(1) Chemical

The object of nutrition is to balance the waste and repair of the body. If, therefore, we can estimate the precise amount of waste matter, calculated as nitrogen, carbon, and oxygen, it is possible to draw up a table of the dietetic requirements of the body for its maintenance in a condition of nutritive equilibrium. For all practical purposes it is sufficient to consider only the nitrogen and carbon of the food and excreta, and a balance-sheet of profit and loss may be constructed on these data. A condition of nitrogen equilibrium is established ideally when as much nitrogen is excreted as is ingested, and, as we shall see later, this is possible with very different quantities of nitrogen. Practically speaking, however, it is usual to regard nitrogen equilibrium as established when the nitrogen intake and output only vary to the extent of a few decigrams. It is usual to estimate only the nitrogen output by faeces and urine, and this would account for a difference of a few decigrams, owing to the loss in the sweat and from the degeneration of epithelial cells, as well as the small amount absorbed by hair and nail growth. The body is said to be in "physiological equilibrium" when on a given diet it neither loses nor gains in weight, and this depends upon the fact that the food intake, as regulated by the appetite, corresponds in the long run to the bodily requirements.

(2) Dynamical

This method of computation, which is now almost universally employed, consists in calculating the amount of potential energy contained in the food, and seeing that it corresponds with the amount of external work transacted, and the amount of heat evolved, for maintaining constant the temperature of the body and warming the breath and other excreta. Each of the alimentary principles, when completely oxidised by being burned in oxygen, gives out an ascertainable quantity of heat. This is known as the thermal or caloric value of the food, and it can be stated per gram of the dry food. The unit employed - called the (large) calorie - is the amount of heat required to raise a kilogram of water through 1° Centigrade. The experiment is conducted in an instrument termed a "bomb" calorimeter, and it is found that -

One gram (or nearly 1/30 of an ounce of dry protein) = 4.1 Calories.

" ( " " fat) = 9.3 "

" ( " " carbohydrate) = 4.1 "

One ounce of dry protein, dry starch, or sugar, produces 116 calories of heat, whilst one ounce of pure fat produces 263 calories, almost two and a quarter times as much as either protein or carbohydrate.

From these data it is quite easy to ascertain a factor by the use of which the calorific value of any food may be determined when its percentage composition is known. When this is stated in grams the calculation is extremely simple and no factor is required. It is only essential when the percentage composition is expressed in ounces. For protein and carbohydrates the factor is 1.16, and for fats it is 2.63. All that is necessary is to multiply the percentage value of any given constituent of the diet by the appropriate factor, and thus estimate the number of calories per ounce of each food principle. For example, the percentage composition of cow's milk is 3.5 of protein, 37 of fat, and 49 of lactose (carbohydrate).

or about 400 calories per pint.

In a similar manner bread will be found to possess 9.3 calories protein, 3.7 calories fat, 63.4 calories carbohydrate, or a total of 76.4 calories per ounce. Butter contains 1.2 calories protein, 226.6 calories fat, and no carbohydrate, a total of 227.8 calories per ounce; moderately fat beef will total 25 calories protein and 32 calories fat per ounce, equal to 812 calories per pound. Conversely, to determine the percentage composition of a food when its caloric value is known, divide its caloric value by the appropriate factor.