Postexercise carbohydrate-protein supplementation improves subsequent exercise performance and intracellular signaling for protein synthesis


Journal Title (Medline/Pubmed accepted abbreviation): J. Strength Cond. Res.
Year: 2011
Volume: 25
Issue:5
Page numbers:1210-1224
doi (if applicable):

Summary of background and research design:

Background: Maximizing the rate of recovery will maximize performance in subsequent events, either 10 min or 2 days later. The combination of protein and carbohydrates (specifically, readily available chocolate milk) has been shown to induce net protein synthesis after exercise.

Hypothesis: After endurance exercise, chocolate milk will aid in the replenishment of post-exercise glycogen, activate the mTOR pathway which signals enzymes involved in protein synthesis, reduce signals involved in protein breakdown, and attenuate muscle damage compared to an isocaloric carbohydrate beverage and a calorie-free placebo.

Subjects:Male (n = 5) and female (n = 5) healthy cyclists trained for distance cycling, age 31.8 ± 1.6 yrs

Experimental design: randomized, double-blind, placebo-controlled, crossover design.

Treatments:1) Chocolate milk, 2) an isocaloric beverage made with water, dextrose, and canola oil, and 3) a placebo (sugar-free Kool Aid). Depending on their body weight, subjects consumed 1.0 to 1.4 L total during the recovery period, equaling 1581-2213.4 kcal (divided into 2 doses). The chocolate milk supplement totaled 36.7-51.4 g protein total.

Protocol: Before the trial, the participants were evaluated for VO2max on a stationary bicycle.  For each trial, the participants reported to the laboratory after an overnight fast.  In order to deplete their glycogen stores, they cycled at 70% VO2max for 90 min on the stationary bicycle, followed by 5 × 1 min intervals alternating between 45% and 90% VO2max.  Water was provided every 15 min.  During the trials, work and power output as well as respired gases were monitored.  After the exercise bout, subjects rested for 4 hrs.  One of the 3 supplements was consumed at the start of the rest period and 2 hrs into the rest period.  Muscle biopsies were acquired at time = 0, 45 min, and 4 hrs during the rest period to assess glycogen content and signaling markers.  After the rest period subjects completed a time trial during which they cycled 40 km with varying intensities as fast as possible.  Their performance during this time trial was one of the major determinants for muscle recovery.  Blood was collected at defined time points throughout the protocol.  Respiratory exchange ratio (RER) was also calculated at given time points.
Summary of research findings
  • The time trial time was significantly shorter with the chocolate milk supplement compared to both the carbohydrate beverage and the placebo (p ≤ 0.05).
  • Average power output and heart rate during the time trial were also significantly higher with chocolate milk compared to the other 2 treatments (p ≤ 0.05), though perceived exertion was not higher.
  • The chocolate milk and the carbohydrate beverage were both more effective at replenishing glycogen compared to the placebo (p ≤ 0.05).  The carbohydrate beverage was more effective than chocolate milk (p = 0.06).
  • At 45 min into the rest period, blood glucose was significantly highest for the carbohydrate trial, and the chocolate milk trial was significantly higher than the placebo trial.  At 120 min, blood glucose was similar for the carbohydrate and chocolate milk trials and the placebo was significantly lower.  Insulin levels mirrored blood glucose levels, except at 120 min insulin remained elevated for the carbohydrate trial.
  • After the time trial, plasma glycerol was significantly lower in the carbohydrate trial compared to the chocolate milk and placebo trials, suggesting that fat is more available for energy in the chocolate milk trial.
  • Plasma myoglobin, cortisol, and measured cytokines rose in all treatments to a similar magnitude, indicating similar amounts of stress.
  • mTOR was activated to a greater extent in the chocolate milk treatment, suggesting that protein synthesis could occur at a faster rate.
  • Protein breakdown was prevented to a greater degree in the carbohydrate and chocolate milk trials, as measured by the phosphorylation of signaling factor FOXO3A (phosporylation of FOXO3A inhibits its ability to increase protein breakdown).

Interpretation of findings/Key practice applications

Chocolate milk improved times in a time trial after glycogen-depleting exercise compared to an isocaloric carbohydrate beverage and a calorie-free beverage. It is likely that the combination of protein availability and sugar allowed the body to recover faster; the sugar aided in the replenishment of glycogen and the protein aided in protein synthesis.

Limitations

The authors suggest that chocolate milk may aid in recovery just as well as commercially designed recovery beverages, but the authors did not compare chocolate milk to commercial recovery beverages. Depending on the manufacturer, recovery formulations often include branched chain amino acids, structured lipids, and other nutrients that additionally aid in recovery. The comparator beverage in this study just contained carbohydrate and a small amount of lipid and this type of beverage would be more suited to consumption during, rather than after, exercise.
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