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Plasma Amino Acids of Infants Consuming Soybean Proteins with and without Added Methionine¹

Departments of Pediatrics and International Health Schools of Medicine and of Hygiene and Public Health, The Johns Hopkins University, Baltimore, Maryland 21205, and Instituto de Investigacion Nutricional, Apartado 55, Miraflores (Lima), Peru

Fasting plasma free amino acids were determined in 54 convalescent malnourished infants: seven infants while consuming a diet based on isolated soybean protein, containing 4.0% to 5.3% of dietary metabolizable energy (calories) as protein (A), 20 at 6.4% to 6.7% protein calories (B), 23 at 6.4% to 6.7% protein calories with added DL-methionine (C), and four with 8.0% to 12.3% protein calories (D). There were no differences in total amino acid concentration (TAA) among the four groups; the molar fraction of essential amino acids (EAA:TAA) was lower for group A; there were no differences among the four groups in Lys:EAA or cystine:EAA ratios or in Met concentration. Met:EAA was higher in C than B, with considerable overlap of individual values. In 10 of 13 infants who were represented in both B and C, Met concentration and Met:EAA ratio were higher in group C. Fasting plasma AA levels are not consistently reliable for field or clinical assessment of dietary Met adequacy. Fasting and postprandial (3- and 4-hour) plasma AA were determined in 29 infants: in 12 the preceding diet and the test meal were both Met-deficient with <6.7% protein calories (E), in five the preceding diet was milk-based but the test meal was Met-deficient at <6.7% (F), in five the preceding diet and test meal were based on isolated soybean protein at <6.7% with DL-Met added (G), and in seven the test meal was soy-based with >9.0% protein calories (H). Plasma Met concentration and Met:EAA fell significantly at 3 and 4 hours in groups E and F, but not in groups G and H, suggesting that a postprandial fall in Met:EAA ratio can be used to identify dietary Met deficiency in field situations.

KEY WORDS: • plasma amino acids • methionine intake

¹ This work was supported by Research Grant AM-09980 and Research Fellowship 1 F22 AM-03252 from the National Institutes of Health, U.S.P.H.S. and by Research Contract csd/2946 with the Agency for International Development, U.S. Department of State.

² Address reprint requests to Dr. George G. Graham, 615 North Wolfe Street, Baltimore, Maryland 21205.

Manuscript received 1 March 1976.

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