Apart from water, the body’s first requirement under all conditions is for an energy source to perform physical and chemical work. ‘Energy nutrition’ explains that the metabolic fuels to provide this energy are derived from fats, carbohydrates, protein, and alcohol in the diet. The constituents of a meal provide these fuels directly for a few hours. Simultaneously, reserves of fat and carbohydrate are laid down for use during fasting between meals. Only about one-third of the average person’s energy expenditure is for voluntary activity; two-thirds is required for maintenance of the body’s functions, metabolic integrity, and homeostasis of the internal environment. Energy expenditure, energy balance, and physical activity are all discussed.