One that feels full of life and is active has energy. The work of the body is done by its energy. Even a well-formed strong body could not work long if food brought no energy-supply. The body would use itself for the energy required in living. Hence the need of heat-energy foods for body-heat and activity, and building foods for tissue-growth and repair. It is thus that the body is aided by food in its living and working and not hindered by unnecessary waste or work.

Energy foods form the largest proportion of daily food, but enter into the body-composition in the least, for they are consumed for current body-heat and activity.

It is activity of body which energy foods (carbohydrates and fats) promote. These provide for the active living of the body itself and its action in the work one does.

Starch, as in starchy vegetables (potato, rice), grains - carbohydrates. Sugar, as in sugar mixtures (cake, candy), fruits (dates) - carbohydrates. Fat (and oil), as in meats, butter, cream, olive and other edible oils - fats.

Starch requires prolonged cooking and longer digestion than sugar or fat. The delayed digestion of starchy food enables it to provide energy longer after eating than sugar and fat energy foods. Starchy food gives endurance in activity. But were it only used for energy, the supply of energy would be too delayed and the digestive tract overworked in securing from starch alone all energy needed. It might also be overburdened with vegetable fiber, with which starch is usually combined in foods.

Sugar and fat are therefore also necessary heat-energy foods. But used alone they would require too continuous eating to sustain energy, because used so quickly. They are not usually as digestible when constantly eaten in appreciable quantities as is the starch in potato and bread. Sugar and starch may store fat in the body. Fat in food probably does not.