β-oxidation

β-oxidation is the main way your mitochondria burn fat for energy. It chops a fatty acid into two-carbon pieces (acetyl-CoA), one cycle at a time. Each cycle runs four steps: oxidation, hydration, another oxidation, and a final cut. Each cycle of an even-chain fat also yields one FADH2 and one NADH. Those feed the electron transport chain. That is why fat is such an energy-dense fuel per gram. Very-long-chain and branched-chain fats need a first round in the peroxisome before they can enter the mitochondria. During fasting, long aerobic exercise, or a ketogenic state, β-oxidation ramps up. It then supplies acetyl-CoA for making ketones and powers your muscles. When it falters, as in fat-oxidation disorders and with the mitochondrial decline of aging, fat builds up where it should not. That drives insulin resistance and metabolic inflexibility.

Last reviewed: June 22, 2026

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