Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans

Purpose: It is usually stated that glycogen is stored in human muscle bound to water in a proportion of 1:3 grams. We investigated this proportion in biopsy samples during recovery from prolonged exercise. Methods: On two occasions, 9 aerobically-trained subjects ( ̇O2max = 54.4±1.05 mL·kg-1·min-1) dehydrated 4.6±0.2 % by cycling 150 min at 65 % ̇O2max in a hot-dry environment (33 ± 4 °C). One hour after exercise subjects ingested 250 g of carbohydrates in 400 mL of water (REHLOW) or the same syrup plus water to match fluid losses (i.e., 3170±190 mL; REHFULL). Muscle biopsies were obtained before, 1 h and 4 h after exercise. Results: In both trials muscle water decrease from pre-exercise similarly by 13±6 % and muscle glycogen by 44±10 % (P < 0.05). After recovery, glycogen levels were similar in both trials (79±15 and 87±18 g · kg wet muscle-1; P = 0.20) while muscle water content was higher in REHFULL than in REHLOW (3814±222 vs. 3459±324 g · kg dm-1, respectively; P < 0.05; ES = 1.06). Despite the insufficient water provided during REHLOW, per each gram of glycogen, 3 grams of water were stored in muscle (recovery ratio 1 to 3) while during REHFULL this ratio was higher (1 to 17). Conclusions: Our findings agree with the long held notion that each gram of glycogen is stored in human muscle with at least 3 g of water. Higher ratios are possible (e.g., during REHFULL) likely due to water storage not bound to glycogen