NUTRITION 217 



weighing sixty kilograms goes up a flight of stairs ten meters 

 high, his muscles do 600 kilogrammeters of work. 1 



Finally, it has been found that the same fuel which when 

 burned will liberate one calorie of heat will supply the power 

 to do 423.985 kilogrammeters of work. By this we mean 

 that not more than 423.985 kilogrammeters of work can be 

 obtained from it. Not every engine is so perfectly con- 

 structed as to get from a certain fuel its full working 

 capacity ; indeed, most engines transform only a small frac- 

 tion of the power of their fuel into work, the rest escaping 

 as heat in the smoke, or by radiation, conduction, and 

 convection from the furnace, boiler, etc. But by the method 

 above outlined it is possible to find the maximum amount 

 of work which can be obtained from a given weight of fuel. 



Applying the same methods to food, we find that 



1 gram of dried protein yields 4.1 calories. 



1 gram of dried carbohydrate yields 4.1 calories. 



1 gram of fat yields 9.3 calories. 



These figures are known as the fuel values of proteins, 

 carbohydrates, and fats. 



But the total possible power which may be obtained by 

 actually burning a certain substance under the most favor- 

 able conditions is one thing, and the amount of power which 

 the muscles may obtain from it is quite another. When coal 

 is burned in an engine it does work, but the human body 

 would get no energy for its muscular work from eating coal. 

 So that we have now to inquire from what nutrients the 

 muscles get their energy for work and from what nutrients 

 the body derives its heat. 



4. The power for muscular work. Few questions in physi- 

 ology have been more thoroughly investigated than this. 

 In the first half of the nineteenth century many investi- 

 gators, impressed with the fact that the muscle fiber yields, 



1 Work may also be expressed in foot-pounds. (How many foot-pounds 

 equal one kilogrammeter ?) 

 p 



