In aerobic respiration, fats and sugars, which are composed of carbon, hydrogen, and oxygen, react with oxygen from the lungs (carried by the blood) to form water and carbon dioxide. The result is a release of energy used to move muscles.
Reacting a carbon-hydrogen fuel with oxygen in this manner is essentially a combustion reaction, so the phrase “burning fat” is more literal than you might have thought. Though in this case the process is controlled by enzymes.
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