It is not only necessary that the body should be supplied with food in order that its natural functions may be performed, but it is equally necelsary that the food supplied should consist of the proper materials. Each animal by instinct seeks those substances which best meet the needs of its own metabolism, and it is a matter of every-day experience that man endeavors to sup ply himself with food suitable to the conditions under which lie lives. and alters his diet with respect to season, latitude, age, activity, and occupation. Food is intended to supply the place of that which is given out by the body. But in the choice of diet this is not enough; the food should be sutlich-nt to meet such needs without waste, and without increasing unduly the out put of excreta, while the organism should be maintained in health. Careful analysis of the excreta shows that they are made up, besides water, chiefly of the chemical elements, carbon, hydrogen. oxygen, and nitrogen, but that they also contain, although in smaller amounts. sul phur, chlorine, potassium. and certain other ele ments. To balance this waste it is evident that food must he supplied containing all these ele ments in something like the proportion in which they are exereted; that is. a 'mixed diet' is neces
sary. Experiments upon the lower animals have shown that a diet composed exclusively of one class of food, such as fat, or sugar, results in the death of the animal after a longer or shorter period of time. See DIET.
The sa jeCt of nutrition is exhaustless. it comprehends all vital phenomena. for 11011C of the functions of life are performed without involving replacement by living matter. and therefore nu trition. The subject of hygiene in all its aspects is connected with it, whether in eating, drinking, exercise. sleeping, or breathing.. A change in each of these processes involves a corresponding ellange in the elaboration and appropriation of new material, and the disassimilation or elimi nation of old. or its revonversion.
Consult the chapters on "Nutrition and Diet," in Eirke. Handbook of Physiology (Philadelphia, 1901:1 and Vieder. Textbooks of Plqmiology (New York, 1 900 ) IZEtiPIRATION: SECRETION.