Research
Macro partitioning
During prolonged low-intensity exercise, the relative contribution of free fatty acids (FFA) to total oxygen metabolism in working muscles increases progressively, eventually exceeding the contribution of glucose.
For long, slow workouts, your body naturally shifts to burning more fat as time goes on. You don't need to force-feed yourself carbohydrates during these sessions; your body will access stored fat efficiently.
GoodSupportsHIGH confidence
As exercise was continued beyond 40 min, the relative contribution of FFA to total oxygen metabolism rose progressively to 62%. In contrast, the contribution from glucose fell from 40% to 30% between 90 and 240 min.
Why this rating
Direct measurement of leg exchange of FFA and glucose using isotopic tracers.
Source
Substrate Turnover during Prolonged Exercise in Man
G. Ahlborg et al. · Journal of Clinical Investigation · 1974
crossover · n=6Cited 786×
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