A 2016 literature review published in the American Journal of Physiology Endocrinology and Metabolism describes the involvement of liver glycogen in prolonged exercise and discusses the most effective nutritional strategies for liver glycogen repletion. Gonzalez et al. report that the total volume of liver glycogen in trained endurance athletes is no different than that of untrained, healthy individuals. However, when untrained individuals participate in exercise, they metabolize liver glycogen much more rapidly than their trained counterparts. They cite one study which found that untrained individuals experienced a rate of liver glycogenolysis (the breakdown of glycogen) of 6.9 mg/kg/min whereas trained individuals experienced a rate of 5.3 mg/kg/min. This means that during moderate-high intensity exercise, untrained individuals had depleted their liver glycogen stores and experienced fatigue at 118 minutes while trained individuals stores weren’t depleted until 153 minutes. The authors conclude, “the lower rate of liver glycogenolysis in the endurance-trained state likely contributes to the greater endurance performance/capacity by facilitating maintenance of (high) carbohydrate oxidation rates and blood glucose homeostasis during the latter stages of exercise.”
Fortunately, glycogen can be easily repleted and maintained during and after exercise. Gonzalez et al. refer to a number of studies that suggest that ingestion of carbohydrates, particularly glucose or sucrose (glucose-fructose) during exercise can attenuate liver glycogen depletion. It is believed that consuming 1.2g/kg of carbohydrate during recovery is ideal for rapid repletion.
While fructose has not been shown to accelerate muscle glycogen repletion post-exercise, it is does augment liver glycogen repletion. Gonzalez et al. note ample research to suggest that co-ingestion of fructose with glucose after a workout can almost double liver glycogen repletion rates independent of the total volume of carbohydrates ingested. Moreover, fructose-glucose combinations are better tolerated and cause less gastrointestinal (GI) stress than glucose alone.
In conclusion, after approximately 90 minutes of moderate-high intensity exercise liver glycogen stores will be depleted. Ingesting carbohydrates, glucose or sucrose, during exercise can attenuate depletion. For post-workout recovery, glucose-fructose or glucose-galactose combinations are recommended for rapid replenishment and GI tolerance.