Hormonal
During submaximal exercise, low muscle glycogen levels increase fat oxidation primarily through heightened AMPK activation, whereas high glycogen levels limit fat oxidation by sequestering free carnitine into acetylcarnitine, despite similar malonyl-CoA reduction in both conditions.
To maximize fat oxidation during exercise, be aware that high muscle glycogen can limit fat burning by consuming free carnitine. While low glycogen boosts fat oxidation via AMPK, high glycogen creates a bottleneck where carnitine is used to buffer acetyl-CoA instead of transporting fat into mitochondria. Moderate intensity exercise with moderate glycogen may offer a balance, but very high intensity with high glycogen may shift fuel use toward carbohydrates due to this carnitine limitation.
The data suggest that a decrease in the concentration of malonyl-CoA, secondary to beta2-AMPK activation and ACC inhibition (by phosphorylation), contributes to the increase in fat oxidation observed at the onset of exercise regardless of muscle glycogen levels. They also suggest that, with high muscle glycogen, the availability of free carnitine may limit fat oxidation during exercise, due to its increased use for acetylcarnitine formation.
Why this rating
Controlled human study with direct muscle biopsy measurements and controlled glucose infusion to isolate glycogen effects.
Source
Malonyl-CoA and carnitine in regulation of fat oxidation in human skeletal muscle during exercise
Carsten Roepstorff et al. · American Journal of Physiology-Endocrinology and Metabolism · 2004
DOI 10.1152/ajpendo.00379.2004
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