Water, water, everywhere,but how much should we drink?

(paraphrase from Samuel Taylor Coleridge)

Fluids have many essential functions in the body: the transport of nutrients to cells and waste from cells, supporting many metabolic reactions, the regulation of body temperature, maintenance of eye health  and the lubrication of joints and digestive and mucous tracts. In fact, total body water forms a large part of body weight: over 70% in the newborn, with a declining percentage in adulthood and old age – 60% in children down to 56% in older men and 47% in older women. Women have a lower percent of body water than men because of a relatively higher percentage of fat (which has a lower water content in comparison to muscle tissue).

Levels of body fluid are tightly controlled, and very small changes in fluid balance can lead to feeling unwell and illness. Fluid intake is influenced predominantly by drinking upon the sensation of thirst. Some fluids come from foods, such as fruits and vegetables/salads, and small amounts are also produced by metabolic reactions in the body. The kidneys mainly control fluid output from the body as urine, but fluid losses also come from losses in faeces and also breathing and from the skin by sweat; amounts can be considerable during fever or during physical activity, especially in heat.

Dehydration occurs in some diseases, and is a particular risk in young children with diarrhoea and vomiting conditions. Long-term low intakes of fluid can lead to constipation, headaches and lethargy, and increases the risk of urinary tract infections and some renal conditions.

Urine colour correlates with urine osmolality (concentration of dissolved particles in a solution ) and urine volume correlates with fluids consumed. The colour and amount of urine excreted is a practical indicator of hydration status in a non-clinical setting: outputs averaging 60 to 100ml/hour indicate a hydrated state, as does urine of a ‘pale straw colour’. Urine outputs averaging less than 30ml/hour, and darker syrup colours, indicate some dehydration and the need for more fluids.

Intakes of oral liquids as water and beverages match volumes of urine output. Studies show that in sedentary people in temperate climates, net water losses range widely between 1.0 to 3.1 litres/day. In practice, intakes of water and beverages of between 1.5 to 2.5 litres/day will be sufficient to achieve adequate hydration in nearly all people. Additional liquids are needed for people who are physically active and in hot environments.

Tap water is the ideal beverage to support fluid requirements. However many other drinks regularly consumed all support hydration: milk, fruit (and vegetable) juices, squash, fizzy drinks and sports drinks. Caffeine is a diuretic (inducing loss of water in the body) in high concentrations, however research shows that a tolerance to caffeine develops with regular consumption; diuretic effects are not significant in cups of tea and coffee typically consumed in the UK, and so such caffeinated beverages also support hydration.

Alcohol, like caffeine, is a diuretic, so drinks containing high proportions of alcohol make no helpful contribution to hydration, and moreover result in temporary dehydration of the body. Drinks low in alcohol, such as many beers or wine spritzers can support hydration, but clearly they should be counted as occasional pleasures rather than quenching beverages.

July 2006