686 ANIMAL HEAT 



WTat has been already said in connection with standard dietaries 

 (p. 624) indicates that the work of the world might possibly be accom- 

 plished as well with a smaller transformation of energy in the human 

 machine, at least in the more prosperous countries, and that in the 

 body, as in an engine, more careful ' stoking ' might result in a saving 

 of fuel. It is extremely improbable, however, that any argument of 

 this sort will have much effect upon the deep-rooted dietetic habits of 

 mankind. 



In any case it must be carefully remembered that the question of 

 the minimum amount of protein necessary in a permanent diet is 

 quite distinct from the question of the minimum heat value of the diet 

 for a man of given body- weight doing a definite amount of work under 

 definite conditions. Whether the protein allowance be scanty or liberal, 

 the total heat value cannot be permanently reduced below a certain 

 minimum depending on the work done, the climate, and other conditions. 



Basal Metabolism. The total metabolism of a normal man or animal 

 as measured by the heat-production is so greatly affected by the taking 

 of food, by the kind as well as the quantity of the food, by the amount 

 and nature of the work done, even by the position of the body, that for 

 many purposes.it is necessary to simplify the conditions in order to 

 disentangle the various factors. The greatest degree of simplification 

 which is practicable is attained when the observations are made upon a 

 subject a considerable time (at least twelve, but better eighteen hours) 

 after the last meal, in the recumbent posture, and in a condition of rest 

 as absolute as possible. The metabolism under those conditions is 

 termed the basal metabolism. In 89 normal men, the basal metabolism, 

 indirectly calculated from the calorific value of the oxygen consumed, 

 was found to vary from 0-8 to 1-3 calories per kilo of body- weight, and 

 from 2 8 -9 to 39-0 calories per square metre of surface per hour (Benedict). 

 The average heat-production per square metre of surface was 34-7 

 calories per hour (or calculated on surface measurements made by newer 

 methods, 40 calories per hour). This agrees very closely with the 

 average obtained by direct determination of the heat with the ' bed ' 

 calorimeter (Du Bois). 



Specific Dynamic Action of the Food-Stuffs. The old idea that the 

 increase in the metabolism above the basal level which follows the inges- 

 tion of food is chiefly due to increased work of the alimentary canal 

 in digestion and absorption has been disproved. A large meal of meat 

 given to a dog may cause the heat- production to be nearly doubled. 

 This phenomenon is spoken of as the specific dynamic action of the 

 food-stuffs. It is especially well marked in the case of the proteins. 

 The best evidence is that the action is due to some kind of stimulation 

 of the protoplasm by decomposition products of the amino-acids; in 

 the case of alanin, for instance, probably lactic acid produced in the 

 process of deaminization. It is not merely the oxidation of the amino- 

 acids themselves which is mainly responsible for the increase in meta- 

 bolism. For it has been found that glycocoll and alanin when fed to 

 phlorhizinized animals, in which they do not yield energy by oxidation, 

 being excreted as sugar and urea, still exert the specific dynamic action, 

 and increase the heat-production (Lusk). 



The Seats of Heat-Production. We have already recognized the 

 skeletal muscles as important seats of heat-production. A frog's 

 muscle, contracting under the most favourable conditions, does not 



