The biology of amino acid requirements: what do they mean and can we measure them?

Identifying recommended dietary allowances for protein and amino acids: a critique of the 2007 WHO/FAO/UNU report

The WHO/FAO/UNU (2007) report examines dietary protein and amino acid requirements for all age groups, protein requirements during pregnancy, lactation and catch-up growth in children, the implications of these requirements for developing countries and protein quality evaluation. Requirements were defined as the minimum dietary intake which satisfies the metabolic demand and achieves nitrogen equilibrium and maintenance of the body protein mass, plus the needs for growth in children and pregnancy and lactation in healthy women. Insufficient evidence was identified to enable recommendations for specific health outcomes. A meta analysis of nitrogen balance studies identifies protein requirements for adults 10 % higher than previous values with no influence of gender or age, consistent with a subsequently published comprehensive study. A new factorial model for infants and children, validated on the basis of the adequacy of breast milk protein intakes and involving a lower maintenance requirement value, no provision for saltatory growth and new estimates of protein deposition identifies lower protein requirements than in previous reports. Higher values for adult amino acid requirements, derived from a re-evaluation of nitrogen balance studies and new stable isotope studies, identify some cereal-based diets as being inadequate for lysine. The main outstanding issues relate to the biological implausibility of the very low efficiencies of protein utilisation used in the factorial models for protein requirements for all population groups especially pregnancy when requirements may be overestimated. Also considerable uncertainty remains about the design and interpretation of most of the studies used to identify amino acid requirement values.

Isotopic studies of protein and amino acid requirements

The quantification of protein and amino acid requirements in health and disease is still an incompletely resolved issue, despite its importance to our knowledge of nutrition, to the clinical management of most health disorders and to food policy. However, the dynamic and adaptive features of protein metabolism render this determination difficult. The first nitrogen balance studies performed have demonstrated their limitations in providing accurate protein and amino acid requirements. Isotopic methods developed over the past 15 years have considerably enhanced the quantification of amino acid and protein requirements and our knowledge of the physiological phenomena underlying these needs. These methods are consistently being improved and producing new estimates for protein and amino acid requirements, together with a clearer understanding of this complex issue.