Changes in plasma amino acid concentrations in man after ingestion of an amino acid mixture simulating casein, and a tryptic hydrolysate of casein.

1. Plasma amino acid levels have been estimated at 0, 15, 30 and 45 min after ingestion of doses of (1) an amino acid mixture simulating casein and (2) a tryptic hydrolysate of casein consisting mainly of oligopeptides. Both doses contained the same amount of nitrogen. 2. After ingestion of both preparations, there was a prompt increase in plasma amino acid levels, followed by a decrease. No such change occurred in fasting subjects. There were no significant differences between increments in plasma amino acid levels after ingestion of the amino acid mixture and the corresponding increments after ingestion of the tryptic hydrolysate. 3. Correlations were found between the areas under the curves for individual amino acid concentrations, after ingestion of the two preparations, and the amino acid composition of casein. The results do not suggest that increases in plasma amino acid levels following small doses of protein digestion products are the result of circadian changes, or that such increases are 'swamped' by absorption of amino acids from endogenous protein in the lumen of the small intestine.