A STUDY OF FOODS 51 



also be oxidized in the body, and give heat and muscular 

 energy ; but the chief fuel nutrients of food will be discussed 

 in the next section. 



Uses of Fats and Carbohydrates. Some of the fat we eat 

 is stored away as fatty tissue and kept for future use. This 

 tissue gives a plump outline to the body, acts as a cushion 

 for many organs, and helps to keep our bodies warm by pre- 

 venting the heat from escaping, and by being oxidized as it 

 is needed. Much of the fat in our foods, however, is prob- 

 ably oxidized to furnish heat and power without being 

 stored within the body, and this class of nutrients furnishes 

 fuel in a most concentrated form. This is the reason why 

 the inhabitants of cold countries eat such large quantities 

 of fatty foods. The starches and sugars of bread, potato, 

 fruits, and milk are also used as fuel. Portions of the car- 

 bohydrates are changed, too, into fat tissue and stored as a 

 reserve of fuel. 



Comparison of Uses of the Nutrients. We have seen that 

 all of the nutrients thus far studied can be used to supply 

 the body with energy. If our diet is deficient in any one, 

 the others supply the need, and are burned instead. For 

 growth and repair, however, proteids are absolutely essen- 

 tial ; neither sugar, starch, nor fat can be transformed into 

 this essential ingredient of protoplasm. An animal soon 

 dies if it is not supplied with a certain amount of proteid. 



The Relative Fuel Values of the Nutrients. We have made 

 frequent reference to the use of food in giving energy to the 

 body. By means of an apparatus called the cal-or-im'e-ter 

 (Latin color = heat -f- metiri = to measure) it is possible to 

 determine the amount of heat that each kind of nutrient 

 will produce, or, in other words, to measure its fuel value. 

 As we measure the quantity of food by the pound or quart, 

 so its fuel value is computed in heat units or cal'o-ries. For 



1 A calorie is "the quantity of heat necessary to raise the temper- 

 ature of a kilogram of water from to 1 centigrade." Century 

 Dictionary. 



