NUTRIENTS 



119 



-Sugar 



Prof-ein 



Minerdls 



Wai-er 



Composition of milk. 

 Why is milk consid- 

 ered a good food ? 



millions of cells of which the body is composed must be given 

 material which will form more protoplasm ; also food material 

 which can be oxidized to release energy when muscle cells move, 

 or gland cells secrete, or brain cells think. 



Nutrients. — Certain nutrient materials form the basis of food 

 of both plants and animals. These, as already stated (page 53), 

 include proteins (such as lean meat, eggs, the 

 gluten of bread) , carbohydrates (starches, sugars, 

 gums, etc.), fats and oils (both animal and vege- 

 table), mineral matter, and water. Not many 

 years ago biologists thought that if the body 

 was supplied with sufficient quantities of water 

 and of proteins, fats or oils, and carbohydrates, 

 it would grow and do its work well. But re- 

 cent scientific work on dietaries shows that 

 other factors also must be considered. Feed- 

 ing experiments with rats and other animals, 

 and with people, have shown that vitamins and 

 certain minerals are necessary. 



The Fuel Value of Food. — In various experiments it has been 

 agreed that the energy stored in foods as a source of heat should be 

 stated in heat units called calories. A calorie is the amount of heat 

 required to raise the temperature of one kilogram of water one degree 

 Centigrade.^ This is about equivalent to raising the temperature 

 of one pound of water four degrees Fahrenheit. The fuel value 

 of different foods may be computed in a definite manner. This is 

 done by burning a given portion of each food (one gram) in the ap- 

 paratus known as a calorim' eter . By this means may be determined 

 the number of degrees the temperature of a given amount of water is 

 raised during the process of burning. It has thus been found that a 

 gram of fat will liberate 9.3 calories of heat, while a gram of starch 

 or sugar liberates only about 4 calories. The burning value of 

 fat is, therefore, over twice that of carbohydrates. In a similar 



^ The calorie thus defined, the one used in food experiments, is sometimes spelled 

 with capital C and is called the " large calorie " to distinguish it from the small calorie 

 which is used for delicate physical measurements. The small calorie is equal to one 

 thousandth of the large calorie. For very precise measurements the small calorie 

 is defined as the amount of heat required to raise the temperature of one gram of 

 water " from 15° C. to 16° C," instead of merely " one degree Centigrade." 



