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 g. We investigated this proportion in biopsy samples during recovery from prolonged exercise. Methods: On two occasions, nine aerobically trained subjects (V˙O_2max = 54.4 ± 1.05 mL kg⁻¹ min⁻¹; mean ± SD) dehydrated 4.6 ± 0.2 % by cycling 150 min at 65 % V˙O_2max in a hot-dry environment (33 ± 4 °C). One hour after exercise subjects ingested 250 g of carbohydrates in 400 mL of water (REH_LOW) or the same syrup plus water to match fluid losses (i.e., 3170 ± 190 mL; REH_FULL). Muscle biopsies were obtained before, 1 and 4 h after exercise. Results: In both trials muscle water decreased 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⁻¹ dry muscle; P = 0.20) while muscle water content was higher in REH_FULL than in REH_LOW (3814 ± 222 vs. 3459 ± 324 g kg⁻¹ dm, respectively; P < 0.05; ES = 1.06). Despite the insufficient water provided during REH_LOW, per each gram of glycogen, 3 g of water was stored in muscle (recovery ratio 1:3) while during REH_FULL this ratio was higher (1:17). 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 REH_FULL) likely due to water storage not bound to glycogen.