Consideration of utilizable amino acid content in the context of dietary protein quality assessment

Summary

Current evaluation systems for protein quality focus only on indispensable amino acids (IAAs) and do not consider the dispensable amino acids (DAAs). However, at excess levels of IAAs, DAAs may become limiting. Here, we used published ileal amino acid digestibility data determined in growing pigs and currently assumed protein and IAA recommendations to estimate utilizable, oxidized, and non-absorbed amino acids in different dietary protein and to compare these utilizable amino acid values to digestible indispensable amino acid score (DIAAS) values. For dietary protein sources with a DIAAS value <90, the amount of calculated utilizable amino acids increased proportionally with the DIAAS value according to “utilizable amino acids (mg/g protein) = DIAAS × 10”; for dietary protein sources with a DIAAS value >90, the amount of utilizable amino acid stays within the range 800–1000 mg/g protein. Cereals contained the lowest levels of utilizable amino acids and the highest amount of oxidized amino acids. Dairy and meat have the highest amount of utilizable amino acids followed by nuts and pulses. For many mixtures of protein sources, the highest value for utilizable amino acids, which considers both IAAs and DAAs, occurred at a different ratio of protein sources than the maximum DIAAS value, which only considers IAAs. From this theoretical study and based on the assumptions made, we conclude that estimation of utilizable amino acids from data typically reported in DIAAS studies further enables complementary insights next to DIAAS values. Careful consideration of protein and IAA requirements is critical in further work.