Macro partitioning
Increasing carbohydrate availability (via glucose ingestion or high glycogen stores) reciprocally downregulates fat oxidation in skeletal muscle during exercise through insulin-mediated suppression of adipose lipolysis and direct inhibition of fatty acid transport proteins.
When you eat carbohydrates before or during exercise, your body prioritizes burning those carbs and burns less fat. This happens because insulin rises (blocking fat release from storage) and transport proteins for fat are inhibited. To maximize fat oxidation during exercise, you would need to exercise in a fasted state or avoid carbohydrate intake, but this comes at the cost of performance and glycogen sparing.
Several studies have demonstrated that increasing the breakdown of muscle glycogen content before exercise, and the availability of exogenous carbohydrate before and during dynamic exercise, increases carbohydrate oxidation and reciprocally decreases fat oxidation [14, 23–25]. The carbohydrate-induced increase in plasma insulin concentration exerts a powerful inhibitory effect on adipose tissue triacylglycerol lipase (ATGL) and hormone sensitive lipase (HSL), decreasing the circulating plasma FFA concentration. This translates into reduced delivery of FFAs to the muscles during exercise and a decrease in FFA uptake and oxidation.
Why this rating
Based on multiple human studies cited (refs 14, 23-25) using tracer techniques and metabolic measurements.
Source
New Insights into the Interaction of Carbohydrate and Fat Metabolism During Exercise
Lawrence L. Spriet · Sports Medicine · 2014
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