The body's total body water content is normally maintained within a small window of fluctuation on a daily basis by intake of food and drink to balance the excretion of urine and other losses. Hyperhydration is corrected by an increase in urine production and hypohydration by an increase in water intake via food or drink consumption initiated by thirst. Most of our water intake is related to habit rather than thirst, but the thirst mechanism is effective at driving intake after periods of deprivation. There are also water losses via the respiratory tract, the gastrointestinal tract, and the skin. The extent of these losses will vary from individual to individual and will be strongly influenced by environmental conditions and physical activity levels, but for a sedentary individual in a cool environment these generally represent only a small proportion of the total body water loss.
All the major textbooks of nutrition and physiology include data on the various components of water intake and output, although it is difficult to find the original data on which the various mean values and ranges are based. The Geigy Scientific Tables suggest that the minimum daily water intake for adults is on the order of 1.51, but others indicate that the minimum intake should be 2 l per day. Body size has a major influence on water turnover, but the total body water content will also be markedly affected by the body composition. Water turnover should therefore be more closely related to lean body mass than to body mass itself. It is expected, therefore, that there will be differences between men and women and between adults and children.
Environmental conditions will affect the basal water requirement by altering the losses that occur by the various routes (i.e., respiration, sweat, and urine). Water requirements for sedentary individuals living in the heat may be two or threefold higher than the requirement when living in a temperate climate, even when not accompanied by pronounced sweating. Transcutaneous and respiratory losses will be markedly influenced by the humidity of the ambient air, and this may be a more important factor than the ambient temperature. Respiratory water losses are incurred because of the humidification of the inspired air with fluid from the lungs. These losses are relatively small in the resting individual in a warm, moist environment
(amounting to about 200 ml per day) but will be increased approximately 2-fold in regions of low humidity, and may be as high as 1500 ml per day during periods of hard work in the cold, dry air at altitude. To these losses must be added insensible water loss through the skin (about 600 ml per day) and urine loss, which will not usually be less than about 800 ml per day.
Variations in the amount and type of food eaten have some effect on water requirements because of the resulting demand for excretion of excess electrolytes and the nonvolatile products of metabolism. An intake of electrolytes in excess of the amounts lost (primarily in sweat and feces) must be corrected by excretion in the urine, with a corresponding increase in the volume and osmolality of urine formed. The daily intake of electrolytes is subject to wide variation among individuals, with strong trends for differences among different geographical regions. Daily dietary sodium chloride intakes for 95% of the young male UK population fall between 3.8 and 14.3 g, with a mean of 8.4 g; the corresponding values for young women are 2.8-9.4 g, with a mean value of 6.0 g. For the same population, mean urinary sodium losses were reported to account for about 175 mmol per day, which is equivalent to about 10.2 g of sodium chloride.
There are also large differences among countries in the recommended intake of salt. The British health authorities advise a maximum of 6 g per day, but in Germany a maximum of 10 g per day is recommended. In contrast, Sweden recommends a maximum of 2 g per day, and Poland recommends a minimum of 1.4 g per day. The differences among countries reflect in part different interpretations with regard to the evidence linking salt intake and health, but also reflect regional consumption patterns dictated by food choices.
A high-protein diet requires a greater urine output to allow for excretion of water-soluble nitrogenous waste; this effect is relatively small compared with other routes of water loss but becomes meaningful when water availability is limited. The water content of the food ingested will also be influenced greatly by the nature of the diet, and water associated with food may make a major contribution to the total fluid intake in some individuals. Some water is also obtained from the oxidation of nutrients, and the total amount of water produced will depend on the total metabolic rate and is also influenced by the substrate being oxidized. An energy expenditure of 3000 kcal (12.6 MJ) per day, based upon a diet composed of 50% carbohydrate, 35% fat, and 15% protein, will yield about 400ml of water per day. Reducing the daily energy expenditure to 2000 kcal (8.4 MJ), but keeping the same diet composition, will yield about 275 ml of water. The contribution of this water-of-oxidation to total water requirements is thus appreciable when water turnover is low but becomes rather insignificant when water losses are high.
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