The immediate cause of obesity is the prolonged consumption of a diet containing more calories than are needed to provide for the body’s tissue repair, vital functions and physical activities. In modern society, food has become very plentiful and attractive, and the physical effort demanded by many occupations has diminished. Most people in civilized communities eat more than they require, and it is surprising that obesity is not more common than it is. It is difficult to escape the conclusion that there exists some unknown mechanism by which the body is enabled to get rid of the surplus calories which would otherwise be stored as fat. If there were not such a mechanism, obesity would be much more common.