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

Abstract

Glycogen supercompensation after 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 are 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 maximal oxygen consumption until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% confidence interval 16, 68] higher than after MIX in type I fibres (P < 0.001), with little to no difference in type II fibres (9% [95% confidence interval −9, 27]). Median particle diameter was 22.5 (interquartile range 20.8-24.7) nm across glycogen pools and fibre types, and 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 I 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. In comparison 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 I fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles.

Muscle Glycogen Assessment and Relationship with Body Hydration Status: A Narrative Review

“…Although the respective effect of glycogen size and amount on bound water content was not revealed, a complicated change in glycogen structure may make it difficult to predict the relationship between muscle glycogen and bound water. Notably, a recent study indicated that glycogen supercompensation in humans increases glycogen number rather than size [ 50 ]. Second, body water flow, which was not associated with glycogen, should be considered.…”

Differential utilisation of subcellular skeletal muscle glycogen pools: A comparative analysis between 1 and 15 minutes of maximal exercise

“…These inherent factors might influence how dietary CHO and energy consumption affect the glycogen storage pattern. In the previous mentioned study by Jensen et al (2021) particle sizes in skeletal muscle fibres are remarkable similar across isocaloric diets differing in CHO content, but a low CHO diet reduces the numerical density in all pools. Therefore, the second aim of the present study was to investigate the effect of lowered carbohydrate and energy intake and subsequent maximal exercise at two different intensities, on glycogen volumetric densities, particle size, numerical density, and localisation within skeletal muscle fibres.…”

Fueling the Female Athlete: Auditing Her Representation in Studies of Acute Carbohydrate Intake for Exercise

“…This suggests that female athletes may need to increase energy intake above their apparent (self-reported) or true habitual intake, both to achieve CHO targets and to allow an energy availability that will support an increase in muscle glycogen. Despite the continued investigation of CHO loading protocols, with topics such as the localization or structure of glycogen particles ( 30 ), these have been undertaken with male-only or mixed-sex cohorts. Indeed, no studies of CHO loading involving separate analysis of female participants have been published over the past 15 yr, despite the likelihood of sex-based differences in muscle fibers ( 10 ).…”