Human adult amino acid requirements: [1-13C]leucine balance evaluation of the efficiency of utilization and apparent requirements for wheat protein and lysine compared with those for milk protein in healthy adults

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Abstract
Background: There is considerable debate about the human lysine requirement and the consequent nutritional value of wheat protein. Objective: We used a novel [1-13C]leucine balance protocol to examine whether adaptive mechanisms to conserve lysine allow wheat to be utilized more efficiently than expected according to current estimates of lysine requirements and wheat utilization. Design: Wheat and milk proteins were compared in 6 adults infused for 9 h with l-[1-13C]leucine in the postabsorptive state (0–3 h), who were fed half-hourly with low-protein (2% of energy, 3–6 h) and isoenergetic higher-protein (12–13% of energy, 6–9 h) meals providing maintenance energy intakes. From acute measurements of [1-13C]leucine balance, we predicted nitrogen balance, the metabolic demand for protein, the efficiency of postprandial protein utilization (PPU), and the requirements for wheat protein and lysine. Results: Leucine balance was higher after the milk than after the wheat feeding because of the greater inhibition of proteolysis by milk. PPU, calculated as the ratio of Δnitrogen balance to Δnitrogen intake between the low-protein and higher-protein periods, was 0.68 ± 0.06 for wheat and 1.00 ± 0.09 for milk (P ≤ 0.001). The estimated average wheat protein requirement (0.6/PPU) was 0.89 ± 0.08 g•kg−1•d−1, indicating a lysine requirement of 23.2 ± 2.0 mg•kg−1•d−1. The measured PPU for wheat, 0.68 ± 0.06, was higher than the value calculated from wheat lysine intake and milk protein lysine deposition, 0.26 ± 0.02, and higher than predicted by most published estimates of lysine requirements, apart from a value of 19 mg/kg indicated by nitrogen balance studies. Conclusions: The results show that adaptive mechanisms of lysine conservation allow wheat protein to be utilized more efficiently than expected.