Water’s role in thermoregulation is vital. Water absorbs heat at the point of generation with little temperature rise and dissipates it throughout the fluid compartments of the body. Thus, damage to enzymes and structural proteins is minimized, and heat-bearing blood plasma can be routed to the skin, where heat is transferred to the environment through conduction, radiation, convection, and evaporation. Panting also dissipates heat, and body heat is transferred to the environment via the moisture carried out by each exhalation. The vaporization of one liter of water at 20°C constitutes the loss of 585 kcal of heat (Kleiber, 1975; Askew, 1996).

The percentage of body water differs among humans according to sex, age, and reproductive stage (Askew, 1996). Because women tend to have less muscle and more adipose tissue than men and adipose tissue has about 10% water compared with about 76% water in muscle, men usually have a higher percentage of body water (about 60%) than women (about 50-55%). A human infant has about 77% body water, whereas an elderly person can have as little as 45%. Pregnancy usually increases blood volume and alters relative water distribution between intracellular and extra-cellular compartments (Southgate, 1987).

Generally for a healthy adult the recommended amount of water is about 1 ml. per kcal so long as the diet is not too high in salt and there is minimal physical exertion and no sweating.

Water Needs Calculations:
The least amount of energy one requires is known as basal metabolic rate (B.M.R.)
To calculate a man's B.M.R.: 1 kcal.x kg. of body weight x 24 hrs. (for men).
To calculate a woman's B.M.R.: 0.95 kcal. x kg. of body weight x 24 hrs.
Sleep requires less energy and therefore must be deducted from the former result. Sleep is figured at (0.1 x kg. of body weight x hrs. of sleep). So a woman that is 5'4" with an ideal body weight of (121 lbs.) 55kg. multiply by 0.95 kcal x 24 hrs = 1254 kcals. This is her B.M.R. minus sleep (0.1 kcals. x 55 kg. x 8 hrs sleep) = 44 kcals = 1210 kcals. Generally women's B.M.R. ranges from 1200 to 1450 kcals. while men's range from 1600 to 1800 kcals. Therefore the minimum water requirement based on B.M.R. = 34 to 68 fluid ounces in a 24-hour period.

Heat and physical exertion will add to the need for water - substantially.