574 METABOLISM 



accuracy of the method for measuring energy output, we shall now con- 

 sider some of the conditions that control it. To study these we must first 

 of all determine the basal heat production that is, the smallest energy 

 output that is compatible with health. This is ascertained by allowing a 

 man to sleep in the calorimeter and then measuring his calorie output 

 while he is still resting in bed in the morning, fifteen hours after the last 

 meal. When the results thus obtained on a number of individuals are 

 calculated so as to represent the calorie output per kilogram of body weight 

 in each case, it will be found that 1 C. per kilo per hour is discharged 

 that is to say, the total energy expenditure in 24 hours in a man of 70 

 kilos, which is a good average weight, will be 70 X 24 = 1,680 C. 



When food is taken the heat production rises, the increase over the 

 basal heat production amounting for an ordinary diet to about 10 per 

 cent. Besides being the ultimate source of all the body heat, food is there- 

 fore a direct stimulant of heat production. This specific dynamic action, 

 as it is called, is not, however, the same for all groups of foodstuffs, being 

 greatest for proteins and least for carbohydrates. Thus, if a starving 

 animal kept at 33 C. is given protein with a calorie value which is equal 

 to the calorie output during starvation, the calorie output will increase by 

 30 per cent, whereas with carbohydrates it will increase by only 6 per 

 cent. Evidently, then, protein liberates much free heat during its as- 

 similation in the animal body; it burns with a hotter flame than fats or 

 carbohydrates, although before it is completely burned it may not yield 

 so much energy as is the case, for example, when fats are burned. This 

 peculiar property of proteins accounts for their well-known heating qual- 

 ities. It explains why protein composes so large a proportion of the diet of 

 peoples living in cold regions, and why it is cut down in the diet of those 

 who dwell near the tropics. Individuals maintained on a low protein diet 

 may suffer intensely from cold. 



If we add to the basal heat production of 1,680 C. another 168 C. (or 

 10 per cent) on account of food, the total 1,848 C. nevertheless falls far 

 short of that which we know must be liberated when we calculate the 

 available energy of the diet, which 'we may take as 2,500 C. What be- 

 comes of the extra fuel? The answer is that it is used for muscular work. 

 Thus it has been found that if the observed person, instead of lying down 

 in the calorimeter, is made to sit in a chair, the heat production is raised 

 by 8 per cent, or if he performs such movements as would be necessary for 

 ordinary work (writing at a desk) it may rise 29 per cent that is to say, 

 to 90 C. per hour. There is, however, practically no difference in the en- 

 ergy output of a person lying flat or lying in a semi-reclining posi- 

 tion, as in a steamer chair. Allowing eight hours for sleep and sixteen 

 hours for work, we can account for about 2,168 C., the remaining 300 odd 



