THE SIGNIFICANCE OF THE FOODSTUFFS 689 



to maintain the temperature of the body, and this must be greater the lower 

 the temperature of the surrounding medium. In cold surroundings a large 

 amount of the food must always be applied to maintain the body tem- 

 perature, and for this purpose the excess heat involved in the ingestion of 

 protein, and to a less extent of other foods, can be, and is, utilised. Hence 

 it comes about that in order to exhibit this specific dynamic action, the 

 animal must be kept at a temperature of about 33 Centigrade. Below this 

 temperature the specific dynamic action of the foodstuffs becomes less 

 and less apparent, and js finally merged in the heat production necessary, 

 whether food is taken or not, to keep up the temperature of the body. 



Even regarded simply as a source of energy, proteins are distinguished 

 from carbohydrates and fats by the fact that no means are provided for the 

 storage of proteins in the body other than by the growth of muscular and 

 other tissues. This growth is normal in the young animal, and in the adult 

 in convalescence from wasting diseases and during recovery from a period 

 of starvation or of insufficient feeding. There is a certain maximum of 

 growth beyond which the living tissues of the body cannot attain. Arrived 

 at this stage the body cannot add to its protein store, and all the protein 

 therefore that is ingested and absorbed from the alimentary canal is at 

 once broken down and burnt up in the body. If we examine the curve 

 of nitrogenous excretion in the urine we find that practically all the nitrogen 

 taken in as protein in the food is turned out within twelve to sixteen hours. 

 Thus, if a man is on a mixed diet sufficient for his daily needs, a doubling 

 of the protein in the diet, leaving the other constituents unchanged, will 

 result in a doubling of the nitrogenous excretion and a storing of the fats 

 and carbohydrates that would otherwise have been consumed, so that these 

 will be laid on as fat in the body. On the other hand, an increase of the fats 

 or carbohydrates will not increase the carbon metabolism of the body, 

 but any excess above the daily needs, if absorbed, is stored up in the body 

 in the form of fat. To increase the protein metabolism therefore, all that 

 is necessary is to increase the protein in the body ; to increase the carbo- 

 hydrate and fat metabolism it is necessary to increase the metabolism as a 

 whole either by increased muscular exercise or by exposure to cold. 



Protein is also distinct from fats and carbohydrates in that it is essential 

 for the repair of the wear and tear of the tissues. We may thus speak of 

 the protein of the food as having a twofold destiny. A certain proportion 

 is used to repair tissue waste; the arnino-acids into which it is resolved 

 in the intestines circulate in the blood, and each living cell picks out those 

 amino -acids which are essential to building up the protoplasm of the cell. 

 Protein with this destiny exerts no specific dynamic action, and this action 

 is therefore lacking in the infant, which is storing up in the form of new tissue 

 the greater proportion of the protein which it takes up in its food. The 

 rest of the protein, after resolution to amino-acids, loses its nitrogen, which 

 appears in the urine chiefly in the form of urea, and the remaining part 

 of the protein molecule then undergoes rapid oxidation. The nitrogen of 

 the urine is therefore derived partly direct from the nitrogen rapidly split 

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