12 WATER 



Water requirement 



The water requirement varies with different conditions. For an adult 

 doing no manual work, and at the time of year when the weather does not 

 induce visible perspiration, it is estimated that an intake of about 3 1. of 

 water per day is needed for good health. Of this quantity, about 2 1. will 

 be contained in the food, leaving 1 1. to be drunk as water. Many health 

 authorities advocate drinking from 5 to 8 glasses of water daily, which 

 is more than most people manage to do. 



Individuals who work under conditions which induce much perspiration, 

 for example, blast furnace workers, may lose many liters of sweat per day 

 and hence must drink large quantities of water. Since the sweat carries 

 with it salts, depletion must be avoided by taking salt tablets or by adding 

 salt to the drinking water. About 0.1 per cent of sodium chloride in the 

 water is scarcely perceptible, and such water quenches thirst about as 

 well as unsalted water. 



The water requirement would be even higher than it is except for the 

 fact that the ingested water is reused several times for different purposes 

 before it is finally lost from the body. Water withdrawn from the blood 

 stream for such secretions as saliva, gastric juice, pancreatic juice, in- 

 testinal juice, and bile is salvaged during absorption and made available 

 for further use. The volume of these secretions amounts to from 3.7 to 

 9.8 1. daily, which means an average reuse of two to three times. 



Formally it was thought that water should not be drunk with meals, 

 but as a result of Hawk's work it appears thaat this idea is erroneous. 

 Under certain conditions such as dropsy, heart and kidney disturbances 

 large quantities of water may be objectionable, but for the normal in- 

 dividual it is probable that too little water is usually consumed. 



Metabolic water 



Water is invariably .one of the end products of oxidation of carbo- 

 hydrates, fats, and proteins. Such water is called "metabolic water." 

 To be specific, let us refer to the equation for complete oxidation of 

 glucose in the body: 



CeHioOe + 6O2 = 6CO2 + 6H2O + 683 Cal.i 



During oxidation of 180 g. (1 mole) of glucose, 108 g. (6 moles) of water 

 is produced with accompanying liberation of 683 Cal. of heat. By the 

 same proportion, 15.9 g. of water would be formed for every 100 Cal. 

 derived by oxidation of glucose. 



^ For a definition of calorie see p. 413. 



