IntroductionEpidemiologyI. Epidemiology of kidney stonesI.1. Prevalence of kidney stonesI.2. An increasing trend in childrenPathophysiology II. Pathophysiology of kidney stonesII.1. LithogenesisII.1.1. Urine supersaturation : the driving force of crystallogenesisII.1.2. Promoters and inhibitors of stone formationII.2. Urine volume and composition: a necessary balanceRisk factorsIII. Risk factors for kidney stonesIII.1. Individual, non-modifiable risk factorsIII.1.1 Family historyIII.1.2. Race and ethnicityIII.1.3. Age and genderIII.1.4. Current change in gender prevalenceIII.2. Lifestyle related factorsIII.2.1. Calcium intakeIII.2.2. Emerging dietary risk factorsIII.2.3. Association with other chronic diseasesDehydrationIV. Dehydration: a risk factor for kidney stonesIV.1. Low urine volume: a key risk factor for kidney stonesIV.2. Environmental factors predisposing to low urine volumeIV.2.1. Occupational risk of kidney stonesIV.2.2. Climate and temperature as risk factorsWater & recurrenceV. Prevention of stone recurrence with high water intakeV.1. Reduction of recurrence rate with increased water intakeV.2. Water intake and urinary parameters in stone formersWater & incidenceVI. Primary prevention of stones with high water intakeVI.1. Reduction of stone incidence with increased water intakeVI.2. Water intake and urinary parameters in healthy subjectsWater & health costsVII. Water intake and health costs of kidney stonesVII.1. Reduction of stone recurrence costs via adequate water intakeVII.2. Reduction of first stone costs with adequate water intakeRecommendationsVIII. Dietary and water recommendations for stone preventionVIII.1. Guidelines for the prevention of recurrence in patientsVIII.2. Dietary and water guidelines for general populationConclusion References
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Chronic dehydration is a confirmed risk factor for kidney stones (Brenner and Rector 2008). This has been clearly observed in a retrospective study investigating causes of urolithiasis on 708 stone formers. In this study, chronic dehydration was defined as history of exposure to heat (e.g. climate and or occupation), or poor drinking, with normal urine and plasma osmolality. Authors concluded that chronic dehydration was the main cause of 19% of all kidney stones incidents (Embon et al. 1990).
Dehydration or low water intake can lead to low urine volume, which is common in stone formers: A urine volume lower than 1.0L/d was reported in 10% of recurrent stone formers (Stitchantrakul et al. 2007), and a urine volume lower than 1.5L/d was found in 40% of recurrent and first time stone formers (Orakzai et al. 2004).
Observational studies have identified low urine volume as a risk factor for kidney stones. In a retrospective study published in 2008, all subjects from NHS I, NHS II and HPFS who provided a 24-h urine collection were evaluated. Among a total of 3,350 subjects, 2,237 had already had at least one episode of kidney stone. Results showed that subjects with a urine volume above 2.5L were at lower risk of developing kidney stones than subjects with a urine volume below 1.0L; the corresponding relative risks (RR) were 0.22 for NHS I subjects, 0.33 for NHS II and 0.26 for HPFS. In all three cohorts, a higher urine volume was associated with a reduced risk of kidney stones (Curhan and Taylor 2008).
Low urine volume leads to higher concentrations of urinary compounds and may promote urine supersaturation. Studies in general population have shown that a low urine volume increases the risk of forming a stone (Borghi et al. 1999c; Curhan 2007; Pak et al. 1980; Trinchieri et al. 2008).
Observational studies have reported higher incidence rates of kidney stones among subjects working in hot environments. In a prospective study from 1993, Borghi et al. compared prevalence of kidney stones between machinists working in hot environment and control subjects working at a mild temperature, and observed higher prevalence among machinists (8.4%, compared to a prevalence of 2.5% in controls)(Borghi et al. 1993). More recently, a cross-sectional study carried out on a population of workers from a steel factory showed that hot-area workers presented a higher risk of kidney stones and lower urine volumes than employees who were stationed in a mild environment (Atan et al. 2005).
Higher incidence rates of kidney stones were also observed in marathon runners. This may be a result of repeated, although short-term episodes of substantial dehydration (Irving et al. 1986; Milvy et al. 1981). Authors observed that in marathon runners, crystalluria and urine supersaturation were similar to stone formers, and significantly different from healthy subjects who did not run (Irving et al. 1986).
Climate and temperature vary greatly between countries and could partly explain how geography influences the risk of developing kidney stones (Figure 1). Other factors such as nutritional and lifestyle habits could also explain these variations between countries. Epidemiological studies have shown variations in prevalence within countries (Soucie et al. 1996). This was observed in 1963 in a hot arid region of Israel, where the highest incidence of stones were noted in the hottest regions (Frank et al. 1963). Variations of prevalence were also observed within the United States: a «stone-belt» was identified after a higher prevalence of kidney stones was observed among the warmest states (Brikowski et al. 2008). Boyce et al. found that prevalence of kidney stones was the highest in the southern states of South Carolina and Georgia, and the lowest in Wyoming and Missouri (Boyce et al. 1956). Likewise, a retrospective analysis of data from NHANES II and from the Cancer Prevention Study II (CPS II), highlighted that the age-adjusted prevalence of kidney stones increased from north to south and from west to east (Soucie et al. 1994). After controlling for other risk factors, prevalence appears to be correlated to ambient temperature and sunlight exposure (Soucie et al. 1996). Overall, the higher prevalence in warmer regions is due to high water losses and inadequate water intake leading to low urine volume (Brikowski et al. 2008; Soucie et al. 1994).
Incidence of kidney stones appears to vary seasonally, with higher incidences generally recorded in the warmer months of spring and summer (Bartoletti et al. 2007; Chauhan et al. 2004; Chen et al. 2008). Yet, in one study carried out in Iran, authors observed the highest incidences in the months of June, July and November (Basiri et al. 2004). This was mostly attributed to positive correlation between mean ambient temperatures and incidence of kidney stones (Chauhan et al. 2004; Chen et al. 2008; Fletcher et al. 2012; Soucie et al. 1994; Soucie et al. 1996).
Higher temperatures can lead to greater water losses through sweating, resulting in a lower urine volume. This could explain the increased risk of urolithiasis (Bartoletti et al. 2007; Frank et al. 1963) (Figure 7).
Figure 7. Conditions related to chronic dehydration, predisposing to a low urine volume
and thereby increasing the risk of kidney stones.
Take home messages
It is essential to avoid dehydration by maintaining an adequate water intake, particularly in situations of acute water losses. In these situations, insufficient water intake leads to uncompensated water losses, a low urine volume and a higher risk of kidney stones.