Plasma Threonine Response Curve and Threonine Requirements of Young Men and Elderly Women

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Abstract
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 × 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.