Dietetics comprise the study of nutrition in health and disease and of the substances serving for this purpose (the diet). All living organisms derive their nourishment from the vegetable kingdom, either directly, or indirectly by living upon animals which in turn live upon a vegetable diet. Foods are substances which are required for the nutrition and maintenance of the body; they replace its wastes and losses.

In studying the normal nutrition of man we perceive quickly that there is a great variety in the food of healthy persons with regard to the quantity as well as to the different food substances. Nevertheless, they all contain the three groups of food-stuffs: Albumin, carbohydrates, and fats. Thus, for instance, vegetarians live and thrive principally on vegetables; the Esquimaux, on the other hand, almost exclusively on animal diet. The golden path, however, lies intermediate, and all authors (Voit, Pettenkofer, Hoffmann, Forster, and Gruber) recommend a combination of animal and vegetable food. E. Virchow likewise is of the same opinion, and expresses himself regarding this question as follows: "Although the Kirghez and the Esquimaux show us that health and life can exist through many generations on an exclusively nitrogenous diet - other tribes (Hindoos) live principally on non-nitrogenous food - still history shows us that the highest attain-8 ments of the human race have emanated from nations who have lived and live on mixed diet." A mixed diet, taken partly from the vegetable and partly from the animal kingdom, is the most suitable form of nourishment.

We obtain the greatest amount of carbohydrates from the vegetable kingdom, while a great deal of the albumin is derived from animal food. The relation between animal and plant albumin, according to Munk and Uffelmann,1 should not be less than three to seven. As regards the quantity of food, according to the same authors, an adult doing a medium amount of work requires daily 118 gm. albumin, 56 gm. fat, and 500 gm. carbohydrates.

Food only in small portions serves the purpose of reconstructing tissue waste; in its largest part, however, it is used for generating the heat requisite for the maintenance of life. For that reason it is customary to speak of the necessary amount of heat units during twenty-four hours instead of the quantity of food. By "heat unit" is meant, as is well known, that quantity of heat which is required to raise the temperature of 1 gm. of water 1° C. "Great heat unit" means the amount of heat required for warming 1,000 gm. of water 1° C. Each kind of food is ultimately oxidized in the body to its end products, and is in greatest part exhaled in the form of carbonic acid; the more carbon atoms a food-stuff contains the more heat units it will generate. In speaking of the heat value of food, the great heat units are used, the term "great," however, being omitted. Thus 1 gm. of albumin generates 4.1, 1 gm. of fat 9.3, and 1 gm. of carbohydrate 4.1 heat unite. If we know the quantity of nourishment taken, the amount of the introduced heat units is easily determined by multiplying the different food-stuffs by the above-given figures.

The daily amount of heat generated by the body, or necessary for the maintenance of the same, has been approximately estimated at twenty-five hundred heat units.1 The heat value of the food taken by an average working person amounts, according to von Noorden,2 to about forty heat units when working, and when resting to about thirty-four heat units per kilogram a day.

1 Munk und Uffelmann: " Die Ern&hrung des gesunden und kranken Menschen," Wien, 1887.