The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults

We have determined whole body protein kinetics, i.e., protein synthesis (PS), breakdown (PB), and net balance (NB) in human subjects in the fasted state and following ingestion of similar to 40 g [moderate protein (MP)], which has been reported to maximize the protein synthetic response or similar to 70 g [higher protein (HP)] protein, more representative of the amount of protein in the dinner of an average American diet. Twenty-three healthy young adults who had performed prior resistance exercise (X-MP or X-HP) or time-matched resting (R-MP or R-HP) were studied during a primed continuous infusion of L-[H-2(5)] phenylalanine and L-[H-2(2)] tyrosine. Subjects were randomly assigned into an exercise (X, n = 12) or resting (R, n = 11) group, and each group was studied at the two levels of dietary protein intake in random order. PS, PB, and NB were expressed as increases above the basal, fasting values (mg.kg lean body mass(-1).min(-1)). Exercise did not significantly affect protein kinetics and blood chemistry. Feeding resulted in positive NB at both levels of protein intake: NB was greater in response to the meal containing HP vs. MP (P < 0.00001). The greater NB with HP was achieved primarily through a greater reduction in PB and to a lesser extent stimulation of protein synthesis (for all, P < 0.0001). HP resulted in greater plasma essential amino acid responses (P < 0.01) vs. MP, with no differences in insulin and glucose responses. In conclusion, whole body net protein balance improves with greater protein intake above that previously suggested to maximally stimulating muscle protein synthesis because of a simultaneous reduction in protein breakdown.