Can we define indispensable amino acid requirements and assess protein quality in adults?

Adult indispensable amino acid (IAA) requirements, as exemplified by lysine, are examined in a metabolic framework in which dietary IAA are needed to replace both regulatory losses (extrinsic and adaptive IAA oxidation) and obligatory losses (intrinsic metabolic consumption). Thus requirements need to be defined as minimal, operational and optimal. For minimal needs, animal studies show different patterns for maintenance 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 less, 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 unsuitable for an adult scoring pattern. Stable isotope lysine balance studies do not provide unequivocal support for an increased minimum requirement value for lysine. As for operational requirements, these are higher because of increased regulatory losses due to the higher intakes of IAA in habitual diets and because of the diurnal feeding pattern. Definition of optimal needs requires value judgements of the beneficial metabolic influence of intakes in excess of minimal. This influence (the 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 1985 FAO values, which may accurately reflect minimal needs.