Differences between Digestibilities of Protein and First Limiting Amino Acid in Some Plant Products

Abstract

Values /percent/ for true digestibility /TD/ of protein and essential amino acids /EAA/ in some cereals, legumes and oilseeds and their mixtures have been determined by the rat or pig fecal /balance/ method. In milling and utility wheats, the TD /bioavailability/ values of lysine /80–85/ were up to 12 percent lower than those of protein /92–95/ and other EAA /90–96/. As in wheat, lysine /the first limiting amino acid in cereals/ had the lowest bioavailability in barley /72/, oat /73/, rye /72/ and sorghum /71/. The TD values of protein and bioavailability of other EAA in barley, oat, rye and sorghum were, 82, 79–89; 79, 77–87; 81, 74–89 and 85, 83–87, respectively. The bioavailability values of methionine /the first limiting amino acid in legumes/ in field pea /75/, fababean /70/, lentil /60/ and navy bean /61/ were markedly lower than the TD values of protein /80–85/ and bioavailability of other EAA /80–96 in field pea, 78–90 in fababean, 67–90 in lentil/. Limiting amino acids were also less digestible than protein in soybean, rapeseed, sunflower and groundnut and blends of wheat with legumes and oilseeds. These data suggest that protein digestibility may not be a good preditor of bioavailability of the first limiting amino acid. Therefore, the rapid in vitro methods based on the amount of first limiting amino acid /amino acid score/ plus enzymatic digestibility may not accurately predict protein quality of foods. A preferable approach would be to obtain information on bioavailability of individual amino acids and then correct amino acid score for bioavailability of individual amino acids. Amino acid score corrected for true digestibility /bioavailability/ of individual amino acids has been termed as “available amino acid score”. The positive correlation /r = 0.92/ between available amino acid score and relative net protein ratio /RNPR, the most suitable rat growth method for evaluating protein quality/ was highly significant. Protein digestibility and amino acid bioavailability has been a problem in preventing widespread acceptance of amino acid score. Available amino acid score provides a solution to this problem.