FOOD AND DIGESTION 71 



any of the three may yield energy, but on an exclusive 

 protein diet, serious disturbances soon arise. There- 

 fore, we may regard protein as the material from which 

 new tissue is built or by which old, worn-out tissue is to 

 be replaced, while fats, and especially carbohydrates, 

 are the fuel foods; all three being needed in order that 

 growth may occur or that health may be preserved. 



It becomes a matter of much practical importance as 

 to how much of each constituent should be included in 

 the diet and what the total should be. Fortunately, 

 most individuals in health instinctively select the proper 

 amounts needed for nutrition, but where it is necessary 

 for the physician to select a diet, the importance of this 

 knowledge is obvious. 



It has been learned that when a substance is burned, 

 it gives off a definite amount of heat, and by suitable 

 means, this heat can be measured. The amount of heat 

 required to raise one gram of water one degree centi- 

 grade is known as the small calorie, and is designated by 

 the symbol "cj" the amount to raise the temperature 

 of a kilogram of water a similar amount is known as 

 the large calorie, and its symbol is "C." If one gram of 

 carbohydrate is burned under suitable conditions, it is 

 found that it yields approximately 4.1 large calories ; 

 while a gram of fat under similar conditions yields over 

 twice as much heat, namely, 9.3 large calories. More- 

 over, the amount of heat produced in the animal body 

 by the oxidation of foods can be measured, and it is 

 found that this is the same for carbohydrate and fat as 

 that produced by burning these substances outside the 

 body. In the oxidation of protein in the body, it has 

 been found that a gram also yields about 4.1 large 

 calories, though the combustion of this amount of pro- 

 tein outside the body gives off more heat. This is due 



