WATER 13 



From fats, proteins, and starch the yield of water per 100 Cal. is some- 

 what less than the amount calculated for glucose. Approximately 12.5 ml. 

 of metabolic water is formed during oxidation of a 100 Cal. portion of 

 a diet in which i)roteins, fats, and carbohydrates contribute 15, 35, and 50 

 per cent, respectively, of the total calorics. On this basis, a person con- 

 suming 3000 Cal. daily would derive 375 ml. of water from oxidation 

 of these energy-yielding foods, an amount approximating one-tenth of 

 the intake of water as a drink. 



Some species of insect, e.g., clothes moth and grain weevil, obtain nearly 

 all of their water from metabolic processes. The clothes moth feeds on 

 wool, which contains about 5-10 per cent of absorbed water. Foi- each 

 gram of dry wool (protein) consumed, ai)proximately 0.4 ml. of watei- is 

 formed by the chemical processes involved in metabolism. 



Water balance 



For a normal man the intake and output of water are so regulated 

 that the amount in the body remains fairly constant. If the intake is 

 increased without any other change in external conditions, the output 

 in the urine is very promptly increased. If the atmospheric temperature 

 rises, or if muscular effort is increased, then more water is eliminated 

 through the skin as insensible or sensible (visible) perspiration and less 

 is put out in the urine. With increased muscular effort there is also more 

 water eliminated through the lungs, but the greatest variation occurs in 

 the volume of urine and perspiration. A typical water balance for an 

 average-sized man of sedentary occupation is about as follows: 



Water intake ml. Water output ml. 



Drinking water 850 In urine 1450 



Water in coffee, milk, soup, and In feces 150 



other fluids 600 Evaporated from the skin 600 



Water in solid foods 700 Vaporized through the lungs 350 



Metabolic water 350 



Total: 2500 2550 



On this particular day there is a negative balance of 50 ml.; on a succeed- 

 ing day the balance might be positive by that much or more. 



Water supplies 



The importance of a pure water supply cannot be overestimated. To 

 determine the potability of a water requires careful chemical and bac- 

 teriological analyses. Pure water in the chemist's sense of the term is 

 not required to furnish a sanitary water supply. All ground and surface 

 waters dissolve more or less salts and other materials. It is only when 



