Original Communication

European Journal of Clinical Nutrition (2003) 57, Suppl 2, S47–S51. doi:10.1038/sj.ejcn.1601901

Fluid intake and epidemiology of urolithiasis

R Siener1 and A Hesse1

1Division of Experimental Urology, Department of Urology, University of Bonn, Germany

Correspondence: Roswitha Siener, Experimentelle Urologie, Klinik und Poliklinik für Urologie, Universität Bonn, Sigmund-Freud-Str. 25, D-53105 Bonn, Germany. E-mail: Roswitha.Siener@ukb.uni-bonn.de

Guarantor: R Siener.

Contributors: Both authors collaborated in writing the paper.

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Abstract

A low urine volume is an important risk factor in urinary stone formation. The present article summarizes available data from epidemiological and clinical studies to elucidate the impact of fluid intake and urine volume on the risk of urinary stone formation and the prevention of stone recurrence. A review of the literature shows that an increased urine volume achieved by a high fluid intake exerts an efficacious preventive effect on the onset and recurrence of urinary stones. A high water intake and urine dilution results in a marked reduction in saturation of lithogenous salts. The type of fluids should be carefully selected to achieve the appropriate change of urine composition depending on stone composition. A sufficient intake of fluid is one of the most important preventive measures for stone recurrence.

Keywords:

fluid intake, urine volume, urinary stone disease

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Introduction

The most important preventive measure for stone recurrence is a sufficient urine dilution accomplished by a generous intake of fluid. Based on the results of various epidemiological and clinical studies, the article summarizes the research data and examines the impact of fluid intake and urine volume on the risk of urinary stone formation and the prevention of stone recurrence.

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Urine dilution and crystallization of lithogenous salts

Theoretically, a high fluid intake could inhibit stone formation by lowering the urinary concentration of stone-forming constituents, but could simultaneously dilute the urinary concentration of inhibitors (Table 1). The effect of urinary dilution on crystallization of stone-forming salts has been evaluated by a study of Pak et al (1980) in normal subjects and stone formers. The dilution of urine, achieved in vitro (1–2 l/day) by addition of distilled water to urine or in vivo (1.0–2.4 l/day) by the intake of distilled water on a constant metabolic diet, reduced the propensity for crystallization of calcium salts in urine by lowering the urinary activity product ratio of calcium oxalate and brushite, and by increasing the formation product ratio, that is, the minimum supersaturation required to elicit spontaneous nucleation of calcium oxalate. Moreover, in vivo dilution of urine did not significantly alter urinary excretion of promotors (calcium, oxalate, phosphorus, uric acid) or inhibitors (magnesium, citrate) of stone formation, or urinary pH value. The results suggest that an increased fluid intake and subsequently urine dilution could have a protective effect on the crystallization of calcium salts (Pak et al, 1980; category of evidence (CE) IIa according to Eccles et al., 1998).


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Chronic dehydration

A consistently low urine volume is mainly due to a low fluid intake or increased respiratory-cutaneous water loss. Results of a retrospective study in 708 patients attending a metabolic stone clinic support the evidence that patients exposed to chronic dehydration have a particularly high incidence of urolithiasis. Chronic dehydration as the cause of stone formation was found in 19% of the patients (Embon et al, 1990, CE III).

Epidemiological studies have shown that populations exposed to chronic dehydration caused by high ambient temperatures, high degree of physical activities and insufficient replacement of water losses have a particularly higher incidence and prevalence of urolithiasis. In Israel, the risk of stone formation was found to be substantially greater in individuals living in villages in the hot arid areas compared with those residing in more temperate regions (Frank et al, 1963, CE III). In the USA, the prevalence of urolithiasis has been reported to be higher in the Southeast than in the Northwest, and in all other regions of the United States combined (Curhan et al, 1994, CE III; Soucie et al, 1996, CE III). A higher incidence of stones occurred in British Naval personnel stationed in a Mediterranean-type climate and in a tropical environment compared with the personnel posted in the United Kingdom (Blacklock 1969, CE IV). Moreover, seasonal variations in the incidence of renal colics and stones, observed during and after a hot season, have been attributed to a reduction in urine volume at higher temperatures (Burkland & Rosenberg, 1955, CE IV; Prince & Scardino, 1960, CE III; Fujita, 1979, CE III).

A survey of participants in the 1977 New York City Marathon found that the incidence of stones in these runners was three to five times greater than in the matched population. The higher risk of urinary stone formation could arise as a result of dehydration during long-distance running (Milvy et al, 1981, CE III; Irving et al, 1986, CE III).

The prevalence of stone disease in workers chronically exposed to dehydration due to heat stress was significantly higher compared to controls working in normal temperature (Borghi et al, 1993, CE IIb).

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Preventive effects of fluid intake

Despite the evidence that the urine volume was the most important risk factor of all parameters evaluated, there are few studies carried out to support the assumption of prophylactic effects of an adequate fluid intake. Frank et al (1966) compared the incidence of urinary stone formation from two desert towns in Israel. In one settlement, the inhabitants participated in an educational program on increased fluid intake as a preventive measure, whereas the control population in a nearby town was not informed about the necessity of an adequate fluid intake for the prevention of urinary stone formation (Frank et al, 1966, CE IIb). Both groups were comparable regarding their living conditions. A follow-up after 3 years confirmed that urine volume was significantly lower in the control population (0.8 vs 1.1 l/day), and the incidence of urolithiasis was significantly higher than among the informed group. The results indicate that the increase in urine volume was capable of preventing urinary stone formation in hot dry desert areas.

A long-term prospective study in patients with idiopathic, recurrent urolithiasis demonstrated that patients with relapse during at least 2 years of follow-up had increased their urine volume to a lower extent (Delta -0.02plusminus0.48 l/24 h) compared with the patients who remained stone-free (Delta 0.23plusminus0.54 l/24 h) (Strauss et al, 1982, CE IIa). A stepwise discriminant analysis for assessment of the contribution of different variables revealed the change in urine volume as an independent risk factor for stone recurrence.

Hosking et al (1983) reviewed the clinical courses of 108 patients with idiopathic calcium stone disease on a conservative management program of increased fluid intake and elimination of dietary excesses alone and with a mean total follow-up of 76.0plusminus3.7 months (Hosking et al, 1983, CE III). There was no evidence of stone growth or new stone formation (metabolic inactivity) in 58.3% of the patients after a mean follow-up of 62.6 months. Comparison of initial and follow-up 24-h urine volumes demonstrated a significant increase in patients who were metabolically inactive at follow-up (2136plusminus109 ml/day; Delta 494plusminus110 ml/day), while no increase was detected in patients who were metabolically active at follow-up (1726plusminus73 ml/day; Delta 95plusminus88 ml/day).

In a prospective study in a cohort of 45 619 men, who had no history of kidney stones, an inverse association between fluid intake and the risk of urinary stones was observed during 4 years of follow-up (Curhan et al, 1993, CE IIa). After simultaneous adjustment for other potentially confounding factors in the multivariate analysis, the relative risk for men dropped significantly from 1.0 in the lowest quintile (< 1275 ml/day) to 0.71 in the highest quintile (greater than or equal to2500 ml of fluid per day). This finding in men was consistent with results of a long-term prospective study conducted on 91 731 women (Curhan et al, 1997, CE IIa). Fluid intake was found to be inversely related to the risk of stone formation: in the multivariate model, the relative risk for women with the highest fluid intake was 0.61, as compared with those with the lowest intake (1.0).

The sole prospective randomized study on the role of fluid intake as a preventive measure in urinary stone formation was performed by Borghi et al (1996). They studied 101 controls and 199 patients from the first idiopathic stone episode. After a baseline study period, the stone formers were randomized into two groups. The first group was precisely instructed to increase fluid intake to achieve a urine volume of at least 2 litres a day without any dietetic change, while the second group did not receive any treatment. The patients were followed prospectively for a period of 5 years with clinical, laboratory and radiological evaluation each year to determine the urinary stone risk profile, recurrence rate and mean time to relapse. It thus emerged that the baseline urine volume was significantly lower in male and female stone patients compared to normal subjects. During the 5-year follow-up period, patients in the intervention group had significantly higher urine volumes (2.1–2.6 vs 1.0–1.2 l/24 h), a 50% lower recurrence rate (27 vs 12.1%), and a longer time to first recurrence (38.7plusminus13.2 vs 25.1plusminus16.4 months). The results corresponded to a significant difference in the saturation of lithogenous salts between the two groups. The study confirms that urine volume is a real risk factor in urolithiasis and that an increase in fluid intake to at least 2 l/day is the initial therapy for the prevention of stone recurrences (Borghi et al, 1996, CE Ib).

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Types of beverages

There are conflicting data from clinical and epidemiological studies on the effect of various beverages on the risk of urinary stone formation. Assessing the results of a prospective postal survey relating to 21 specific types of beverages, the risk of stone formation was found to be reduced by the daily intake of caffeinated and decaffeinated coffee by 10%, tea (14%), beer (21%) and wine (39%) (Curhan et al, 1996, CE III). In a case–control study of six beverages, the intake of beer has already been reported to be associated with a decreased risk of stone formation (Krieger et al, 1996, CE III). In contrast, the risk of stone formation markedly increased by 35% for apple juice and 37% for grapefruit juice (Curhan et al, 1996, CE III).

However, according to the results of controlled clinical trials, alkalizing beverages, that is, citrus fruit juices, have been found to be suitable for the prevention of calcium oxalate, uric acid and cystine stones. The ingestion of 1.2 litres orange juice resulted in a significant increase in urinary pH and citrate excretion (Wabner & Pak, 1993). Whereas Wabner and Pak (1993) observed an increase in urinary oxalate excretion, we could not confirm a change in urinary oxalate excretion in healthy subjects (Hesse et al, 1993). The intake of lemon juice, with a nearly five times higher citrate concentration compared to orange juice, led to a two-fold rise in urinary citrate levels in patients with hypocitraturic calcium nephrolithiasis (Seltzer et al, 1996, CE IIa).

Moreover, studies on the effect of fluids containing lithogenic agents which may increase the risk of stone formation have been carried out. In healthy subjects, cola has been found to significantly increase urinary oxalate excretion in both sexes (Rodgers, 1999), and the consumption of beer resulted in a reduction of urinary pH and an increase in urinary uric acid excretion (Hesse et al, 1993). In a randomized controlled trial in male stone patients, Shuster et al (1992) were able to demonstrate a strong association between the amount of soft drink consumption (acidified with phosphoric acid) and the recurrence of urinary stone formation in the course of a 3-year follow-up (Shuster et al, 1992, CE Ib).

In Europe, mineral waters enjoy general popularity. Since the mineral and bicarbonate content vary greatly, the composition of different mineral waters has to be taken into account. Two bicarbonate-rich mineral waters (1715 and 3388 mg/l, respectively) have been found to significantly increase urinary pH and citrate excretion in healthy subjects (Hesse et al, 1993). These effects have been shown to be similar to that of sodium–potassium citrate, a well-established therapy for urinary stone disease (Kes zligler & Hesse, 2000). The alkalizing effect due to the high bicarbonate content of mineral water is desired in the treatment of calcium oxalate, uric acid and cystine stones, whereas it is contraindicated in struvite stones. Considering the effect of a high calcium content of water on urine composition and the risk of calcium stone formation, the majority of the controlled clinical trials in calcium stone patients observed increases in urinary calcium excretion (Jaeger et al, 1984, CE IIb; Ackermann et al, 1988, CE IIa; Marangella et al, 1996, CE IIa; Caudarella et al, 1998, CE IIa). However, this lithogenic effect was counterbalanced by a decrease in urinary oxalate excretion, presumably by an increased oxalate complexation by calcium in the gut and subsequent reduced intestinal absorption of free oxalic acid. The results of retrospective trials on the relationship between drinking water hardness and urinary stone disease have been inconsistent. Whereas a higher incidence of urinary stones has been reported in soft-water areas in the USA (Sierakowski et al, 1979, CE III), no significant correlation between water hardness and urinary stone disease has been found after adjusting for other environmental factors (Shuster et al, 1982, CE III).

Analysis of 150 urinary calculi showed that the fluoride content of water results in the inclusion of fluoride in the calculus substance. In areas with fluoridation of drinking water, a significantly higher fluoride content was measured in calcium oxalate stones, whereas the degree of crystallization of carbonate apatite stones increased with increasing percentage of fluoride content of the stones (Hesse et al, 1978, CE IIb).

If the stone composition is so far unknown, beverages should be urine neutral. This means that urine dilution must be accomplished without changing the quantitative composition of urine. Suitable beverages are therefore mineral waters with a low content of mineral salts and bicarbonate.

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Conclusions

In conclusion, a sufficient dilution accomplished by an adequate intake of fluid is the most important therapeutic measure irrespective of stone composition or the cause of stone formation. The findings from epidemiological studies support evidence that a sufficient urine dilution is achieved with a urine volume of at least 2 l/day (Consensus Conference, 1988, CE IV). Depending on the environmental temperature and the degree of physical activity, it is usually necessary to drink between 2 and 3 l/day to achieve this urine flow. The long-term efficacy of some fluids for the prevention of different types of stone is still to be assessed.

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