Elite swimmers typically undertake 4000-20,000 m per day in training, burning thousands of calories. However, the typical body fat levels of these athletes are significantly higher than runners or cyclists who expend similar or even smaller amounts of energy in their training. It has been suggested that swimming doesn't cause the appetite drop that accompanies heavy running and cycling training. Some research suggests that this is due to the cool temperatures in which swimmers train. By contrast, runners and cyclists usually experience an increase in body temperature during training, which may serve to suppress appetite - at least in the short term.
Swimmers are less active outside their training sessions.
They are so tired from the hours spent training that they sleep, sit or otherwise avoid any real energy expenditure outside their sessions. Two studies from Costill's Human Performance Laboratory at Ball State University have tried to address the energy balance oddity of swimmers. Ten athletes of each sex from each sport participated in the study. These records noted the amount of time each individual spent sleeping, sitting, walking, standing or training. The energy cost of these activities was estimated individually for each athlete by duplicating the activity in the laboratory and collecting oxygen consumption data.