Lean body mass includes muscle tissue which tends to use up food energy. Long-distance athletes are the leanest people in town because sustained exertion causes muscle cells to use fat as the primary fuel. If you train long enough, most of your stored body fat is burned as fuel. It seems both ironic and fitting, then, that corpulence now poses a growing threat to the health of many inhabitants of the richest nations. 59 percent of the adult population meets the current definition of clinical obesity, according to a 1995 report by the Institute of Medicine, easily qualifying the disease for epidemic status.
Body fat is energy storage which acts like a savings account.
Food surplus tends to be saved with interest and stored as fat. People who remain fat have a frugal metabolism and it is difficult to withdraw and spend the savings. One pound of fat is worth at least one day's hard physical labor. Reduced food energy intake tends to induce energy conservation and body weight is maintained until severe food shortage results in weight loss. Energy conservation in overweight people has a significant behavioral component.