Blood amino acid studies: Prediction of limiting amino acids in mixtures of dietary proteins

Diets containing mixtures of cereals and protein supplements were fed ad libitum to male weanling rats 18 h daily for 5 days. The cereals consisted of rice, wheat, and corn, and the supplements included peanut, soy, cottonseed, and chick pea flours. Levels of plasma lysine, methionine, threonine, trytophan, leucine, and isoleucine were determined. Two methods designated as plasma amino acid (PAA) score and PAA ratio were tested for predicting the limiting amino acids in the diets utilizing the PAA data. Chemical score values were calculated from the amino acid content of the mixtures. Growth tests were also carried out with the diets, with or without a supplement of the amino acids indicated to be limiting by PAA score and PAA ratio methods. PAA ratios, PAA scores, and protein efficiency ratio data of rats indicated that lysine and threonine were equally limiting in rice–peanut, rice–soy, rice–cottonseed, rice – chick pea, wheat–peanut, and wheat–cottonseed diets. Plasma methods were also correct in showing that lysine was the limiting amino acid in corn–peanut, corn–soy, and corn–cottonseed mixtures. Plasma methods did not indicate a methionine deficiency when this amino acid was co-limiting with lysine and threonine. Chemical score failed to detect the threonine deficiency present in 8 of 12 diets. Limiting amino acids were not established for the corn – chick pea diet. It was interesting that the effective amino acid supplement for 8 of 12 diets required threonine in addition to lysine. It was concluded that the PAA methods are effective for predicting a deficiency of a single amino acid or of two co-limiting amino acids, but less satisfactory for predicting multiple amino acid deficiencies.