The ordinary food material, such as breads, grains, eggs, vegetables, fruit, etc., consists of, first, the refuse, such as the shells of eggs and bran of wheat; and second, the edible portion, as the yolk of the egg and the flour of wheat.

In the following table only the edible portion is considered. This consists of the non-nutrients and nutrients. Water, refuse, and the salt of salted meats are called non-nutrients; while protein, fats, carbohydrates, and ash (mineral matter) are the nutrients:-

Protein. - 1. Albuminoids: albumen (white of egg; casein (curd of milk); myosin, the basis of muscle (lean meat); gluten of wheat, etc. 2. Gelatinoids : as collagen of tendons, ossein of bones which yield gelatin, or glue.

Fats. - Fats of meat, butter, nuts, corn, olive, etc.

Carbohydrates. - Sugar, starch, cellulose (woody fiber).

Ash, or Mineral Matter. - Calcium phosphate, or phosphate of lime; sodium chloride (common salt).

Fuel Value. - Fuel value represents the number of calories of heat which one pound of a given material would yield upon combustion, allowance being made for the nitrogenous products of metabolism of protein compounds, which are not consumed in the body. The fuel value, or potential energy, is estimated in calories. The calory is the heat which would raise a kilogram of water one degree centigrade (or one pound of water about four degrees Fahrenheit). A foot-ton is the energy (power) which would lift one ton one foot. One calory corresponds to 1.53 foot-tons. A gram of protein or carbohydrates is assumed to yield 4.1 calories, and a gram of fats 9.3. A pound of fats is equivalent in fuel value, on the average, to a little over two pounds of protein or carbohydrates. The figures for fuel value (potential energy) in the table is calculated by multiplying the number of grains of protein and of carbohydrates in one pound (1 lb. = 453.6 grains) by 4. 1, and the number of grains of fat by 9.3, and taking the sum of these three products as the number of calories of fuel value in a pound of the material.

The average composition of these compounds is about as follows : -

Protein contains 53 per cent. carbon, 7 per cent. hydrogen, 24 per cent. oxygen, 16 per cent. nitrogen.

Fats contain 76.5 per cent. carbon, 12 per cent. hydrogen, 11.5 per cent. oxygen, no nitrogen.

Carbohydrates contain 44 per cent. carbon, 6 per cent. hydrogen, 50 per cent. oxygen, no nitrogen.

It is to be noted that the fats and carbohydrates contain no nitrogen, while the protein does.

The albuminoids and gelatinoids are frequently classed together as proteids. The term "proteids" is also used to include all the nitrogenous ingredients; i. e., synonomous with protein.