tangible phenomenon in growing animals, but is yet to be identified in humans. Thus optimal requirements are not yet definable, and there is no suitable scoring pattern to replace the 19 85 FAO values, which may accurately reflect minimal needs.