320 HUMAN PHYSIOLOGY 



How THE Nutritive Value of Food has been Discovered. — For a 

 number of years, experiments have been in progress in different parts of 

 the civilized world which have led to the beliefs regarding food that have just 

 been quoted. One of the most accurate and important series of experi- 

 ments was made a few years ago by the late Professor W. O. Atwater of 

 Wesleyan University, in cooperation with the United States Department of 

 Agriculture. By means of a machine called the respiration calorimeter 

 (Lat. caZor= heat + mefrwm= measure) which measures both the products of 

 respiration and the heat given off by the body, it has been possible to deter- 

 mine accurately the value of different kinds of food, both as fuel and as 

 tissue builders. This respiration calorimeter is described by Professoi 

 Atwater as follows : — • 



" Its main feature is a copper-walled chamber 7 feet long, 4 feet wide, and 

 6 feet 4 inches high. This is fitted with devices for maintaining and measur- 

 ing a ventilating current of air, for sampling and analyzing this air, for 

 removing and measuring the heat given off within the chamber, and for 

 passing food and other articles in and out. It is furnished with a folding 

 bed, chair, and table, with scales and appliances for muscular work, and 

 has telephone connection with the outside. Here the subject stays for a 

 period of from three to twelve days, during which time, careful analyses 

 and measurements are made of all material which enters the body in the 

 food, and of that which leaA^es it in the breath and excreta. 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 gain- 

 ing 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 correctly measured, should 

 equal the energy of the body material gained or lost. With such appara- 

 tus it is possible to learn what effect different conditions of nourishment 

 will have on the human body. In one experiment, for instance, the sub- 

 ject 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 compar- 

 ing 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 slightly 

 changed in the one experiment, and the effect of this on the balance of 

 matter or energy, observed. Such methods and apparatus are very costly 

 In time and money, but the results are proportionately more valuable than 

 those from simpler experiments." 



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 of heat required to raise the temperature of one kilogram of 

 water from zero to one degree centigrade. This is about equivalent te 



