Journal Title (Medline/Pubmed accepted abbreviation): J Appl Physiol
Page numbers: 431-438
doi (if applicable):
Summary of Background and Research Design
Hypothesis: Exercise with low glycogen would increase the net negative protein balance at the whole body and skeletal muscle levels compared with the high glycogen condition and this would be primarily due to an increase in protein breakdown with an additional reduction in protein synthesis.
Subjects:6 healthy men (mean age 24 y, mean body weight 80 kg) who were recreationally active
Experimental design:;Randomized, crossover
Diets:High carbohydrate (6837 kcal per 44 h, 71% CHO, 19% fat, 10% protein) and low carbohydrate (6172 kcal per 44 h, 11% CHO, 64% fat, 25% protein)
Protocol:Subjects initially underwent a VO2 max test and baseline kicking VO2 max tests before the protocol. For the first treatment period, subjects underwent a glycogen depletion ride (75% VO2 max to fatigue) and then began either the high carbohydrate or low carbohydrate diet with rest for the next 43-45 h. Following completion of the diet, there was an exercise test coupled with infusion of labeled amino acids to measure protein turnover and leucine balance and oxidation (d5-phenylalanine, 13C-leucine). The exercise test comprised 2 h of leg extensions at 45% of kicking VO2 max. Catheters placed in the radial artery and femoral vein allowed researchers to measure arterio-venous differences in amino acid concentrations. Multiple measures blood, breath, and muscle were made during the exercise and also there was a measurement at 1-h of recovery from exercise. There was a 7-day washout period and then the subjects underwent the same treatment period with the opposite diet.
Summary of research findings:
- The low CHO diet was associated with lower muscle glycogen concentrations at rest, during 2 h of exercise, and at 1 h of recovery after exercise.
- Leucine balance was significantly more negative both during the leg extension exercise for the low vs. high CHO diet. At the same time, leucine oxidation was higher, by roughly the same magnitude, for the low vs. high CHO diet.
- Phenylalanine balance across the leg followed a pattern similar to leucine, being lower at all time points during the leg exercise for the low vs. high CHO diet.
- During the later portion of the leg exercise (90 and 120 min time points), the rate of phenylalanine disappearance from the plasma, an indicator of phenylalanine incorporation into muscle tissue, was reduced for the low vs. high CHO diet.
- These findings suggest that carbohydrate availability affects both protein synthesis and degradation rates in the muscle.
- The low CHO diet was associated with higher leg glucose uptake, indicating that lower muscle glycogen levels may lead to an increased ability of muscle to take up glucose.
Interpretation of findings/Key practice applications:
This paper presents important data regarding the relationship of muscle glycogen levels and protein synthesis. A key limitation of this paper is that protein intake as well as carbohydrate intake varied between the low and high CHO diets. In the case of the low CHO diet, protein intake was about 2.4 g/kg body weight/d, while in the high CHO diet, protein intake was 1.1 g/kg body weight/d. Thus, glycogen levels in the muscle might not be the sole factor responsible for the changes in protein synthesis and degradation in the muscle. However, it is interesting to note that the low CHO diet, despite containing over double the protein of the high CHO diet, still was associated with more negative amino acid balance. This suggests that carbohydrate intake, as well as protein intake, could be an influential factor in regulating protein synthesis.