Caffeine Ingestion Does Not Accelerate Muscle Glycogen Resynthesis Following 90-minutes Of Running In Active Adults

PURPOSE: Muscle glycogen has vast physiological effects influencing exercise and metabolic health. Caffeine reportedly accelerates muscle glycogen synthesis, with rates exceeding all other human literature. However, these effects are inconsistent, and mechanisms remain equivocal. Therefore, this study attempts to assess the influence of caffeine (CAFF) addition to carbohydrate (CHO) supplementation on muscle glycogen resynthesis following running exercise and identify any potential mechanisms of action. METHODS: Eleven active adults (mean age: 22 (SD 5), BMI: 21.4 (2.1) kg/m2, V̇O2max 57.0 (6.6) mL/kg/min) completed two trials separated by >7 d in a double-blind, randomised cross-over design. Trials involved a 90-min treadmill run at 70% V̇O2max followed by a 4 h recovery. Throughout recovery, every hour, participants consumed beverages containing 1.2 g/kg/hr CHO with or without 8 mg/kg CAFF. Muscle biopsies and blood samples were obtained regularly throughout recovery. RESULTS: Muscle glycogen did not differ following 90-mins of running (mean [normalised confidence intervals] for CHO and CHO + CAFF respectively; 203.79 [149.3 - 258.3] vs 240.3 [185.8 - 294.81] mmol·L-1·kg dm-1). Following 1 h of recovery, muscle glycogen accumulation did not differ (8.5 [-50.6 - 67.7] vs 39.2 [-19.87 - 98.35] mmol·L-1·kg dm-1). After 4 h of recovery, muscle glycogen synthesis was higher in CHO, although this was not statistically significant (85.1 [40.5 - 129.6] vs 41.8 [-2.76 - 86.31] mmol·L-1·kg dm-1). Blood glucose did not differ between conditions until 4 h of recovery when concentrations were raised in CHO + CAFF compared to CHO (5.53 [5.09 - 5.97] vs 4.86 [4.42 - 5.30] mmol·L-1). Glucose iAUC did not differ between conditions (297.68 [226.3 - 368.97] vs 285.48 [214.19 - 356.77] mmol·L-1 x 240 min). Similarly, Plasma NEFA tAUC showed no discernable differences (147.91 [127.26 - 168.56] vs 132.84 [112.19 - 153.49] mmol·L-1 x 240 min). Insulin iAUC was raised in the CHO + CAFF condition, although this did not reach statistical significance (21.74 [10.88 - 32.62] vs 31.79 [20.92 - 42.66] nmol·L-1 x 240 min). CYP1A2 genotype did not influence any outcomes. CONCLUSION: These data do not provide any evidence to suggest caffeine addition to carbohydrate increases muscle glycogen synthesis post-running exercise.