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Plasma Threonine Response Curve and Threonine Requirements of Young Men and Elderly Women^{1 ,2}

Department of Nutrition and Food Science, and the Clinical Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Studies were conducted in eight male M.I.T. students and 13 subjects 72 ± 5 years of age on the relationship between plasma threonine concentration, threonine intake and threonine requirement. The formula diet contained an L-amino acid mixture, patterned as in egg protein, and provided N equivalent to 0.5 g protein (N x 6.25) per kilogram body weight per day with graded reductions in threonine intake from 18 to 1 mg/kg/day. Fasting and 3.5-hour postprandial plasma threonine levels were measured at the end of each consecutive dietary period, usually lasting 5 days. The fasting plasma threonine data were evaluated statistically to identify the point at which plasma threonine remained constant despite further reductions in intake. This "lower breakpoint" on the plasma threonine response curve was assumed to occur at the intake of the amino acid which just meets the minimum physiological requirement. It was estimated to be 7 ± 2 mg/kg/day for eight young males and for 11 elderly women it was essentially identical, being 8 ± 2 mg/kg/day. Alternative nutritional interpretations of the "lower breakpoint" on the plasma amino acid response curve in adult human subjects are discussed.

KEY WORDS: • plasma threonine • threonine requirement • young adults • elderly • human

¹ Supported by USPHS grant (AM-15892). The MIT Clinical Research Center is supported through a grant (RR-88) from the General Research Centers Program of the Division of Resources, National Institutes of Health, Bethesda, Maryland 20014.

² Contribution no. 2246 from the Department of Nutrition and Food Science, MIT.

³ Recipient of a Rockefeller Foundation Fellowship for advanced study. Present address: Children's Hospital Medical Center, Boston.

Manuscript received 9 October 1973.