Research
Hormonal
During prolonged low-intensity exercise (4 hours at 30% VO2max), blood glucose levels fall because hepatic glucose output fails to keep pace with augmented glucose utilization by working muscles, leading to significant liver glycogen depletion.
If you exercise for long durations (over an hour) at a moderate pace, expect your blood sugar to drop as your body depletes liver glycogen. This is a normal metabolic shift, not a failure. Ensure you are well-fed beforehand, but be aware that your body will increasingly rely on fat and gluconeogenesis as glycogen runs low.
GoodSupportsHIGH confidence
It is concluded that during prolonged exercise at a low work intensity (a) blood glucose levels fall because hepatic glucose output fails to keep up with augmented glucose utilization by the exercising legs; ... Total estimated splanchnic glucose output was 75 g in 4 h, sufficient to deplete approximately 75% of liver glycogen stores.
Why this rating
Direct measurement of splanchnic and leg exchange in healthy subjects using isotopic tracers.
Source
Substrate Turnover during Prolonged Exercise in Man
G. Ahlborg et al. · Journal of Clinical Investigation · 1974
DOI 10.1172/jci107645
crossover · n=6Cited 786×
Read the paper DOI resolved against Crossref · corpus check 2026-06-10
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