Threonine requirements of healthy Indian men, measured by a 24-h indicator amino acid oxidation and balance technique

Background: We previously questioned the validity of the 1985 FAO/WHO/UNU upper requirement value for threonine (7 mg · kg(−1) · d(−1)) and proposed a tentative mean requirement of 15 mg · kg(−1) · d(−1). Objective: In this study we used a 24-h indicator amino acid oxidation and balance technique, with [1(-13)C]leucine as the indicator amino acid, to assess threonine adequacy at 6 test intakes (7, 11, 15, 19, 22, and 27 mg · kg(−1) · d(−1)) with a 6-d dietary adaptation phase in healthy, well-nourished Indian men. Design: Sixteen men were randomly allocated to 3 of 6 test intakes and were studied after 6 d of adaptation to the experimental diets. Diets were based on an l-amino acid mixture in which the threonine content was varied. At 1800 on day 6, a 24-h intravenous [13C]leucine tracer infusion protocol was conducted to assess 24-h leucine oxidation and daily leucine balances. Results: Leucine balances differed significantly (P = 0.02) between the different intakes of threonine. Two-phase linear regression analysis from 12-h and 24-h leucine oxidation and 24-h leucine balance gave a breakpoint at a threonine intake of 15 mg · kg(−1) · d(−1), with 95% CIs ranging from 11 to 27 mg · kg(−1) · d(−1). There was no significant effect of threonine intake on 24-h leucine flux. Conclusion: The results of the 24-h indicator amino acid oxidation and balance experiments indicate that the current FAO/WHO/UNU threonine recommendation of 7 mg · kg(−1)· d(−1) is inadequate. A mean threonine intake of 15 mg · kg(−1) · d(−1) is sufficient to achieve the indicator (leucine) amino acid balance in healthy Indian men.