Water UK

Water and the urinary tract

The majority of the water that our bodies need is taken in as liquids, but 20-40% or more of dietary fluids is derived from solid foods. We lose water through the skin, lungs and in faeces, however most water is lost as urine.(i) In a temporate environment, adults normally pass about 2 litres of urine per day. Urine is formed by the kidneys in order to excrete a number of water-soluble compounds which if retained by the body would be toxic and lethal. The kidneys also control the amount of water in the body, by increasing urine dilution to remove excess water and reducing urine dilution to retain water and prevent dehydration. The kidneys also control blood pressure via the production of hormones which influence the tension in blood vessels, as well as by controlling the amount of salt and water in the body.(ii)

The risk of urinary tract disease has been shown to be affected by fluid intake. One of the actions recommended by the National Kidney Research Fund (NKRF) to prevent urine infections and kidney stones, and to reduce the risk of bladder cancer, is to "drink plenty of fluid - about 2 litres or 8 glasses a day."(ii)

Urinary incontinence

Research has demonstrated a link between fluid consumption, bladder filling and voiding frequency. For example, Fitzgerald MP et al(iii) found that the volume of fluid intake was positively correlated to both diurnal and nocturnal voiding frequency of urinary asymptomatic women. Although increased voiding has been associated with urinary incontinence there is no evidence to suggest that dehydration and associated reduced voiding frequency can reduce urinary incontinence. In contrast, it has been hypothesised that dehydration may act as a risk factor for urinary incontinence and a number of studies have shown that ensuring adequate fluid intake does not increase the frequency of urinary incontinence episodes.(iv,v) Gray and Krissovich(vi) recommend that patients with urinary incontinence should be advised to drink sufficient fluids to meet daily requirements and that restriction of overall fluid intake does not reduce urinary incontinence frequency or severity.

Urinary tract cancer (see Cancer)

Urinary tract infection (UTI) in adults and children

UTI in adults

Urinary tract infection is more common in women than men, with some 5-10% of women suffering from recurrent attacks. In the majority of cases, the infection is confined to the lower urinary tract, the bladder and urethra. Although this type of UTI is uncomfortable, it does not cause serious harm. However, in a small proportion of cases, the infection may spread to the kidneys and in a small proportion of these cases (less than 1%), other complicating factors such as kidney stones or urinary obstruction can lead to kidney damage.(vii)

Studies have shown that restricting fluid intake increases the risk for urinary tract infection in adults. For example, Nygaard and Linder(viii) found that teachers who reduced fluid intake had a 2.2-fold greater risk of urinary tract infection compared to teachers who did not restrict fluid consumption or micturition frequency. As part of the self-help treatment for a single, unrepeated, urinary tract infection in adults, the National Kidney Research Foundation (NKRF) recommends embarking on a high fluid intake (at least 2 litres of fluid in 24 hours), in addition to any ‘over-the-counter’ preparation. One of the most important measures for preventing recurrent UTI is to establish a regime of high fluid intake, with frequent and complete bladder emptying to prevent the establishment of bacteria in the bladder. NKRF advocates drinking sufficient fluid to ensure that individuals pass two litres of urine daily.(vii)

Other studies suggest that it is the maintenance of adequate levels of fluid intake that is important in the prevention of urinary tract infection, rather than high fluid intake, per se. Eckford et al(ix) showed that women who maintained an adequate level of hydration (via the use of a self-monitoring tool to test urine osmolality) had a statistically significant decrease in urinary tract infection frequency during the four month period of the study. The authors concluded that adequate hydration was able to reduce urinary osmolality and incidence of urinary tract infections.

However, whilst adequate hydration appears to be preventative, there is no definitive evidence that the susceptibility to urinary tract infection is dependent on fluid intake. Susceptibility depends on aspects of the bladder and host defence mechanisms and not simply hydration levels. More research is needed, in the form of controlled prospective randomised clinical studies, to determine whether mild dehydration has any deleterious effect and whether high fluid intake might be efficacious in the prevention of urinary tract infection.(x)

There has been some discussion to suggest that excessive dilution of the urine may impair the urinary tract's ability to resist infections. This may be particularly important if it affects the concentration of an antibiotic within the urine and hence reduces the drugs efficacy in treating the infection. Two in vitro studies have shown reduced gentamicin activity in highly dilute urine, although these findings need to be verified in a clinical setting.(xi,xii)

UTI in children

The NKRF estimates that 1% of boys and 3% of girls experience a urinary tract infection during the first ten years of life. Urinary tract infection can be serious in children if it is associated with reflux of urine back up the ureter to the kidneys (vesicoureteric reflux), where it can damage the kidneys, causing scarring and high blood pressure. If both kidneys are severely damaged, chronic kidney failure can occur. Urine infections can also cause serious problems such as fevers and failure to grow in small infants, as well as bed- or pant-wetting and poor school attendance in older children. Children should be encouraged to drink plenty of fluids and empty their bladder adequately (about every 2-3 hours), in order to prevent recurrent infection.(xiii)

For more information about the importance of water for children see the Water is Cool in School campaign.

Urinary tract stones

The prevalence of kidney and bladder stones is higher in populations with low urinary volume.(xiv) Chronic dehydration has been identified as a cause of stone formation in 19% of patients attending a metabolic stone clinic.(xv) Epidemiological studies have also identified chronic dehydration as a factor in the development of urolithiasis in populations exposed to high ambient temperatures, high degrees of physical activities and as a result of insufficient replacement of water losses.(xvi)

In hot climates, there appears to be an alteration in the thirst threshold – possibly as a result of tolerance to the constant thirst stimulation that occurs in such conditions. Many people living in such climates (eg south of Israel and parts of the US) have been found to have maximally concentrated urine which coincides with the higher incidence of urinary tract stones in these populations.(xvii)

Curhan et al(xviii) conducted a prospective study of men with no history of kidney stones and found an inverse association between fluid intake and the risk of urinary stone formation during 4 years of follow-up. Consumption of 2.5 litres or more of fluid per day reduced the risk of stone formation by 29%. A further study by Curhan et al found that in women, higher levels of fluid intake reduced the risk of stone formation by 39%.(xix) Borghi et al(xx) conducted a prospective randomised study on the role of fluid intake as a preventative measure in urinary stone formation and recurrence. The results showed that baseline urine volume was significantly lower in male and female stone forming patients, compared to normal subjects. After an initial adjustment period, the stone forming patients were then randomised into two groups. Patients in group 1 (intervention group) were instructed to increase fluid intake to achieve a urine volume of at least 2 litres per day. Stone forming patients in group 2 did not receive any such instruction. During the 5-year follow-up period patients in the intervention group (group 1) had a 50% lower recurrence rate for stones, and a longer time to first recurrence, compared with group 2. The authors concluded that urine volume is a real risk factor in stone formation and that an increase in fluid intake to at least 2 litres per day is an important method by which to prevent stone recurrence.

There is some evidence to show that the type of fluid consumed is also an important consideration in urinary stone prevention. Different effects have been observed for fluids other than water. For example, in a case-control study of six beverages, a decrease in the risk of stone formation was observed for coffee, tea, beer and wine. Whilst apple juice and grapefruit juice were found to increase the risk of stone formation.(xxi) However, other studies have shown that citrus fruit juices are suitable for the prevention of calcium oxalate, uric acid and cystine stones.(xvi)

Water composition has been specifically examined in some studies. In particular bicarbonate-rich mineral waters have been found to increase urinary pH and citrate excretion in healthy subjects. These effects are desired in the treatment of calcium oxalate, uric acid and cystine stones, whereas they are contraindicated in struvite stones. To date, the results of trials examining the relationship between drinking water hardness and urinary stone disease have been inconsistent. Siener and Hesse recommend that if stone composition is unknown, urine dilution should be accomplished without changing the composition of the urine. A suitable beverage would therefore be water with a low content of mineral salts and bicarbonate.(xvi)

The best treatment for urinary tract stones is prevention. Adequate intake of fluid is important in the prevention of stone formation because urine dilution has a protective effect on the crystallization of stone-forming salts.(xxii) Since stone formation may begin early in life, Rapoport(xvii) has recommended an education programme for children in at risk groups (such as those living in hot climates), to teach them about the importance of adequate fluid intake. Ample hydration, avoidance of urinary tract infection, and good voiding habits minimize the change of initial and recurrent stone formation.(xiii) The Consensus conference of 1988(xxiv) recommended that a sufficient urine dilution is achieved with a urine volume of at least 2 litres per day. Depending on environmental conditions and degree of physical activity, Siener and Hesse(xvi) propose that it would be necessary to drink between 2 and 3 litres per day to achieve this urine flow.

Last updated: November 2004

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