Fat people and thin people have different body temperature changes in water with the same temperature. People with thick subcutaneous fat have the ability to withstand low temperatures. Someone has done an experiment: in water with water temperature 18 degrees, a thin man with subcutaneous fat thickness of only 5.3 mm swims for 20 minutes, and the oral temperature will drop by 1.6 degrees; But for a fat person with subcutaneous fat thickness of 14 mm, the oral temperature will not drop, but will increase by 0.7 degrees. Frequent swimming can thicken subcutaneous fat, but this thickening is not obesity. Human obesity is mostly caused by endocrine, nutrition, heredity and other factors, and people who lack exercise are more likely to be obese. Thin people often take part in swimming, and their bodies are often stimulated by low temperature, which will consume more energy and material. So ... The need of exercise and his digestive function are contradictory, and there is a phenomenon of "demand exceeds supply". In order to balance supply and demand, he eats a lot at this time. The nutrition obtained from food exceeds the needs of the total body mass. Over time, the proportion of muscle in the whole body mass will become larger and larger, and the relative thickening of subcutaneous fat will make thin people gradually muscular.