328 PHYSIOLOGICAL CHEMISTRY 



portion of the proteids we absorb are derived from bread and meat. 

 The foods richest in proteids are meat, fish, eggs, cheese, milk, etc.; 

 in short, those foods having an animal origin. The older physiology 

 considered the material composition of the food as the essential char- 

 acteristic, although even Liebig recognized metabolism as a process 

 of combustion, and it is the work of the Voit school which has caused 

 the calorimetrical value of food to attain its present central position. 

 By its combustion, the nutriment absorbed supplies the energy which 

 is required by the human body for its various purposes. The value 

 of a food, then, can be expressed by the amount of energy it can pro- 

 duce, and this value can be stated clearly and accurately in the ordi- 

 nary terms of energy, i. e., in units of heat, or calories. As the result 

 of years of work by various investigators it has been found that the 

 individual foods can be almost completely represented by their 

 calorimetrical values. Rubner, Zuntz, and Atwater, by differing 

 methods, have all come to the same conclusion, viz. that for purposes 

 of heat and muscular action, i. e., for its principal requirements, 

 proteids, fats, and carbohydrates, the organism can employ vegetable 

 and animal foods equally well. That the civilized nations of Europe 

 and America employ bread and meat as the principal source, while 

 the Indians and Chinese use rice exclusively, and the Esquimos fat, 

 is not due to any difference in physiological organization, or to differ- 

 ing needs of the body, but simply to the more or less easy attainment 

 of the substances, fruitfulness of the soil, and other secondary cir- 

 cumstances. 



The law of the calorimetrical equivalency of all food-stuffs has but 

 one notable exception. So far as investigation has been carried out it 

 has been found that the dietary of any man or race always contains 

 a certain and apparently similar amount of proteinaceous material. 

 The kind of material seemingly has little influence, but about 100 

 gr. of protein is found with great constancy in the daily food of 

 the individual. In the food of a powerful man, who exerts a fairly 

 large amount of muscular effort, Voit found 118 gr. of proteids per 

 day, and he assumes this as a basis for the dietary of a soldier. 

 Weaker men, doing less muscular work, require, according to Voit, a 

 smaller quantity of proteids. For the poorly nourished, and also for 

 those who are incapable of any intense effort, the hand-loom weavers 

 of Zittau, the poor of Naples, and the poorest negroes of Alabama, 

 von Rechenberg, Manfredi, and Atwater have found much lower 

 amounts. During comparatively short laboratory experiments, 

 Munk, Hirschfeld, Kumagawa, and especially Siven have found con- 

 siderably smaller quantities. For well-nourished men, during long 

 periods, Chittenden, only, found less proteids; otherwise, physio- 

 logical investigation, as well as the experience of daily life, has shown 

 that it is not well to consume less than 100 gr. of proteids per day. 



