Effects of threonine supplementation on the slope assay for protein quality

Several diets considered to be marginally deficient in threonine were fed to young male rats at protein levels ranging from 3 to 9%. Protein sources included lactalbumin (reference standard), soy protein with added methionine, pea protein with added methionine, rice-casein, and peanut-sesame-fish. Chemical scores and plasma amino acid scores indicated that these diets were limiting in threonine at the 3--4% protein level; increased growth resulted from supplementation with threonine. After threonine supplementation, all protein sources except pea were limiting in lysine. Increased growth at low protein levels with little or no extra growth at high protein levels due to threonine supplementation resulted in decreased slopes in the relative protein value (RPV) assay. Consequently, threonine supplementation apparently decreased RPV values for these samples but the protein efficiency ratio was not affected. It was concluded that the RPV assay underestimates the protein quality of lysine-deficient proteins.