THE ABSORPTION AND USE OF FOODS 507 



The Metabolism of Muscular Work. The total energy turn- 

 over per day of any individual is made up, as we have just seen, 

 of his basal metabolism, together with the metabolism of his 

 active muscles. The first factor is practically constant; the second 

 is extremely variable. Some average figures may, however, be 

 presented. If we reckon the muscular efficiency at 20 per cent 

 every Calorie of energy manifested in the form of muscular work 

 means a consumption of 5 Calories altogether, and a liberation 

 of 4 Calories as the by-product of heat. A man who leads a de- 

 cidedly sedentary life, making no more movements than necessary, 

 is calculated to do an amount of work in a day equivalent to about 

 40 Calories (120,000) foot-pounds. This work consists in large 

 part of the labor involved in the maintenance of the sitting and 

 standing positions. The performance of 40 Calories of muscular 

 work requires, on account of the bodily inefficiency, previously 

 noted, an energy liberation of 200 Calories. This, added to the 

 practical basal metabolism of 1,900 Calories, brings the total to 

 2,100 Calories. When allowance is made for a moderate amount 

 of exercise; no more than must be taken if good health is to be 

 maintained; the daily metabolism amounts to 2,500 Calories. This 

 figure is believed to represent the average for adults of all classes 

 other than manual laborers. An interesting fact is that calcula- 

 tions of the average daily metabolism per individual of the in- 

 habitants of cities, based on estimates of the amounts of food 

 brought in each day to the markets, indicate this same figure, 

 2,500 Calories, as the average metabolism for the city dweller. 

 The energy liberation of the manual laborer varies greatly, of 

 course, with the nature of the toil. The range is usually set at 

 3,500 to 5,000 Calories per day. The latter figure probably repre- 

 sents a high limit which is rarely exceeded by any worker day 

 after day for long periods, although trained men may show a 

 much greater metabolism for a day or two. An output at the 

 rate of 10,000 Calories is believed not to be impossible for a brief 

 spurt. 



The Relative Food Values of Proteins, Carbohydrates and Fats. 

 Disregarding the use of protein as a tissue-repairer, and consider- 

 ing all three varieties of food simply as furnishers of energy, we 

 may inquire whether any one of them is superior to the others, 

 or whether any particular proportion of the three food stuffs is 



