NUTRITION LABORATORY. 13^ 



however, invariably found in the ordinary mixed diets. Olive oil as a pure 

 fat and cane-sugar as a pure carbohydrate and other similar materials are 

 used to a certain extent in our diets, but usually the nutrients are mixed in 

 varying proportions, dependent upon the nature of the food and the dietetic 

 habits of people. Pure protein is almost never eaten, as it is difficult to pre- 

 pare in large quantities. 



Experiments on the influence of the ingestion of food soon showed that 

 there were marked differences in the effect of the different nutrients on the 

 general metabolism. When relatively pure protein was ingested it was fol- 

 lowed by a considerably greater increase in the bodily activities than was the 

 ingestion of an equal quantity of pure carbohydrate or fat. So great was 

 this apparent stimulus to metabolism that Rubner first considered it a 

 "specific dynamic" action of protein. Zuntz has found a similar stimulus 

 to metabolic activity, though of less intensity, in the case of the other two 

 nutrients. Few of the experiments made by these investigators were con- 

 ducted upon men, and in attempting to study the fundamental laws governing 

 metabolism in man it is obviously desirable to institute an extended series of 

 experiments to study the influence of the ingestion of food upon metabolism. 

 The experiments were planned to include a study not only of the effect of the 

 ingestion of varying amounts of the ordinary mixed diets, but also a study of 

 the influence of ingesting varying amounts of relatively pure nutrients. 



While as has been shown by the long series of experiments on inanition 

 the true fasting metabolism is not reached till the second or third day 

 of a fast, for purposes of comparison, at least at the present stage of the 

 development of the experimental technique, the metabolism during rest, 12 

 hours after the last meal (which is usually a very light one), is reasonably 

 constant with the majority of subjects. Consequently our experiments are 

 planned as follows : The evening before the experiment begins the subject is 

 given a very light supper. The next morning at 7 he enters the chamber 

 of the respiration calorimeter, sits comfortably in an arm chair and reads. 

 About an hour is required to adjust the heat-measuring devices and secure 

 thermal equilibrium in the chamber. An unbroken series of four 2-hour 

 experiments then follows. Without leaving the arm chair the subject is 

 weighed accurately at the beginning and end of each period. By means of 

 a pneumograph the pulse and respiration rates are recorded. An electrical 

 thermometer permits the observation of the rectal temperature of the subject 

 as often as desired (usually every 4 minutes), and there are measured simul- 

 taneously the carbon dioxide and water-vapor output, the heat production, 

 and the oxygen consumption. The four 2-hour experiments usually show a 

 remarkable agreement, thus attesting to the uniformity of the metabolism 

 under these conditions. Duplicate experiments made with the same subject 

 after several intervening weeks or months likewise show uniformity. Thus 

 the standard of comparison may be said to be well established. 



