FOOD AND FEEDING 483 



is the circulating medium or currency of proteids, as glucose is the cur- 

 rency of carbohydrates. Proteids in excess are not stored as amino- 

 acids but as body protein or tissue. 



The following role is played by protein as a nutritive: — 



1. It is the building material for cell protoplasm of tissues of all 

 kinds. 



2. It forms energy directly by undergoing destruction without 

 tissue-building. 



3. It may be transformed (through its CHO) into sugar or gly- 

 cogen but can not form fat in the body contrary to former belief. 



Carbohydrates of the food, as starch and sugar, are converted by 

 the enzymes of the intestinal mucosa into sugars — as dextrose, levulose 

 and galactose, which pass into the portal circulation to tRe liver where 

 they are converted into glycogen, and stored as- such, except a part of 

 the sugars which are carried to the muscles and there converted into 

 glycogen. 



The glycogen undergoes combustion in the liver and muscles with 

 a production of heat equal to 4.18 calories for each gram of carbohy- 

 drates. CO2 and water are end-products. Sugar metabolism is under 

 the control of the internal secretions of the pancreas, adrenal, thyroid 

 and pituitary glands. 



These either aid in the storage of sugar as glycogen in the liver 

 (pancreas), or, in excessive and abnormal amount (adrenal, pituitary 

 and thyroid glands), hinder this action and allow sugar to pass through 

 the liver unchanged and escape into the blood and urine, thus causing 

 diabetes. The heat production, as in the case of the combustion of 

 proteids, is of great importance in sustaining nerve action. 



Fat in the food is emulsified by the bile and by the pancreatic 

 and intestinal secretions and is absorbed as fatty acids, glycerol and 

 soaps by special cells of the intestinal mucous membrane and passes 

 into the lymphatics as emulsions of fat; thence into the thoracic duct, 

 and is finally oxidized into carbonic dioxide and water with produc- 

 tion of heat and energy. In what part of the body oxidation of fat 

 occurs is unknown. In the combustion of 1 gram of fat heat equal 

 to 9,4 calories is liberated. 



Carbohydrates do not directly furnish tissue elements, but do so 

 indirectly in preventing decomposition (to some extent) of protein in 

 the body, and in lessening" its consumption. In this way the compara- 

 tively inexpensive carbohydrates will compensate for an insufiicient ra- 

 tion of costly proteids. 



It is asserted that this action of carbohydrates is explained by the 

 fact that these nutrients have a greater affinity for oxygen than proteids 

 and so are the first to undergo combustion. 



The term albuminoid was formerly used as synonymous with pro- 

 teid, but is now employed to include nitrogenous bodies (gelatin) de- 

 rived from protein in the body but not convertible into proteids. These 

 bodies do not take the place of proteids, but appear to fulfil the functions 

 of circulating protein, and, like carbohydrates, conserve protein con- 

 sumption, but are not tissue builders. 



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