The nutritional role of indispensable amino acids and the metabolic basis for their requirements

Short-term balances and metabolic studies suggest that current estimates of adult indispensable amino acid (IAA) requirements are too low. In addition, the observed biological value of protein is less than that predicted from its amino acid score. We propose that these discrepancies reflect the fact that IAA requirements are complex, and can only be defined under specific artificial conditions. This is because consumption of protein usually results in oxidative losses of IAA and nitrogen, which are important to the organism and variable according to dietary composition. It is likely that the age-related fall in the current values for IAA requirement primarily reflects different dietary designs in the original balance studies, which induced different rates of oxidative losses. A new model for amino acid utilization is proposed which takes account of such losses, and factors that influence them are reviewed. We propose that dietary IAAs serve an important transient function prior to their oxidation, exerting a regulatory influence on maintenance and growth which we call the anabolic drive. The model also allows for diurnal cycling of body protein, in which oxidative losses of amino acids occurring during the postabsorptive phase must be balanced by net protein deposition during feeding if overall balance is to be attained. Diurnal cycling is likely to be nutritionally sensitive, influenced by the anabolic drive and could affect the relationship between total protein and IAA requirements. We argue that oxidative losses should be included in the requirement; this will account for apparent discrepancies in the observed efficiency of protein utilization. We propose that IAA requirements (R) can only be unambiguously defined in terms of Rmin, a value obtainable under artificial conditions when oxidative losses, Lr, are minimal (Lr min); current estimates of adult requirements may be close to this level. An operative requirement of practical value, Rop, will only be defineable when the value to the organism of the oxidative losses induced by usual diets, (Lr op), can be evaluated.