1004 EXPERIMENT STATION RECORD. 



protein, fat, and carbohydrates, and various mineral salts. In addition 

 to these water is essential, as is also the oxygen from the air. The 

 nutrients are composed of carbon, oxygen, hydrogen, nitrogen, sulphur, 

 phosphorus, potassium, and other elements. Substances composed of 

 these elements undergo numberless changes, synthetical and analytical, 

 in the processes of digestion, assimilation, and respiration. All the 

 manifold changes of matter and energy are covered by the term metab- 

 olism. It signifies the transformation of matter and energy in the 

 animal organism. Practically all material which has undergone metab- 

 olism (and which is not stored in the body) is excreted in the urine 

 and respiratory products, although the feces contain metabolic prod- 

 ucts along with undigested residue of the food consumed. 



As previously stated, food is required for the building and repair of 

 tissue and as a source of energy. In the young organism growth is the 

 important consideration. The skeletal framework, tissues, organs, and 

 fluids must be formed from materials supplied in the food. The prob- 

 lem of feeding the young animal is entirely different from that of feed- 

 ing the adult. It has been found that the digestibility of certain foods 

 is more complete by young than by adult animals, and there are other 

 differences equally marked. 1 n this discussion reference is always made 

 to the adult man or animal unless otherwise stated. 



Laws of nutrition.^ — Experiments in metabolism in which the income 

 and outgo of matter and energy are determined depend upon certain 

 fundamental facts. From these facts certain theories of nutrition have 

 been deduced. The more important of these facts and theories follow: 



(1) All nitrogen is supplied from the food; that is, no nitrogen is 

 taken from the air. 



(2) Nitrogen is excreted only in the urine and feces; that is, no 

 nitrogen is excreted in the gaseous excretory products. 2 



(3) When the animal is supplied with sufficient food the amount of 

 nitrogen excreted in the urine is dependent upon and, under ordinary 

 circumstances, readily adjusts itself to the amount consumed; that is, 

 the amount excreted in the urine becomes approximately the same as 

 that consumed. The greater part of the nitrogen excreted in the urine 

 is in the form of urea. Some is excreted as uric acid (in herbivora as 

 hippuric acid), and a small amount as nitrogen of extractives, etc. 

 There are numerous conditions apparently causing the retention of 

 nitrogen in the body. The gains of nitrogen, however, are small in the 

 adult in health and are seldom long continued. The tendency is toward 

 nitrogen equilibrium. 



(4) A certain amount of food material, i. e., protein, fat, and carbo- 

 hydrates, is required for maintenance. Mineral matter is also essential, 



1 In the discussion which follows little reference is made to controversial points. 

 The attempt has been made to state theories in accord with the consensus of opinion 

 of the majority of investigators. 



2 The small amount of nitrogen lost in hair, hoofs, etc., or excreted in perspiration 

 and in volatile organic compounds is left out of account. 



