Background: Casein and whey proteins differ in amino acid composition and absorption rate; however the absorption rate of casein can be increased to mimic that of whey proteins by exogenous hydrolysis. In view of these compositional differences we studied the metabolic responses to intake of casein, hydrolyzed casein and whey proteins in overweight and moderately obese men and women by investigating select urinary and blood plasma metabolites.

Results: A total of 21 urinary and 23 plasma metabolites were identified by NMR spectroscopy. The postprandial plasma metabolites revealed a significant diet-time interaction for isoleucine (P = 0.001) and tyrosine (P = 0.001). The level of isoleucine and tyrosine peaked 90 min postprandial with a 1.4-fold difference following intake of whey proteins compared to casein and hydrolyzed casein, respectively. A 1.2-fold higher urinary level of lactate was observed after intake of whey proteins compared to intake of intact casein (P < 0.01).

Conclusions: The plasma metabolites revealed different amino acid profiles reflecting the amino acid composition of casein and whey proteins. Furthermore, the results support that casein hydrolysates do neither affect the postprandial amino acid absorption rate nor the amino acid level compared to that of intact casein. The urinary lactate increases following whey protein intake might indicate a higher metabolism of glucogenic amino acids.