Protein utilization in rats receiving a low-protein diet with various limiting amino acids

1. A study was made of protein utilization in rats given a variety of low-protein diets containing (g/kg) egg albumin 18, casein 49, gluten 50, or mixtures of either maize protein and gelatine 58, or casein and gelatin 37, each with supplemental methionine. The diets were limiting in leucine, tryptophan or lysine, or in both tryptophan and the sulphur-containing amino acids. 2. Values obtained for net protein utilization (NPU) at these low levels of nitrogen intake were markedly higher than the amino acid score calculated for the different test proteins, except with casein for which the two values were similar. The NPU values agreed more closely with chemical score values based on the content of S-amino acids or isoleucine in the dietary protein.