Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men

Abstract

Background: Increased amino acid availability stimulates muscle protein synthesis, however, aged muscle appears less responsive to the anabolic effects of amino acids when compared to the young. We aimed to compare changes in myofibrillar protein synthesis (MPS) in elderly men at rest and after resistance exercise following ingestion of different doses of soy protein and compare the responses to those we previously observed with ingestion of whey protein isolate.

Methods: Thirty elderly men (age 71 ± 5 y) completed a bout of unilateral knee-extensor resistance exercise prior to ingesting no protein (0 g), or either 20 g or 40 g of soy protein isolate (0, S20, and S40 respectively). We compared these responses to previous responses from similar aged men who had ingested 20 g and 40 g of whey protein isolate (W20 and W40). A primed constant infusion of L-[1-13 C]leucine and L-[ring-13 C6]phenylalanine and skeletal muscle biopsies were used to measure whole-body leucine oxidation and MPS over 4 h post-protein consumption in both exercised and non-exercised legs.

Results: Whole-body leucine oxidation increased with protein ingestion and was significantly greater for S20 vs. W20 (P = 0.003). Rates of MPS for S20 were less than W20 (P = 0.02) and not different from 0 g (P = 0.41) in both exercised and non-exercised leg muscles. For S40, MPS was also reduced compared with W40 under both rested and post-exercise conditions (both P < 0.005); however S40 increased MPS greater than 0 g under post-exercise conditions (P = 0.04).

Conclusions: The relationship between protein intake and MPS is both dose and protein source-dependent, with isolated soy showing a reduced ability, as compared to isolated whey protein, to stimulate MPS under both rested and post-exercise conditions. These differences may relate to the lower postprandial leucinemia and greater rates of amino acid oxidation following ingestion of soy versus whey protein.

Figure 1

Plasma insulin in the 0 g protein group, and in the whey (W20, W40) and soy (S20, S40) groups. Means with different letters are significantly different from time 0 and from each other (P < 0.05). Data are means ± SD.

Figure 2

Blood leucine in the 0 g protein group, and in the whey (W20, W40) and soy (S20, S40) groups (no statistical analysis is shown except on the area under the concentration-time curve, see inset). Means with different letters are significantly different (P < 0.05). Data are means ± SD.

Figure 3

Area under the blood amino acid concentration time curves (AUC) in the 0 g protein group, and in the whey (W20, W40) and soy (S20, S40) groups for summed total of branched-chain amino acids (BCAA), summed total of the essential amino acids (EAA, including His) and summed total of all amino acids (total AA, excluding Cys and Trp). Means with different letters are significantly different (P < 0.05). Data are means ± SD.

Figure 4

Whole-body leucine oxidation at different protein doses expressed relative to total body weight (left) and to lean body mass (right) in the 0 g protein group, and in the whey (W20, W40) and soy (S20, S40) groups. Means with different letters are significantly different (P < 0.05). Data are means ± SD.

Figure 5

Myofibrillar protein fractional synthetic rate (%·h^-1) for whey and soy (20 g and 40 g) groups and a group who consumed no protein (0 g) at rest and following resistance exercise (Ex) as described. There was a significant dose by condition by protein source interaction (P < 0.001). † Indicates a significant (P < 0.05) difference from the 0 g dose within the Rest condition; * indicates a significant (P < 0.05) difference from the 0 g dose within the Ex condition; and ‡ indicates a significant (P < 0.05) difference from the 20 g Ex condition within the same protein source. Data are means ± SD.