FEEDING EXPERIMENTS WITH ISOLATED FOOD 



SUBSTANCES. 



Although the proteins have long had attention centered upon 

 them because of their commanding position in relation to nutrition, 

 it is only in very recent years that the progress of chemical investi- 

 gation, fostered by the introduction of new methods of research, has 

 begun to make our conception of these fundamentally important 

 substances more exact. The role of the nitrogenous substances in 

 both plants and animals has gradually been brought to light by a 

 series of effective researches; but even to-day it is still common, 

 despite the newer knowledge in the field of biochemistry, to read 

 of the part played by proteins, fats, and carbohydrates in nutrition, 

 as if these groups of compounds were each chemically homogeneous 

 and, within wide limits, physiologically interchangeable. Not only 

 have these food-stuffs been discussed in the past without more than 

 the scantiest consideration of possible specific differentiations within 

 the individual groups, but in nearly every investigation on nutrition 

 the nutrients have been employed in those complex and often little- 

 understood mixtures which make up the common foods. 



That any such indefinite combination of known and unknown 

 organic compounds is an unsatisfactory and unideal starting-point 

 in any adequate study of the laws of nutrition was appreciated by 

 Carl Voit, the foremost student of this subject of the past generation. 

 In discussing the necessity for an accurate knowledge of the food 

 intake in the study of metabolism he says: 



Zu dem Zwecke ware es unstreitig am besten, konnte man nur reine, 

 chemische Verbindungen (die reinen NahrstofYe) z.B. reines Eiweiss, Fett, 

 Zucker, Starkemehl, Aschebestandtheile, oder Gemische derselben geben. 

 Da aber die Menschen und auch die Thiere nur selten solche geschmacklose 

 Gemenge auf die Dauer aufzunehmen oder zu ertragen vermogen, so bleibt 

 fiir die meisten Falle niehts anderes iibrig als schon durch die Natur zusam- 

 mengesetzte Mischungen (die Nahrungsmittel) zu wahlen. Jedoeh ware es 

 wohl moglieh und ganz verdienstvoll, die Grundversuche, nachdem vorher 

 der Weg mit Hilfe der letzteren Mischungen gefunden worden ist, mit den 

 reinen Stoffen zu wiederholen, obwohl sich dabei sicherlich im Wesentlichen 

 keine anderen Resultate ergeben werden.* 



Note. The expenses of this investigation were shared by the Connecticut Agri- 

 cultural Experiment Station and the Carnegie Institution of Washington, D. C. In these 

 experiments we have been further assisted by M. S. Fine and C. S. Leavenworth. 



* C. Voit: Hermann's Handbueh der Physiologie, 1881, vi, (1), p. 19. 



