ENERGY RELATIONSHIPS. 351 



as a member of human society in such a state would be negligible. 

 It is therefore only as the cellular activity increases that we find him 

 becoming more and more of service to humanity, and not until he is 

 erect and ready to perform active external muscular work is he in a 

 condition to live on a basal plane that is of practical value. 



The ingestion of food with its attendant increase in metabolism 

 appears at first thought like a highly inefficient process, this increase 

 being comparable to the extra energy required by a donkey engine to 

 stoke the boilers in a large factory ; so far as the direct mechanical out- 

 put of the factory is concerned, the energy thus used appears as waste, 

 and yet it is necessary in order to secure a supply of fuel to the boilers. 

 The increment in the metabolism or excess energy given off by the body 

 as a result of the ingestion of food may be considered as the energy re- 

 quired for the preparation of material for use in the body tissues, and 

 on this basis may be regarded as waste energy. Indeed, it is the belief 

 of some writers that heat is invariably a waste product and that this 

 factor has interest only in that it is developed in connection with mus- 

 cular or glandular activity. Another phase of the situation appears, 

 however, when we consider that the extra heat developed under these 

 conditions may possibly be looked upon as a normal physiological stim- 

 ulus to cellular activity. In this connection the practical experience 

 of many investigators may be mentioned, especially those making ob- 

 servations with severe muscular work in studies with a protein diet and, 

 in many cases, with a carbohydrate diet, such as sugar or sweet choco- 

 late. If it be true that the increase in the metabolism resulting from 

 the ingestion of such diets has a specific influence in stimulating the 

 whole cellular system of the body to greater activity, then we may not 

 properly regard this excess heat as a waste product. 



Continuing the discussion in the terms of the efficiency engineer, it 

 may be possible to consider the increase in heat production due to food 

 as a measure of the " cost of digestion." For instance, the ingestion of 

 1,000 calories of food in the form of sugar requires the excess production 

 of 60 calories of heat in order to have the sugar ready for an actual 

 share in the muscular work. On this basis, one might compute that 

 this excess heat was lost and that when 1,000 calories in the form of cane 

 sugar are transformed into material ready for combustion in the body 

 only 940 calories are available for such use. If, then, the increments in 

 heat production obtained in our various experiments are computed and 

 compared with the fuel value of the food ingested, .the proportion of the 

 energy in the ingested food which was given off as excess heat may be 

 determined. One great difficulty in securing such data is the fact that 

 in many instances the experiments did not continue long enough to 

 include the entire heat increment. This is particularly true in the pro- 

 tein experiments, for frequently (see table 215, page 284) the basal value 

 was not reached before the end of the experiment. 



