Beta-Oxidation

Medium-chain fatty acids bypass the carnitine shuttle and enter mitochondria directly, but this does not substitute for a functional carnitine shuttle in the broader diet. Insulin simultaneously stimulates fat storage and inhibits fat burning, so excess carbohydrate intake produces a double penalty of concurrent fat synthesis and blocked fat mobilization. At rest, 70–90% of the body's energy is derived from fat oxidation. Elevated lactate acutely suppresses fat burning by forcing a switch to glycolytic fueling, meaning exercising above Zone 2 reduces fat burned during and after the session. Approximately two-thirds of circulating fat in a person eating a high-carbohydrate diet is synthesized in the liver, not derived from dietary fat.