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Threonine requirement of healthy adults, derived with a 24-h indicator amino acid balance technique^1,2,3

¹ From the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge (SB, MMR, S-HO, MC, and VRY), and Chonnam National University, Kwang-JU, Korea (S-HO).

Background: Because we question the validity of the 1985 FAO/ WHO/UNU upper requirement for threonine of 7 mg·kg^-1·d^-1, we proposed a tentative mean requirement of 15 mg·kg^-1·d^-1.

Objective: Our goal was to assess threonine adequacy at 3 test intakes and the consequences of a 6-d compared with a 13-d dietary adaptation phase.

Design: We used a 24-h indicator amino acid balance technique ([1-¹³C]leucine as indicator) to assess the threonine requirement. Fifteen healthy adults were randomly assigned to receive 7, 15, or 46 mg threonine·kg^-1·d^-1 and were studied after 6 and 13 d of adaptation to the experimental diets. Diets were based on an L-amino acid mixture in which the threonine content was varied. At 1700 on days 6 and 13, a 24-h intravenous [¹³C]leucine tracer infusion protocol was begun to assess leucine oxidation and daily leucine balances.

Results: There was no detectable effect of duration of dietary adaptation in leucine oxidation or balance, but the 24-h leucine oxidation and balances differed significantly between the 7-mg intake and each of the 2 higher intakes (P < 0.05). The latter were not significantly different. The 24-h leucine oxidation rate decreased across threonine intakes (P < 0.01 for main effect of diet, independent of infusion day). Leucine oxidation was highly correlated (r = 0.80) between the 2 dietary adaptation phases across all test intakes.

Conclusion: The 1985 FAO/WHO/UNU threonine recommendation is inadequate, and 15 mg·kg^-1·d^-1 is sufficient to achieve mean indicator (leucine) amino acid balance.

Key Words: Threonine requirements • indicator amino acid oxidation • amino acid balance • leucine • healthy adults

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