Journal Title (Medline/Pubmed accepted abbreviation): IAm. J. Physiol. Endocrinol. Metab.
Summary of background and research design
Background: Exercise stimulates protein breakdown, and protein ingestion post-exercise is necessary to achieve net protein synthesis.
Hypothesis: A protein-carbohydrate beverage during endurance exercise will further increase net protein synthesis immediately after exercise.
Subjects:20 trained male cyclists, age 22 ± 1 yr.
Experimental design:randomized, double-blind, cross-over
Treatments:1.0 g carbohydrate per hr per kg body weight OR 0.8 g protein and 0.2 g carbohydrate per hr per kg body weight during exercise.
Protocol:At an initial session, the subjects were evaluated for body composition, VO2max, and workload capacity (Wmax). Diet and exercise were loosely controlled before the testing days. On testing days, the participants arrived in the morning after an overnight fast. They rested for 4 hrs, during which 2 muscle biopsies were taken (t = 2 and 4 hrs) and infusions with stable isotopes were made in order to assess muscle protein synthesis and breakdown. Then, they consumed an initial 6.0 mL/kg body weight of the beverage and began to cycle on a stationary bicycle. They rode for 2 hrs at 55% of their individual Wmax. Every 15 min, they consumed 2 mL/kg of the test beverage. After the cycle bout, a third muscle biopsy was acquired. Blood samples were also taken throughout the protocol and into the recovery phase.
Summary of research findings
- There were no differences in blood glucose concentrations, though blood insulin rose higher after the CHO+PRO beverage compared to the CHO beverage. This is likely due to the fact that protein itself stimulates insulin secretion.
- Amino acid concentrations in the blood were higher with in the CHO+PRO trial compared to the CHO trial. Plasma lactate levels were not different between treatments.
- Rates of whole body protein synthesis and oxidation were greater with the CHO+PRO treatment versus CHO.
- Net whole body protein balance was positive with CHO+PRO and negative with just CHO just after exercise.
- Exercise itself did increase the fractional synthetic rate (FSR) of mixed muscle protein versus rest. However, there was no difference in mixed muscle FSR between the CHO and CHO+PRO treatments.
Interpretation of findings/Key practice applications
This study showed that the addition of protein to carbohydrate (isocaloric treatments) did not further increase mixed muscle FSR during exercise. However, it did appear, from Figure 5 in the paper, that there may have been a trend toward a greater increase in mixed muscle FSR, albeit not statistically significant, in the CHO+PRO vs. CHO group. Further, the CHO+PRO was associated with higher whole body protein synthesis and net protein balance than CHO. Thus, it appears that there is potential for at least some benefit from CHO+PRO vs. CHO consumed during exercise. Even so, it is not clear why the authors choose to study protein synthesis during exercise. The “during exercise” period is typically a catabolic phase and one would not expect much in the way of protein synthesis when compared with the postexercise recovery period. This calls into question to some degree the practical significance of these findings to the athlete.