Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans.
Level 2 - randomized trial
Randomized crossover trial in humans evaluating hydration protocols post-exercise.
PubMed 25911631 · doi:10.1007/s00421-015-3175-z
What was done
Nine aerobically trained individuals (mean VO2max 54.4 ± 1.05 mL·kg⁻¹·min⁻¹) dehydrated by 4.6 ± 0.2% during 150 minutes of cycling at 65% VO2max in a 33 ± 4 °C environment across two experimental trials. One hour post-exercise, participants consumed 250 g of carbohydrates dissolved in 400 mL of water (REHLOW) or the same carbohydrate amount with fluid volume matched to total body mass loss (3170 ± 190 mL; REHFULL). Muscle biopsies were taken before exercise, 1 hour post-exercise, and 4 hours post-exercise to assess muscle water and glycogen concentrations.
What was found
Exercise decreased muscle water by 13 ± 6% and muscle glycogen by 44 ± 10% across both conditions (P < 0.05). After 4 hours of recovery, muscle glycogen concentrations were not significantly different between REHLOW and REHFULL (79 ± 15 vs. 87 ± 18 g·kg⁻¹ dry muscle, respectively; P = 0.20). Muscle water content was significantly higher following REHFULL compared to REHLOW (3814 ± 222 vs. 3459 ± 324 g·kg⁻¹ dry muscle; P < 0.05; effect size = 1.06). The resulting glycogen-to-water storage ratio was 1:3 g in REHLOW and 1:17 g in REHFULL.
Why it matters
This study provides direct human biopsy verification that muscle glycogen resynthesis incorporates at least 3 g of water per gram of glycogen, while showing that full fluid replacement allows substantial intramuscular water storage beyond what is bound directly to glycogen.
Limits
The sample size was small (n = 9) and limited to aerobically trained individuals, with participant sex not reported in the abstract. Measurements were restricted to an acute 4-hour recovery window and did not distinguish between intracellular and extracellular muscle fluid compartments.
Cited by
- supports The human body retains approximately 4 grams of water per gram of carbohydrate stored.