Energy takes different forms. Our supply comes from the sun in the forms of heat and light, and plants store it up in the form of chemical energy when they build carbohydrates, fats, and proteins. This may be changed into the forms of work or of heat when we eat the food. Whenever an attempt is made to change chemical energy to work, some of it will
change to heat. So in our bodies, the fuel foods, which enable us to do both involuntary and voluntary work, furnish heat at the same time, to keep our bodies warm. When we are too cold, we can shiver or run or jump, and thus, by doing more work, get more heat too.
The unit of fuel value.—In our studies of food materials, we must find out just how much energy, or working power, can be obtained from each kind. We must have a measure of energy or fuel value; and just as the inch is a measure of length, and the pound of weight, so the Calorie serves as a measure of fuel value. This unit[2] measures energy as heat, being the amount of heat required to raise 1 kilogram of water 1° C. (or 1 pound of water about 4° F.), but we can express it also as work, being sufficient energy to raise a 1-pound weight 3087 feet into the air (or 1 ton about 11⁄2 feet) if it were possible to convert it into mechanical work without loss. By burning foods in pure oxygen in a vessel placed in water so that all the heat is given off to the water, and then noting the change in temperature of the water, it is possible to find out just how much energy each will yield. Such a device is called a calorimeter. In the body there is usually a small portion of each kind of foodstuff which escapes digestion, and protein is not quite so completely burned as in the calorimeter. When allowance for the probable loss is made, the energy values of the fuel foodstuffs are as follows:
| Protein | 4 Calories per gram or 1814 per pound. |
| Fat | 9 Calories per gram or 4082 per pound. |
| Carbohydrate | 4 Calories per gram or 1814 per pound. |
The standard portion.—Knowing the composition of any food material, it is possible from these figures to calculate
the total fuel value, or we can refer to tables in which this has been calculated, and save ourselves labor. For comparison of different foods the Standard, or 100-Calorie, Portion is used, as this corresponds very closely with the amount of food for a single serving in many cases. In the sections treating of different foods the Standard Portion will be stated.
Food for body building.—Every living cell has a little life history of its own, and constantly demands a certain amount of new material to replace old which it has worn out. Besides this, old cells die, and new ones have to be made to replace them. Hence even a full-grown person needs building material, and much more is required in proportion when the person is growing and perhaps adding several ounces a week to his weight. The foodstuffs which have especial value as building material are protein and mineral matter.
Food for body regulating.—To help in the digestion of food, to keep the blood in proper condition, the muscles supple, and all the processes of the body at their best, ash constituents and water in the diet are necessary. A tabular summary of the functions of the foodstuffs and an outline of the changes which take place in digestion will be found in the appendix.
We are now able to give a more complete answer to our question, “What is Food?”
Food has been said to be that which taken into the body builds tissue or yields energy, or both. The food as a whole must contain all the chemical elements needed by the body, these elements being supplied in substances known as foodstuffs, viz., protein, fat, carbohydrates, mineral matter or ash, water. To be a food, a substance must contain one or all of the foodstuffs.