In general, body fat consumption begins after 30-40 minutes of fasting and appropriate intensity of exercise. If the body glycogen is not consumed, fat may not be consumed for a long time. Under normal circumstances, the order of energy metabolism in the body is carbohydrate, fat and protein. Sugar is the main energy supplier of the body and is the first to be consumed to replenish the body’s energy; when the body’s blood sugar drops, the body will mobilize fat to replenish blood sugar or generate ketone bodies for the body’s energy supply; the last to be consumed is protein. After 30-40 minutes of fasting, blood glucose level starts to fall, and then fat starts to be consumed to maintain blood glucose level. However, in fact, the metabolism of sugar, fat and protein is mixed, and the consumption of the three is accompanied by a certain period of time and the consumption of a certain substance dominates. Aerobic exercise after 30-40 minutes of fasting is good for weight loss, but long-term hunger is not good for health.