At low speeds, the body burns fat; at high speeds, it burns sugar. Thus, running too fast causes the body to rely solely on sugar, preventing weight loss. Conversely, running too slowly allows continuous fat burning but at a limited pace, meaning even prolonged sessions may yield minimal weight loss results.
We can conclude that for exercise-based weight loss, moderation is key—excessive intensity typically fails to burn fat effectively.During intense exercise, the body easily shifts into anaerobic mode. In anaerobic exercise, muscle glycogen undergoes anaerobic glycolysis, producing lactic acid. Most of this lactic acid is broken down in the liver into carbon dioxide and water, while some is converted into liver glycogen. A small portion is metabolized and resynthesized into fat. This explains why vigorous exercise can sometimes paradoxically increase fat accumulation in the body.