Higher protein intake during a mixed meal ingestion increases net protein accretion through a reduction in protein breakdown

Abstract

It has been reported that the maximal protein synthetic response to protein intake is achieved with an intake of 20 ‐ 35 g of protein. However, in these studies account was not taken of possible changes in protein breakdown. In the present study we determined whole body protein kinetics [protein synthesis (PS), breakdown (PB), and net balance (NB), mg/kg ffm/min] in the fasted state and after ingestion of 4 oz (LP, 30g protein) or 10 oz (HP, 75g protein) beef in isocaloric mixed meals. Eleven healthy young adults were studied on two occasions in a randomized crossover design with primed continuous tracer infusions of L‐[2H5]phenylalanine & L‐[2H2]tyrosine on day 4 following 3 days of diet habituation. Basal whole body protein kinetics was not different between HP and LP [PS, 3.5 ± 0.1 vs. 3.5 ± 0.1; PB, 4.2 ± 0.1 vs. 4.2 ± 0.1; NB, ‐0.7 ± 0.0 vs. –0.7 ± 0.0]. Feeding increased NB in both treatments but to a greater extent in HP vs. LP [3.4 ± 0.3 and 1.0 ± 0.1; for all, p<0.001]. PS was increased similarly in both treatments [HP, 4.2.0 ± 0.2 and LP, 4.1 ± 0.2; for all, p<0.001]. PB was decreased in both treatments (for all, p<0.005) but to a greater extent in HP vs. LP [0.8 ± 0.2 vs. 3.1 ± 0.1; p<0.001]. In conclusion, whole body net protein accretion was increased with a protein intake above “optimal” protein intake primarily through further reductions in protein breakdown.