SECTION V 

 THE SIGNIFICANCE OF THE FOODSTUFFS 



IN order to keep an animal alive a certain amount of food must be supplied 

 to replace the loss to the body of its tissues and of its reserves of carbo- 

 hydrates and fat, which are being continually consumed for the provision of 

 the energy necessary to keep the animal warm and for the performance of 

 the internal and external movements of the body. The actual amount 

 of food required will depend on the size of the animal, on the temperature 

 of the surrounding medium and on the muscular work performed. For 

 supplying this energy any of the three classes of foodstuffs may be utilised, 

 the value of each being given by the Calories which are evolved when the 

 foodstuff is oxidised to the end stages which it attains in the body. Thus 

 carbohydrates and proteins are isodynamic i. e. give rise to the same amount 

 of energy in the body for a unit weight one gramme of each giving out 

 4*1 Calories. Weight for weight fat has double the value of either of these 

 two classes, its Calorie value being 9-3. This isodynamic equivalence of the 

 foodstuffs is however interfered with to a certain extent by the specific 

 dynamic action already spoken of, in virtue of which protein has an excitant 

 action on the processes of oxidation occurring in the body, so that an animal 

 fed on a largely protein diet will give out more heat and consume more food 

 than if it were fed on a diet of the same Calorie value but in which carbo- 

 hydrates preponderate. This specific dynamic action is not altogether 

 lacking to carbohydrates and fats. Thus, according to Rubner's experi- 

 ments, for every 100 Calories of protein ingested there is an increased 

 heat production over the fasting level of 30 to 35 Calories, while fat 

 gives an increased heat production of about 12 per cent., and sugar 

 of about 5 per cent. On account of this specific dynamic action an 

 average mixed diet, in order to maintain the body in a state of 

 equilibrium, must have a Calorie value of 13 to 14 per cent, higher than 

 the sum total of the Calories given off during fasting. Thus, supposing 

 a man while fasting and in bed gives off 1800 Calories a day, which would 

 be derived from using up the tissues of his body, he would under the same 

 circumstances give off about 2000 Calories if he received a minimum mixed 

 diet just sufficient to maintain his body weight constant. This specific 

 dynamic action of protein nii^ht be regarded as mere waste or ' luxus 

 consumption* since it results only in the production of heat and cannot 

 be utilised for the performance of muscular work. It must be remembered 

 however that a certain amount of heat production is necessary in order 



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