Chocolate milk and endurance exercise recovery: Protein balance, glycogen, and performance


Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Year: 2012
Volume: 44
Issue: 4
doi: 10.1249/MSS.0b013e3182364162
 

Summary of Background and Research Design

Background: Efficient recovery is important for athletes so that they can get the most out of their workout and so they can perform optimally at their next training session.  It has been shown that recovery beverages that include both carbohydrate and protein contribute to recovery by restoring muscle glycogen and increasing the ratio of protein synthesis to breakdown during recovery.  Chocolate milk, a natural source of protein with added carbohydrate, had not been assessed specifically for glycogen restoration, protein turnover, or subsequent exercise performance.

Hypothesis: Compared to an isocaloric carbohydrate control beverage, chocolate milk will increase the rate of net protein synthesis, help retain muscle glycogen, and improve performance on an endurance test performed 3 hrs after the initial exercise routine.

Subjects: Study 1: 8 recreational runners, age 23.7 ± 1.6 y.
Study 2: 6 recreational runners, age 21.3 ± 1.2 y.

Experimental design: randomized, placebo-controlled, cross-over trial

Treatments: Fat free chocolate milk (480 mL, 296 kcal) or a sweetened grape-flavored beverage (also 296 kcal, all carbohydrates)

Protocol: This experiment was divided into 2 studies.  The goals of study 1 were to determine the post-exercise rate of glycogen synthesis, the fractional rate of muscle protein synthesis, and the phosphorylation state of several enzymes involved in muscle synthesis (indicates if they are “on” or “off”) after drinking chocolate milk.  The participants were first evaluated for VO2peak (also known as VO2max) and resting energy expenditure (REE).  During the 14 day study, their diet was controlled at 1.5 g protein/kg body weight per day, carbohydrates at 6 g/kg/day and fat at ≤ 30% of total caloric intake.  All meals were prepared for them.  On the days of the study, the participants arrived at the laboratory after an overnight fast.  They then received a primed, intravenous infusion of isotopically labeled phenylalanine (an amino acid that behaves equivalently to the unlabeled amino acid but its fate is able to be traced).  After resting for 75 min, they ran at 65% their previously determined VO2max for 45 min.  After exercise, a second catheter was inserted into a vein for blood sampling.  A muscle biopsy was acquired from the vastus lateralis (thigh) for quantification of glycogen and for calculation of muscle protein fractional synthesis rate.  The participants then consumed one of the test beverages.  Blood and muscle biopsies were acquired periodically throughout the 3 hr recovery period.  Exactly one week later, the participants returned to repeat the protocol but with the other test beverage.
For study 2, the performance during a subsequent exercise session and protein turnover was assessed.  The control of the diet and pre-trial protocol was the same as study 1.  On the day of the study, the participants received a primed infusion of isotopically labeled bicarbonate and leucine.  The participants then completed the exercise and recovery protocol as in study 1, but instead of muscle biopsies, breath was collected and assessed for CO2 production.  Time to exhaustion was assessed after the recovery, with the speed set at the same speed used to evaluate VO2max.  After 4 min, the incline on the treadmill was increased and the participants ran until exhaustion.
 

Summary of Research Findings
  • The fractional rate of synthesis (i.e. rate of muscle growth) was 38% higher after consuming chocolate milk than the control beverage.
  • Several, but not all, enzymes exhibited differences in their phosphorylation state between treatments during the recovery period.  The differences seen between treatments suggest that there was more protein synthesis occurring under the chocolate milk treatment.
  • There was greater activity of enzymes involved in protein breakdown after the carbohydrate beverage compared to chocolate milk.
  • Muscle glycogen content was not different between treatments.
  • Net leucine balance was 77% more negative with chocolate milk compared to the carbohydrate beverage, indicating that there was less protein being broken down.
  • In study 2, time to exhaustion was 23% greater with chocolate milk compared to the control (250 ± 43 sec with milk, 203 ± 31 sec with the placebo).

Key practice applications

As a post-exercise supplement, chocolate milk increased the rate of protein synthesis and decreased the rate of protein breakdown compared to an isocaloric carbohydrate beverage. This led to better performance at a sprint test 3 hrs after the first exercise bout. The exercise protocol was not adequate to deplete glycogen stores, so it is unknown if chocolate milk could aid in the rate of recovery if glycogen was depleted.


Limitations

It was impossible to blind the participants as to which beverage they were consuming, which may have played a psychological role in the time to exhaustion test.
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