Skeletal Muscle Glycogen Content at Rest and During Endurance Exercise in Humans: A Meta-Analysis.
Level 1 - systematic review of randomized trials
Meta-analysis of 181 human biopsy studies
PubMed 29923148 · doi:10.1007/s40279-018-0941-1
What was done
PubMed was searched through February 2018 for English-language human studies reporting skeletal muscle glycogen concentrations at rest and during continuous or intermittent cycling and running measured via biochemical biopsy analysis. Data from 181 studies were analyzed using mixed-effects meta-regression models with random effects for between- and within-study variance. Models assessed resting glycogen and exercise-induced glycogen utilization across fixed effects including fitness status (maximal oxygen uptake, VO2max), acute carbohydrate (CHO) availability (low, normal, high), exercise duration, exercise intensity (%VO2max), exercise mode, CHO ingestion during exercise, sex, and muscle group.
What was found
Resting vastus lateralis glycogen in males with normal CHO availability and average fitness (VO2max 53 ± 8 mL·kg⁻¹·min⁻¹) was 462 ± 132 mmol·kg⁻¹ dry mass (DM). Compared to normal CHO availability, high CHO increased resting glycogen (102 ± 47 mmol·kg⁻¹ DM; mean ± 90% confidence limits), while low CHO availability reduced it substantially (-253 ± 30 mmol·kg⁻¹ DM). Each 10 mL·kg⁻¹·min⁻¹ higher VO2max increased resting glycogen by 29 ± 44 (low CHO), 67 ± 15 (normal CHO), and 80 ± 40 mmol·kg⁻¹ DM (high CHO). During exercise, an increase in intensity of 30% VO2max increased glycogen utilization by 41 ± 20 mmol·kg⁻¹ DM at 5 min and by 87–134 mmol·kg⁻¹ DM at all subsequent timepoints. An increase in resting glycogen of 200 mmol·kg⁻¹ DM increased utilization by 28–59 mmol·kg⁻¹ DM at 5–23 min, 104 ± 15 mmol·kg⁻¹ DM at 116 min, and 143 ± 33 mmol·kg⁻¹ DM at fatigue. CHO ingestion during exercise, exercise mode (continuous vs. intermittent), and VO2max had trivial effects on glycogen utilization rates. Small reductions in glycogen utilization were noted in females compared to males (-30 ± 29 mmol·kg⁻¹ DM) and in running compared to cycling (-70 ± 32 mmol·kg⁻¹ DM).
Why it matters
This meta-analysis provides normative reference values for resting human muscle glycogen and quantifies how dietary carbohydrate, aerobic fitness, exercise intensity, and initial glycogen stores dictate glycogen use during endurance exercise.
Limits
The search was restricted to English-language studies indexed in PubMed. The abstract does not report the total number of human participants. The meta-regression relied on study-level aggregate data rather than individual participant data, and data were largely derived from the vastus lateralis during cycling and running, limiting direct generalization to other muscle groups or exercise modalities.
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