Consumption of Milk Protein or Whey Protein Results in a Similar Increase in Muscle Protein Synthesis in Middle Aged Men

Abstract

The differential ability of various milk protein fractions to stimulate muscle protein synthesis (MPS) has been previously described, with whey protein generally considered to be superior to other fractions. However, the relative ability of a whole milk protein to stimulate MPS has not been compared to whey. Sixteen healthy middle-aged males ingested either 20 g of milk protein (n = 8) or whey protein (n = 8) while undergoing a primed constant infusion of ring (13)C₆ phenylalanine. Muscle biopsies were obtained 120 min prior to consumption of the protein and 90 and 210 min afterwards. Resting myofibrillar fractional synthetic rates (FSR) were 0.019% ± 0.009% and 0.021% ± 0.018% h(-1) in the milk and whey groups respectively. For the first 90 min after protein ingestion the FSR increased (p < 0.001) to 0.057% ± 0.018% and 0.052% ± 0.024% h(-1) in the milk and whey groups respectively with no difference between groups (p = 0.810). FSR returned to baseline in both groups between 90 and 210 min after protein ingestion. Despite evidence of increased rate of digestion and leucine availability following the ingestion of whey protein, there was similar activation of MPS in middle-aged men with either 20 g of milk protein or whey protein.

Keywords: amino acids; fractional synthetic rate; milk protein; muscle; stable isotopes; whey protein.

Figure 1

Plasma Total Amino Acids (AAs). Time course of plasma total AA concentration following the ingestion of 20 g of MPC or WPC (main panel). Solid line and filled circles resent WPC; dashed line and open squares represent MPC. The insert shows the area under the curve (AUC) for 210 min following the ingestion of the protein beverage. *, different from MPC at the same time point p ≤ 0.05. Error bars represent standard error of the mean.

Figure 2

Plasma Essential Amino Acids (EAAs) excluding Leucine. Time course of plasma EAA excluding leucine concentration following the ingestion of 20 g of MPC or WPC (main panel). Solid line and filled circles resent WPC; dashed line and open squares represent MPC. The insert shows the area under the curve (AUC) for 210 min following the ingestion of the protein beverage. *, different from MPC at the same time point p ≤ 0.05. Error bars represent standard error of the mean.

Figure 3

Plasma leucine. Time course of plasma leucine concentration following the ingestion of 20 g of MPC or WPC (main panel). Solid line and filled circles resent WPC; dashed line and open squares represent MPC. The insert shows the area under the curve (AUC) for 210 min following the ingestion of the protein beverage. *, different from MPC at the same time point p ≤ 0.05. Error bars represent standard error of the mean.

Figure 4

Plasma ¹³C₆ enrichment. The tracer enrichment of ¹³C₆ phenylalanine over the infusion period is shown. The closed circles represent WPC and the open square represent MPC. *, different from MPC at the same time point p ≤ 0.05; #, main effect for difference from 0 time point p ≤ 0.05. Error bars represent standard error of the mean.

Figure 5

Muscle anabolic signaling. (A) Change in p70S6K phosphorylation at threonine 421 and serine 424 sites from the fasted state, solid bar represent WPC ingestion and open bars represent MPC ingestion. (B) Change in Akt phosphorylation at the serine 473 site from the fasted state. (C) Change in ribosomal protein S6 (rpS6) phosphorylation at the serine 235/236 sites from the fasted state. (D) Change in ribosomal protein S6 (rpS6) phosphorylation at the serine 240/244 sites from the fasted state. The solid horizontal line represents a main effect for time. *, different from fast p ≤ 0.05. Error bars represent standard error of the mean.

Figure 6

Muscle fractional synthetic rate (FSR). A) The course of myofibrillar protein synthesis following the ingestion of 20 of MPC (open bars) or WPC (black bars). B) The aggregate myofibrillar protein synthesis following the ingestion of 20 of MPC (open bars) or WPC (black bars). The solid horizontal line represents a main effect for time. *, different from fast p ≤ 0.05. Error bars represent standard error of the mean.