Water

BACKGROUND

Water is defined as an essential nutrient because it is required in amounts that exceed the body's ability to produce it. All biochemical reactions occur in water. It fills the spaces in and between cells and helps form structures of large molecules such as protein and glycogen. Water is also required for digestion, absorption, transportation, dissolving nutrients, elimination of waste products and thermoregulation (Kleiner 1999).

Water accounts for 50–80% of body weight, depending on lean body mass. On average, men have a higher lean body mass than women and higher percentage of body mass as water than in women. The relative mass of water decreases in both men and women with age. Human requirements for water are related to metabolic needs and are highly variable. They depend to some extent on individual metabolism.

Solid foods contribute approximately 20% of total water intake or about 700–800 mL (NNS 1995). The remainder of the dietary intake comes from free water and/or other fluids (NHMRC 2003). An additional 250 mL or so of water is also made available to the body from metabolism (water of oxidation). The body must retain a minimal amount to maintain a tolerable solute load for the kidneys. Excluding perspiration, the normal turnover of water is approximately 4% of total body weight in adults. In a 70 kg adult, this is equivalent to 2,500–3,000 mL/day.

Water losses from lungs and skin (insensible losses) are responsible for 50% of the total water turnover. They are sensitive to environmental conditions and can be increased at high temperatures, high altitude and low humidity. During summer, when heat stress may be high, water depletion can lead to heat exhaustion, loss of consciousness and heat stroke (Cheung et al 1998, Hubbard & Armstrong 1988). Unfit, overweight, older people may be especially at risk, particularly if they are subjected to strenuous exercise. Infants and dependent children may also be at risk if not offered sufficient fluids. The remainder of the losses are from urine and stools.

Dehydration of as little as 2% loss of body weight results in impaired physiological responses and performance. The reported health effects of chronic mild dehydration and poor fluid intake include increased risk of kidney stones (Borghi et al 1996, Hughes & Norman 1992, Iguchi et al 1990, Embon et al 1990), urinary tract cancers (Bitterman et al 1991, Wilkens et al 1996, Michaud et al1999), colon cancer (Shannon et al 1996) and mitral valve prolapse (Lax et al 1992) as well as diminished physical and mental performance (Armstrong et al 1985, Brooks & Fahey 1984, Brouns et al 1992, Cheung et al 1998, Kristel-Boneh et al 1988, Torranin et al 1979, Sawka & Pandolf 1990).

Oral health may also be affected by fluid consumption. Apart from the beneficial effects of fluoride added to tap water in many communities in Australia and New Zealand, fluid intake can affect saliva production. Saliva, which is primarily water, is essential for maintenance of oral health. Decreased body water has been associated with salivary dysfunction, especially in older adults. However, one investigation (Ship & Fischer 1997) found that decreased salivary gland function was associated with dehydration, independent of age.

Several factors increase the possibility of chronic, mild dehydration, including a poor thirst mechanism (Sagawa et al 1992, Sansevero 1997), dissatisfaction with the taste of water (Meyer et al 1994, Weissman 1997), consumption of common diuretics such as caffeine (Meyer et al 1994) and alcohol, participation in exercise (Convertino et al 1996) and environmental conditions (Sagawa et al 1992). Kidney function can decline as part of the normal ageing process with decrease in kidney mass, declines in renal blood flow and glomerular filtration rate, distal renal tubular diluting capacity, renal concentrating capacity, sodium conservation and renal response to vasopressin. This decline in kidney function together with hormonal changes and factors such as decreased thirst perception, medication, cognitive changes, limited mobility and increased use of diuretics and laxatives make older adults a group of particular concern (NHMRC 1999). Numerous studies have shown diminished thirst sensations in the elderly. Despite the fact that these changes may be normal adaptations of the ageing process, the outcomes of dehydration in the elderly are serious and range from constipation to cognitive impairment, functional decline, falls or stroke.

Hydration status, assessed by plasma or serum osmolality is the indicator of choice to assess water requirements. However, the body's needs vary widely according to environmental conditions, physical activity and individual metabolism. The body can also compensate in the short term for over or underhydration, so it is difficult to establish an EAR experimentally. There is no single level of water intake that would ensure adequate hydration and optimal health for half of all the apparently healthy people in the population, in all environmental conditions. Thus an AI has been established based on median population intakes in Australia