GENERAL METHODS HISTORY OF PROTEIN FOOD. 905 



is not as a source of energy and probably not as direct building- 

 stones. Just how they influence metabolism cannot yet be stated. 

 It is possible that there may be substances in our foods other than 

 the vitamines, as above described, which are essential in some way to 

 metabolism. This possibility is indicated at least by a series of 

 observations reported by several observers, especially by Osborne 

 and Mendel.* They found that young rats fed on a diet in which 

 the protein was furnished by purified casein, the additional energy 

 material by starch and lard, and the inorganic salts by milk from 

 which the protein had been removed (protein-free milk), grew 

 remarkably well for a period of many days, but subsequently 

 showed a sudden loss of weight. If the diet was changed to one 

 consisting of dried whole milk together with starch and lard, normal 

 growth was resumed, and a similar result was obtained if purified 

 butter-fat, free from nitrogen and phosphorus, was added to the 

 former diet. They interpret these experiments to mean that the 

 butter-fat supplies some essential non-nitrogenous material not 

 contained in the lard-fat. A similar material is contained in the 

 fat of the egg-yolk or of the liver (cod-liver oil). 



The Specific Dynamic Action of Proteins. This somewhat 

 indefinite term is used by Rubner to designate the fact that pro- 

 tein foods seem to increase the metabolic processes of the body 

 to a greater extent than the fats or carbohydrates. This pecu- 

 liarity may be demonstrated, for instance, in the case of an animal 

 that has been starved (eighteen hours) until the gastro-intestinal 

 tract is free from food, f If the heat- production of such an ani- 

 mal is determined at hourly periods, it gives an index of what may 

 be called its basal metabolism when living on the material within 

 its body. If in this condition the animal is fed with carbohydrate 

 (glucose) there will be an increase in heat production, lasting for 

 three or four hours, which may amount to as much as 30 to 40 

 per cent. Feeding with meat or with some of the amino-acids 

 (glycin, alanin) causes a similar but more marked increase in 

 metabolism. After a large diet of meat the increase may amount 

 to as much as 90 per cent. This effect upon the metabolism is 

 exhibited especially by proteins. Many explanations of it have 

 been proposed, but recent experiments indicate that it is probably 

 due to some direct stimulating action of the intermediary acids, 

 such as lactic or pyruvic acid, which are formed during the further 

 metabolism of the amino-acids in the body. This stimulating 

 effect of proteins may furnish a physiological explanation of the 

 fact that in hot summer weather it is advisable to avoid a meat diet. 



* Osborne and Mendel, "Journal of Biological Chemistry," 15, 311, 1913; 

 16, 423, 1913. 



t See Lusk, "Journal of the American Medical Association," September 5, 

 1914. 



