Post-exercise ketosis was first described in 1909 by Forssner, who found that his daily excretion of acetone in the urine was increased on days when he had undertaken a brisk walk in the morning. Athletes on unrestricted diets eat more, particularly carbohydrates, than non-athletes, and have lower post-exercise ketone body levels. They also have lower plasma glucagon, growth hormone, catecholamine, and free-fatty acid levels during sub-maximal exercise. On a standard 12 MJ/d diet, however, non-athletes have the lower post-exercise ketone body levels. The hormone response of athletes and non-athletes to exercise can similarly be reversed by dietary manipulation. The carbohydrate status of the body is, therefore, probably the main determinant of the type of blood hormone profile that develops during exercise, and the degree of ketosis that develops after exercise.