366 



AX AMERICA X TEXT-BOOK OE PHYSIOLOGY 



has been successfully performed by Rubner; 1 his experiments were made with 

 the greatest accuracy and with careful attention to all the possible sources of 

 error, and it was found that the quantities of heat as determined by the two 

 methods agreed to within less than 0.5 per cent. These experiments are note- 

 worthy because they furnish us with the first successful experimental demon- 

 stration of the accuracy of the general principles, stated above, upon which 

 the available energy of foods is calculated. 



Dietetics. — The subject of the proper nourishment of individuals or col- 

 lections of individuals — armies, inmates of hospitals, asylums, prisons, etc. — 

 is treated usually in books upon hygiene, to which the reader is referred for 

 practical details. The general principles of dieting have been obtained, how- 

 ever, from experimental work upon the nutrition of animals. These principles 

 have been stated more or less completely in the foregoing pages, but some 

 additional facts of importance may be referred to conveniently at this point. 

 In a healthy adult who has attained his maximum weight and size the main 

 object of a diet is to furnish sufficient nitrogenous and non-nitrogenous food- 

 stuffs, together with salts and water, to maintain the body in equilibrium — 

 that is, to prevent loss of proteid tissue, fat, etc. In speaking of the nutritive 

 value of the food-stuffs it was shown that in carnivora (dogs) this condition 

 of equilibrium may be maintained upon proteid food alone, putting aside all 

 consideration of salts and water, or upon proteids and fats, or upon proteids and 

 carbohydrates, or upon proteids, fats, and carbohydrates. When proteids alone 

 are used, the quantity must be increased far above that necessary in the case of 

 a mixed diet, and it is doubtful whether, in the case of man or the herbivora, 

 a healthy nutritive condition could be maintained long upon such a diet, owing 

 to the largely increased demand upon the power of the alimentary canal to 

 digest and absorb proteids, to the greater labor thrown on the kidneys, etc. 

 The experience of mankind, as well as the results of experimental investiga- 

 tion, shows that the healthy diet is one composed of proteids, fats, and carbo- 

 hydrates. The proportion in which the fats and the carbohydrates should be 

 taken — and, to a certain extent, this is true also of the proteids — may be 

 varied within comparatively wide limits, in accordance with the law of " iso- 

 dynamic equivalents/' provided that the total amount of potential energy repre- 

 sented in the food does not fall below a certain amount, on the average about 

 10,000 calories per kilo, of body weight. This is illustrated by the fol- 

 lowing "average diets" calculated by different physiologists to indicate 

 the average amount of food-stuffs required by an adult man under normal 

 conditions of Life : 



1 ZriUrhnjt Jiir Jlmh./ie, 1893, Bd. XXX. S. 73. 



