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Original Research Communication |
1 From the Department of Physiology and the Division of Nutrition (AVK, TR, JV, BC, and DN) and the Department of Biochemistry (JG), St John’s Medical College, Bangalore, India, and the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA (MMR and VRY).
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 [1-13C]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.
Key Words: Threonine requirements • indicator amino acid oxidation • indicator amino acid balance • leucine kinetics • healthy Indian men
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