The scientific study of "the sum of processes concerned in the growth, maintenance, and repair of the living body as a whole or of its constituent organs" [Graham Lusk, 1928]. These processes include ingestion and digestion of food, the conversion of food into chemical energy and other materials that the body can use or store, and excretion. The scope of the field also includes how these processes are different between organisms of different age, healthfulness, species, or environments.