in its broadest signification includes all those substances which when taken into the body build tissues, restore waste, furnish heat and energy, and provide appropriate condiments. The building of tissues is especially an important function during the early life of animals as it is through this building of tissues that growth takes place. The restoration of waste of tissues assumes special importance during that period of life when the weight of the body is supposed to be reasonably constant. At this time the waste of tissue in the natural processes is restored by the assimilation of new material in the same proportion.
If the assimilation of new material goes on at a greater rate than the waste of old material it manifests itself during the period of expected equilibrium in the deposition of adipose tissue and a consequent abnormal increase in weight.
In the after period of life the process of waste is naturally more vigorous than that of assimilation, and the tendency is manifested, which is wholly in harmony with the laws of Nature, to gradually diminish the weight of the body, and this continues to the extreme emaciation of old age.
It is evident, therefore, that the food consumed should be adapted to these changing periods. The growing animal needs a larger quantity of food in proportion to its actual weight than the animal which is in a state of equilibrium, that is, of mature age, and the animal which is entering upon the period of old age needs a less quantity of food in proportion to its weight than in either of the other periods of life. Thus, the rations of infants and children should be generous, the rations of mature man sufficient, and the rations of old age limited.
The food should also contain the various elements which enter into nutrition in the proper quantity. The nitrogenous constituents in food, when subjected to the ordinary process of digestion, yield a certain quantity of heat and energy but their more important function is to nourish the nitrogenous elements, of the body of which the muscles, hair, skin, and finger-*nails are types. The mineral constituents of food, especially phosphorus and lime, have a general utility in promoting the metabolic functions, especially in the movement of the fluids of the body through the cells walls, and at the same time are actual nourishing materials, entering particularly into the composition of the bones and teeth.
The fats and oils which are present in the foods have the capacity of producing large quantities of heat and energy during their combustion in the body, and thus serve as a source of animal heat and muscular activity.
The starches and sugars which are the most abundant elements of our food, although they have a heat-forming power of less than one-half that of fats, are largely utilized in the production of heat and energy and in the formation of animal fat.
