Long sustained exercise burns fat. Short bursts of activity burn sugar derived from your food, and leave the fat stores intact. The cyclist who trains 2-3 hours a day with sustained exertion becomes the leanest person in town, by slowly, progressively burning up fat stores. Human Energy Human action is an expression of biological energy derived from food. Food-derived energy allows us to move, to do work by muscle contraction, and to keep warm.
Body heat is generated by the metabolic activity of every cell.
Nutrient molecules supply the fuel for our metabolic processes. Carbohydrates and fats are the principle sources of energy, although amino acids may be utilized as energy. The amount of energy in food is expressed as calories. Not all food calories are metabolically equivalent. The energy requirement of any individual is determined by physical activity.