Carbohydrate intake of 10 g/kg body mass rapidly replenishes liver, but not muscle glycogen contents, during 12 h of post-exercise recovery in well-trained cyclists

Abstract: Post-exercise carbohydrate intake is required to replenish endogenous glycogen stores. Although adequate carbohydrate consumption has been reported to restore muscle glycogen contents within 24 h of post-exercise recovery, the time required to replenish liver glycogen contents is less evident. Twelve well-trained male cyclists (age: 25 ± 5 years; V̇O ₂peak: 67 ± 5 ml/min/kg; W _max: 5.8 ± 0.7 W/kg) participated in this trial. Ultra-high-field (UHF) ¹³C magnetic resonance spectroscopy ( ¹³C MRS) was applied to assess muscle and liver glycogen concentrations before and immediately after glycogen-depleting exercise on two test days. This was followed by 12 h of recovery during which participants remained fasted (CON) or consumed 10 g carbohydrate per kg body mass (BM) in the form of sucrose-containing beverages (1.2 g/kg BM/h over the first 6 h) and carbohydrate-rich meals (CHO). Muscle and liver glycogen concentrations were again assessed at 6 and 12 h into recovery using ¹³C MRS. Furthermore muscle biopsies were collected to assess muscle glycogen concentrations using biochemical analysis. Exercise significantly reduced muscle (from 159 ± 32 to 56 ± 19 mmol/l; −64%) and liver (from 166 ± 40 to 110 ± 44 mmol/l; −34%) glycogen concentrations (P < 0.05), with no significant differences between test days (P > 0.05). In the absence of carbohydrate intake (CON) muscle and liver glycogen concentrations remained unchanged during recovery. After carbohydrate intake (CHO) muscle glycogen concentrations increased from 56 ± 19 to 88 ± 16 and 110 ± 19 mmol/l (or from 269 ± 90 to 418 ± 78 and 523 ± 92 in mmol/kg dry mass), after 0, 6 and 12 h of recovery, respectively, remaining well below pre-exercise values (i.e. 55% of pre-exercise at 6 h and 69% at 12 h). Liver glycogen concentrations increased from 110 ± 44 to 236 ± 39 and 258 ± 40 mmol/l, after 0, 6 and 12 h of recovery, respectively, exceeding pre-exercise values within 6 h of recovery (i.e. 142% of pre-exercise). A very strong correlation (r = 0.89, P < 0.001), good agreement (ICC: 0.87) and low bias (4.1 ± 23.7 mmol/l) were observed when comparing muscle glycogen concentrations assessed using ¹³C MRS and biochemical analyses. Sucrose ingestion (1.2 g/kg BM/h) fully restored liver glycogen concentrations well within 6 h of post-exercise recovery. Ingesting large amounts of carbohydrate (10 g/kg BM) did not allow muscle glycogen stores to be replenished within 12 h of post-exercise recovery. (Figure presented.). Key points: Carbohydrate ingestion is required to replenish muscle and liver glycogen stores after a strenuous bout of exercise. Carbon-13 magnetic resonance spectroscopy at ultra-high-field (7T) allows non-invasive measurement of both muscle and liver glycogen contents before and after exercise, and at 6 and 12 h of recovery with (CHO) and without (CON) carbohydrate ingestion (10 g per kg body mass). Exercise strongly reduces glycogen contents of both muscle and liver tissue. Without carbohydrate ingestion muscle and liver glycogen levels remain depleted. After ingestion of large amounts of carbohydrate both muscle and liver glycogen contents increase rapidly, with liver glycogen stores being fully repleted within 6 h. Ample carbohydrate ingestion allows rapid replenishment of liver but not muscle glycogen stores within 6 and 12 h of post-exercise recovery.