Habituation to low or high protein intake does not modulate basal or postprandial muscle protein synthesis rates: a randomized trial

Background: Muscle mass maintenance is largely regulated by basal muscle protein synthesis rates and the ability to increase mus-cle protein synthesis after protein ingestion. To our knowledge, no previous studies have evaluated the impact of habituation to either low protein intake (LOW PRO) or high protein intake (HIGH PRO) on the postprandial muscle protein synthetic response. Objective: We assessed the impact of LOW PRO compared with HIGH PRO on basal and postprandial muscle protein synthesis rates after the ingestion of 25 g whey protein. Design: Twenty-four healthy, older men [age: 62 6 1 y; body mass index (in kg/m2): 25.9 6 0.4 (mean 6 SEM)] participated in a parallel-group randomized trial in which they adapted to either aLOWPRO diet(0.7g $ kg–1 $ d21; n = 12) or a HIGH PRO diet (1.5 g $ kg–1 $ d–1; n = 12) for 14 d. On day 15, partici-pants received primed continuous L-[ring-2H5]-phenylalanine and L-[1-13C]-leucine infusions and ingested 25 g intrinsically L-[1-13C]-phenylalanine– and L-[1-13C]-leucine–labeled whey pro-tein. Muscle biopsies and blood samples were collected to assess muscle protein synthesis rates as well as dietary protein digestion and absorption kinetics. Results: Plasma leucine concentrations and exogenous phenylala-nine appearance rates increased after protein ingestion (P , 0.01) with no differences between treatments (P . 0.05). Plasma exoge-nous phenylalanine availability over the 5-h postprandial period was greater after LOW PRO than after HIGH PRO (61% 6 1% com-pared with 56% 6 2%, respectively; P , 0.05). Muscle protein synthesis rates increased from 0.031% 6 0.004% compared with 0.039% 6 0.007%/h in the fasted state to 0.062% 6 0.005% com-pared with 0.057% 6 0.005%/h in the postprandial state after LOW PRO compared with HIGH PRO, respectively (P , 0.01), with no differences between treatments (P = 0.25). Conclusion: Habituation to LOW PRO (0.7 g $ kg–1 $ d–1)compared with HIGH PRO (1.5 g $ kg–1 $ d–1) augments the postprandial avail-ability of dietary protein–derived amino acids in the circulation and does not lower basal muscle protein synthesis rates or increase postprandial muscle protein synthesis rates after ingestion of 25 g protein in older men. This trial was registered at clinicaltrials.gov as NCT01986842. Am J Clin Nutr 2017;105:332–42.