Journal Title (Medline/Pubmed accepted abbreviation): J Strength Cond Res
Page numbers: 2577-2586
Summary of Background and Research Design
Background: Ingesting carbohydrate (CHO) plus protein (approximately 25% to 30% of total energy content from protein) during prolonged variable-intensity exercise has demonstrated improved aerobic endurance performance beyond that of a CHO supplement alone and may reduce muscle damage. Previous work has demonstrated that lower-CHO beverages with added protein (CHO-PRO) were as effective in extending time to exhaustion (TTE) as higher-calorie CHO-only beverages. Further research suggests that the efficacy of supplements can be maintained despite a reduction in total CHO and total energy content with the substitution of a small amount of protein.
Hypothesis: The authors sought to determine if a supplement containing a lower amount of CHO (3% vs 6%) with a moderate amount of PRO could be optimized using a mixture of different CHO (glucose, maltodextrin, and fructose) rather than a single CHO (dextrose).
Subjects:Fifteen trained endurance athletes, both male and female (mean age, 28.7 yr; mean body weight, 65.9 kg) participated in the study. All subjects were accustomed to riding 3 to 6 hours in a single ride and had a mean 6.5 years of cycling training.
Experimental design: Randomized, double-blind
Treatments and protocol: Prior to the trials, subjects reported to the same laboratory and used the same ergometer as trial use to determine baseline maximal oxygen consumption (VO2max). All trials were performed after a 12-hour, overnight fast. Three to 5 days after the VO2max test, cyclists performed a familiarization ride that followed the same protocol as the experimental ride except that only water was provided and no blood samples were collected. The experimental protocol consisted of cyclists exercising on 2 separate occasions on a cycle ergometer at intensities alternating between 45 and 70% VO2max for 3 hours, after which the workload increased to ~74% to 85% VO2max until exhaustion. Each trial was performed on the same day of the week and at the same time of day in a room ~21°C. Trials were separated by >= 7 to <= 14 days. Supplements (275 mL) were provided every 20 min during exercise and consisted of either a 3% CHO/1.2% PRO supplement (MCP) or a 6% CHO supplement. The TTE after cycling and the ventilatory threshold (VT) at which subjects cycled to exhaustion were evaluated. Additional outcome measures of interest were substrate use, and response to insulin, glucose, lactate, and myoglobin. Constant verbal encouragement was given to the subjects during each trial, and the same investigators were present during all trials for consistency.
Summary of research findings:
- For the combined male and female group (n = 15), TTE in MCP did not differ from CHO (31.06 ± 5.76 min vs 26.03 ± 4.27 min, respectively; P = .064).
- For subjects cycling at or below VT (n = 8), TTE in MCP was significantly greater than for CHO (45.64 ± 7.38 min vs 35.47 ± 5.94 min, respectively; P = .006).
- There were no significant differences in TTE subjects for subjects cycling above VT (n = 7).
- Plasma glucose was significantly lower in the combined MCP group than CHO (P = .005).
- No significant overall treatment differences in plasma insulin were found among treatments; however, a significant treatment-by-time difference was observed at 177 min, with plasma insulin higher in the combined-CHO group and in the at-or-below-VT group vs the corresponding MCP groups.
- There were no significant differences in blood lactate levels or plasma myoglobin levels between treatments.
- There were no significant treatment differences in either CHO oxidation rates or fat oxidation rates.
Interpretation of findings/Key practice applications:
The results suggest that, compared with a traditional 6% CHO supplement, a 3% CHO plus 1.2% protein beverage can: 1) support exercise time to exhaustion on the whole just as well; and 2) improve aerobic endurance (28.7% improvement in TTE) at exercise intensities near the VT, despite containing lower total CHO and caloric content. The VT was chosen post priori as a surrogate marker for lactate threshold (LT) as they have both been shown to occur near the same VO2max. Using VT levels to group data is a novel approach, as studies typically use VO2max. However, other studies do not relate the VO2max to VT or LT, and exercise intensity may be a factor in CHO and MCP efficacy. Additionally, the ability to exercise for long periods near an individual's LT is a critical component of performance in long events such as marathons, longer cycling races, and long-distance triathlons. The results of this trial are significant to athletes who want to improve their endurance performance yet try to maintain or improve their body composition; these athletes may be more interested in supplements with lower caloric content.