Glycogen supercompensation is due to increased number, not size, of glycogen particles in human skeletal muscle.

NEW FINDINGS What is the central question of this study? Glycogen supercompensation following glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles is unknown. What is the main finding and its importance? Using transmission electron microscopy to visually inspect individual glycogen particles, we show that glycogen supercompensation is achieved by increasing the number of particles while keeping them at submaximal sizes. This may be a strategy to ensure that glycogen particles can be utilised fast as too large particles may impair utilisation rate. ABSTRACT Glycogen supercompensation following glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles is unknown. We investigated how a glycogen loading protocol affects fibre type-specific glycogen volume density, particle diameter and numerical density in three subcellular pools: between (intermyofibrillar) or within (intramyofibrillar) the myofibrils or beneath the sarcolemma (subsarcolemmal). Resting muscle biopsies from 11 physically active men were analysed using transmission electron microscopy after mixed (MIX), LOW, or HIGH carbohydrate consumption separated by glycogen lowering cycling at 75% of VO2 max until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% CI 16;68] higher than after MIX in type 1 fibres (P<0.001) with little to no difference in type 2 fibres (9% [-9;27]). Median particle diameter was 22.5 (IQR 20.8;24.7) nm across glycogen pools and fibre types, while the numerical density was 61% [25;107] and 40% [9;80] higher in the subsarcolemmal (P<0.001) and intermyofibrillar (P<0.01) pools of type 1 fibres, respectively, with little to no difference in the intramyofibrillar pool (3% [-20;32]). In LOW, total glycogen was in the range of 21-23% lower, relative to MIX, in both fibre types reflected in a 21-46% lower numerical density across pools. Compared to MIX, particle diameter was unaffected by other diets ([-1.4;1.3] nm). In conclusion, glycogen supercompensation after prolonged cycling is exclusive to type 1 fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles. This article is protected by copyright. All rights reserved.