THE DIGESTIVE SYSTEM 107 



THE PHYSIOLOGY OF NUTRITION 



Nutrition is the process of assimilating or transforming food 

 into living tissue. It has been called constructive metabolism 

 or tissue building. A feeder of animals without a knowledge of 

 the principles of animal nutrition is unable intelligently to feed his 

 live stock. 



Not until very recently has much progress been made in the 

 study of animal nutrition. This was due to the fact that no 

 method had been found by which it was possible to control ex- 

 perimental animals and their rations. In 1906 Dr. E. V. Mc- 

 Collum, who was then connected with the Wisconsin College of 

 Agriculture, started investigations designed to show more clearly 

 just how rations of different kinds affected nutrition. He experi- 

 mented largely with rats but used hogs and cattle when advisable. 



One of the first important things that scientists discovered 

 in the study of nutrition was that purified foodstuffs containing 

 all the supposedly essential substances for nutrition, viz., pro- 

 teins, carbohydrates, fats, water and salt mixtures, when fed 

 experimental animals not only do not induce growth, but also fail 

 to sustain life for any great length of time, not over two months. 



In his experiments McCollum found that the addition of a 

 small amount of egg yolk or diied milk to the above mentioned 

 purified mixture caused animals to grow satisfactorily. Also 

 that the addition of wheat, corn, or even meat was unable to 

 make the animals grow or maintain them in a healthy condition. 

 Furthermore, when all the fat was extracted from the egg yolk 

 or dried milk, the residue would not make the animals grow. 

 However, when the egg fat or butter-fat was replaced in the 

 purified ration, the ration became complete again. 



The next surprising discovery was that with the fats of the egg 

 yolk as the only addition to the purified foodstuffs, the animals 

 would not grow. But by adding a little of the egg yolk, which 

 is not fat, and leaving in the egg fats, growth at once started. 



This led to carrying out an experiment in which to a mixture 

 of the pure food was added the egg yolk fats, and instead of all 

 that part of the egg yolk that was left after extracting the fat, 

 only those substances that are water soluble were added. This 

 ration caused growth just as if the whole yolk were in the food. 



Before McCollum had completed the investigations just men- 



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