Liver glycogen metabolism during and after prolonged endurance-type exercise

Jul 21, 2016American journal of physiology. Endocrinology and metabolism

How the liver uses stored sugar during and after long endurance exercise

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Abstract

Endurance-trained athletes do not exhibit elevated basal liver glycogen concentrations compared to untrained controls.

Carbohydrate is the primary energy source during moderate- to high-intensity exercise.
Endogenous carbohydrate stores in the liver and muscle are limited, potentially affecting performance.
Liver glycogenolysis may be lower in endurance-trained athletes during exercise compared to untrained individuals.
High-rate carbohydrate ingestion (>1.5 g/min) can prevent liver glycogen depletion during moderate-intensity exercise.
Coingesting glucose with fructose or galactose can double postexercise liver glycogen replenishment rates.
No established guidelines currently exist for optimizing liver glycogen repletion through carbohydrate intake.

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Carbohydrate and fat are the main substrates utilized during prolonged endurance-type exercise. The relative contribution of each is determined primarily by the intensity and duration of exercise, along with individual training and nutritional status. During moderate- to high-intensity exercise, carbohydrate represents the main substrate source. Because endogenous carbohydrate stores (primarily in liver and muscle) are relatively small, endurance-type exercise performance/capacity is often limited by endogenous carbohydrate availability. Much exercise metabolism research to date has focused on muscle glycogen utilization, with little attention paid to the contribution of liver glycogen. (13)C magnetic resonance spectroscopy permits direct, noninvasive measurements of liver glycogen content and has increased understanding of the relevance of liver glycogen during exercise. In contrast to muscle, endurance-trained athletes do not exhibit elevated basal liver glycogen concentrations. However, there is evidence that liver glycogenolysis may be lower in endurance-trained athletes compared with untrained controls during moderate- to high-intensity exercise. Therefore, liver glycogen sparing in an endurance-trained state may account partly for training-induced performance/capacity adaptations during prolonged (>90 min) exercise. Ingestion of carbohydrate at a relatively high rate (>1.5 g/min) can prevent liver glycogen depletion during moderate-intensity exercise independent of the type of carbohydrate (e.g., glucose vs. sucrose) ingested. To minimize gastrointestinal discomfort, it is recommended to ingest specific combinations or types of carbohydrates (glucose plus fructose and/or sucrose). By coingesting glucose with either galactose or fructose, postexercise liver glycogen repletion rates can be doubled. There are currently no guidelines for carbohydrate ingestion to maximize liver glycogen repletion.

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Liver glycogen metabolism during and after prolonged endurance-type exercise

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