The digestion rate of protein is an independent regulating factor of postprandial protein retention

To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of id entical amino acid composition and nitrogen contents but of different diges tion rates. Four groups of five or six young men received an L-[1-C-13] leu cine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking c asein composition (AA), a single meal of rapidly digested whey proteins (WP ), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rat e. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) m eals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). T he fast meals induced a strong, rapid, and transient increase of aminoacide mia, leucine flux, and oxidation. After slow meals, these parameters increa sed moderately but durably. Postprandial leucine balance over 7 h was highe r after the slow than after the fast meals (CAS: 38 +/- 13 vs. AA: -12 +/- 11, P < 0.01; RPT-WP: 87 +/- 25 vs. WP: 6 +/- 19 mmol/kg, P < 0.05). Protei n digestion rate is an independent factor modulating postprandial protein d eposition.