FOODS AND DIETARIES 333 



Record is also kept of the energy given off from the body as heat and 

 muscular work. The difference between the material taken into and that 

 given off from the body is called the balance of matter, and shows whether 

 the body is gaining or losing material. The difference between the energy 

 of the food taken and that of the excreta and the energy given off by the 

 body as heat and muscular work, is the balance of energy, and, if cor- 

 rectly measured, should equal the energy of the body material gained or 

 lost. AYith such apparatus it is possible to learn what effect different con- 

 ditions of nourishment wiU ha^e on the human body. In one experiment, 

 for instance, the subject might be kept quite at rest, and in the next do a 

 certain amount of muscular or mental work with the same diet as before, 

 then by comparing the results of the two, the use which the body makes 

 of its food under the different conditions could be determined ; or the 

 diet may be sHghtly changed in the one experiment, and the effect of this 

 on the balance of matter or energy, obser\ed. Such methods and appa- 

 ratus are ^■ery costly in time and money, but the results are proportionately 

 more valuable than those from simpler e.xperiments." 



Fuel.. Values of Nutrients. — In experiments performed by 

 Professor Atwater and others, and in the appended tables, the 

 value of food as a source of energy is stated in heat units called 

 calories. A calorie is the amount af heat required to raise the tem- 

 perature of one kilogram of water from zero to one degree Centi- 

 grade. This is about equivalent to raising one pound 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 a food (say one poimd) in the apparatus known as a calori- 

 meter. By this means may be determined the number of degrees 

 the temperattire of a given amount of water is raised during the 

 process of burning. 



The Best Dietary. — Inasmuch as all living substance contains 

 nitrogen, it is evident that proteid food must form a part of the 

 dietary ; but proteid alone is not usable. If more proteid is eaten 

 than the body requires, then immediately the liver and kidneys 

 have to work overtime to get rid of the excess of proteid which 

 forms a poisonous waste harmful to the body. We must take foods 

 that will give us, as nearly as possible, the proportion of the dif- 

 ferent chemical elements as they are contained in protoplasm. It 

 has been found, as a result of studies of Atwater and others, that 

 a man who does muscular work requires a little less than one quarter 

 of a pound of proteid, the same amount of fat, and about one pound 



