818 NUTRITION AND HEAT REGULATION. 



derived from a metabolism of the living muscular tissue, but no 

 entirely satisfactory demonstration of the correctness of these 

 views has been obtained (see p. 780). There is evidence to show 

 that increased muscular activity may be associated with an increased 

 formation of creatin. 



The Specific Dynamic Action of Proteins. This somewhat 

 indefinite term is used by Rubner to designate the fact that protein 

 foods seem to stimulate the metabolic processes of the body to a 

 greater extent than the fats or carbohydrates. This peculiarity 

 may be demonstrated, for instance, in the case of a starving animal 

 living upon its own substance and metabolizing a certain amount 

 of its tissues daily. The amount thus metabolized may be expressed 

 in terms of the heat production and it represents the minimal con- 

 sumption of material requisite for the maintenance of body-tempera- 

 ture. If this minimal daily consumption is expressed in calories 

 and the animal is given food containing the same amount of avail- 

 able energy the consumption of body material is not completely 

 covered, since the food leads to an increased total metabolism 

 which is especially marked in the case of proteins. Expressing the 

 minimal daily metabolism during starvation by 100, Rubner esti- 

 mates that the minimal amounts of the three foodstuffs requisite 

 to give a heat equilibrium would be for proteins 140.2, for fat 114.5, 

 and for sugar 106.4. The fact that protein food tends to increase 

 the total metabolism of the body has been explained by some 

 authors on the assumption that a greater amount of secreting and 

 muscular work is required for its digestion. The proteins are 

 retained for a longer time, for instance, in the stomach, and the 

 continued muscular movements of the organ involve, of course, a 

 consumption of tissue material. This explanation does not seem 

 to be entirely satisfactory and the term "specific dynamic action 1 ' 

 expresses the view that the protein food actually stimulates the 

 tissues to a greater metabolism, in somewhat the same way as in 

 the case of increased muscular work. On the general basis of the 

 theory of protein metabolism given above, its specific dynamic 

 action would mean simply that the body stores protein with rela- 

 tive difficulty, either as so-called circulating protein or in the form 

 of glycogen or fat, so that in cases of abundant protein feeding a 

 greater portion of the material is promptly consumed. With abun- 

 dant feeding of fats or carbohydrates, on the contrary, a larger por- 

 tion of these substances is retained as storage in the form of body 

 fat or glycogen.* 



Nutritive Value of Albuminoids. The albuminoid most fre- 

 quently occurring in food is gelatin. It is derived from collagen 



* For further discussion see Rubner, " Gesetze des Energieverbrauchs, 1902, 

 or Lusk, "Elements of the Science of Nutrition, Philadelphia, 1906. 



