Foods are composed of a great many chemical elements, as nitrogen, carbon, Hydrogen, oxygen, sulphur, phosphorus, calcium, sodium, potassium. These so unite as to form the very complex food-constituents, protein, carbohydrates, fats, and the simpler mineral salts and water.

As it is through the oxidation of food that it comes into use in the body, the fuel value - that is, the amount of heat produced as the food is oxidized - has been determined for all common foods. The amount of heat that foods yield as they unite with oxygen is measured in heat units called calories. A calorie is the quantity of heat which will raise 1 pint of water 40 F (or 1 liter 1° C). Calculation of fuel value, p. 223.

Adults need from their food 2000 - to 3000 + calories a day according to their age, sex, size, work (see p. 223). A man at very hard work needs food that will yield heat enough daily to raise 1/7 bbl. of water from freezing to boiling, or heat enough in a week to convert 1 bbl. (63 gal.) from ice to steam.

Fuel Value Common Foods

An inactive person weighing 150 pounds needs daily 1800 + calories to repair tissues, supply energy, maintain body temperature.