FOOD AND DIGESTION 



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2. Inorganic Salts. The most important of these is sodium 

 chloride, which is essential for the maintenance of the 

 chemical changes in the body. When it is not freely sup- 

 plied in the food, it is retained in the tissues, and hence 

 animals can, when necessary, live with a comparatively small 

 supply. 



3. Salts of Organic Acids. The sodium and potassium salts 

 of citric, malic, and tartaric acid, which are found abun- 

 dantly in various vegetables, when taken into the tissues, are 

 oxidised into carbonates which are strongly alkaline salts. 

 The proteins which are decomposed in the body contain sulphur 

 and phosphorus, and these are oxidised into sulphuric and 

 phosphoric acids. In herbivorous animals the prejudicial 

 effect of such acids is counteracted by the formation of these 

 alkaline carbonates which neutralise the acids. In carnivorous 

 animals these salts are not so necessary, since ammonium 

 is formed from the nitrogen of the proteins in sufficient 

 quantity to neutralise the acids produced. Man occupies a 

 position midway between the herbivora and the carnivora. 

 The amount of energy yielded by the breaking down of 

 these salts into carbonates is so small that it is of no 

 importance. 



B. Food-stuffs yielding Energy. These are complex com- 

 binations of carbon, hydrogen, and oxygen, with or without 

 nitrogen, sulphur, phosphorus, and iron. They are of the 

 same nature as the materials which are found on analysis 

 of dead protoplasm. They are commonly spoken of as the 

 Proximate Principles of the food, and they may be classified 

 as follows : 



1. Nitrogen-containing Proteins and Sclero-proteins. 



2. Non-nitrogenous Fats and Carbohydrates. 



In studying the value of these food-stuffs it is necessary to 

 consider their Energy Value that is, the amount of energy 

 which can be yielded by the decomposition of a definite 

 quantity of each in the body. 



The fats and carbohydrates leave the body as carbon dioxide 

 and water, the proteins leave it partly as carbon dioxide and 

 water, partly as urea CO(NH 2 ) 2 . 



Such a body as glucose, C 6 H 12 O 6 , by being oxidised to C0 2 



