FOODS AND THE THEORY OF DIGESTION 123 



Importance of Water. Water finds its way into the 

 body as a pure liquid, as a part of such mixtures as coffee, 

 chocolate, and milk, and as a constituent of all our solid 

 foods. (See table of foods, page 126.) It is also formed in 

 the body by the oxidation of hydrogen. It passes through 

 the body unchanged, and is constantly being removed by all 

 the organs of excretion. Though water does not liberate 

 energy in the body nor build up the tissues in the sense 

 that other foods do, it is as necessary to the maintenance 

 of life as oxygen or proteids. It occurs in all the tissues, 

 and forms about 70 per cent of the entire weight of the 

 body. Its presence is necessary for the interchange of 

 materials at the cells and for keeping the tissues soft and 

 pliable. As it enters the body, it carries digested food 

 substances with it, and as it leaves it is loaded with wastes. 

 Its chief physiological work, which is that of a transporter 

 of material, depends upon its ability to dissolve substances 

 and to flow readily from place to place. 



Relative Quantity of Nutrients Needed. Proteids, 

 carbohydrates, and fats are the nutrients that supply most 

 of the body's nourishment. The most hygienic diet is the 

 one which supplies the proteids in sufficient quantity to 

 rebuild the tissues and the carbohydrates and fats in the 

 right amounts to supply the body with energy. Much ex- 

 perimenting has been done with a view to determining these 

 proportions, but the results so far are not entirely satis- 

 factory. According to some of the older estimates, a per- 

 son of average size requires for his daily use five ounces of 

 proteid, two and one half ounces of fat, and fifteen ounces 

 of carbohydrate. Recent investigations of this problem 

 seem to show that the body is as well, if not better, nour- 

 ished by a much smaller amount of proteid not more 

 than two and one half ounces (60 grams) daily. 1 



i Chittenden, The Nutrition of Man. 



