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October 2002, Volume 56, Number 10, Pages 1020-1023
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Original Communication
Effect of blackcurrant-, cranberry- and plum juice consumption on risk factors associated with kidney stone formation
T Kelera,b, B Jansenb and A Hesseb

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

Correspondence to: T Keler, Division of Experimental Urology, Department of Urology, University of Bonn, Sigmund-Freud-Str. 25, 53105 Bonn, Germany. E-mail:

aGuarantor: T Keler.

bContributors: All authors contributed to the design and the performance of the study and to the writing of the paper.


Objective: To evaluate the influence of plum-, cranberry- and blackcurrant juice on urinary stone risk factors.

Design: Investigations were carried out in 12 healthy male subjects aged 18-38 y. All subjects received a standardized diet formulated according to the dietary recommendations of the German Society of Nutrition. The subjects provided 24 h urine collections in a control, three loading phases. In each loading phase a neutral mineral water was substituted for 330 ml of the particular juice.

Results: Cranberry juice decreased the urinary pH, whereas the excretion of oxalic acid and the relative supersaturation for uric acid were increased. Blackcurrant juice increased the urinary pH and the excretion of citric acid. The excretion of oxalic acid was increased too. All changes were statistically significant. The plum juice had no significant effect on the urinary composition.

Conclusion: It is concluded that blackcurrant juice could support the treatment and metaphylaxis of uric acid stone disease because of its alkalizing effect. Since cranberry juice acidifies urine it could be useful in the treatment of brushite and struvite stones as well as urinary tract infection.

Sponsorship: Funded by our own Division respectively the University.

European Journal of Clinical Nutrition (2002) 56, 1020-1023. doi:10.1038/sj.ejcn.1601442


blackcurrant-, cranberry-, plum juice; urolithiasis; kidney stone; fluid intake


Although urolithiasis is a multifactorial disease, nutrition and especially fluid intake is one of the most important factors involved. A high fluid intake is therefore the first general advice for prevention of stone recurrences in stone-forming patients, irrespective of the stone composition (Pak et al, 1980; Borghi et al, 1996). An epidemiological study by Curhan et al (1996) has confirmed that increased fluid intake is associated with a reduced risk for kidney stone formation. Owing the dilutional effect, a large fluid intake in the treatment of stone diseases is irrespective of the stone composition. The risk for recurrent stone formation is greatly reduced when the urinary volume exceeds 2.0 l/day. To reduce urine concentration consistently it is recommended that the fluid intake should be distributed more or less evenly over 24 h (Hesse & Siener, 1997). Diluting urine has two major consequences. First, by increasing urine output, the concentration of constituent ions and the saturation of stone-forming salts are lowered. Second, fluid composition has a direct influence on urine composition and crystal formation (Hesse et al, 1993). Depending on the stone composition, several fluids have been found to be suitable, eg mineral water (Hesse et al, 1993; Borghi et al, 1996; Rodgers, 1997), orange juice (Hesse et al, 1993; Wabner & Pak, 1993), apple juice, fruit and herbal teas (Vahlensieck, 1986). Care should be taken to avoid fluids containing lithogenic agents which may increase the risk of stone formation, such as coffee, black tea, alcohol (Vahlensieck, 1986) and cola (Weiss et al, 1992; Rodgers, 1999).

Little is known about the influence of plum-, cranberry-, and blackcurrant juice on risk factors for kidney stone formation. A protective effect in the therapy of urinary tract infection was attributed to cranberry juice (Bodel et al, 1959; Avron et al, 1994). Bodel et al (1959) and Kahn et al (1967) observed an acidifying of the urine after ingestion of large amounts of cranberry juice and Howell et al (1988) identified proanthocyanidins as the compounds in cranberries that are responsible for the inhibition of the adherence of Escherichia coli to uroepithelial cells. Klingeberg (1972) found a slight decrease in urinary pH immediately after supply of 100 ml of blackcurrant juice. Blatherwick and Long (1923) showed an acidifying effect in human urine after consumption of prunes. However these studies had some shortcomings. The studies of Kahn et al (1967) and Bodel et al (1959) were limited to only four, respectively five, subjects and they ingested an unphysiologically high dosage of 1200-4000 ml per day. Blatherwick and Long (1923) studied only two persons. Finally Klingeberg (1972) estimated the pH values using indicator strips with a systematic error greater then the observed change in the mean pH value.

The present study was undertaken to overcome these shortcomings by evaluating the influence of plum-, cranberry- and blackcurrant juice on the urinary composition and therefore on multiple risk factors of kidney stone formation. The study was carried out under standardized condition in a relatively larger group of subjects and with a physiological ingestion of 330 ml of each juice daily.


All investigations were carried out using 12 healthy male subjects with no previous history of urolithiasis or other renal disorders. The mean age was 29.3 (range 18-38) y, their mean body mass index was 24.1 (range 20.1-26.3). In a pre-phase each subject was required to provide two 24 h urine collections under normal dietary conditions (urinalysis see below). Subjects with values out of the reference range were excluded from the study.

The test series were divided into four consecutive phases of 5 days each. The phases were equal but on the loading phases a neutral mineral water with no influence on the urinary composition (unpublished own results) was substituted for plum-, cranberry- and blackcurrant juice. Urine samples (24 h) were collected each day. All subjects received a standardized diet (Table 1) formulated according to the dietary recommendations of the German Society of Nutrition (Deutsche Gesellschaft für Ernährung, 1996). The diet consisted of normal food items to ensure consistency of the investigation results. The subjects were required to eat all of the food offered, and the same type and amount of food was consumed every day. Sport and other extreme exercise were not permitted during the experimental phase. A 4 day adaptation period on the standardized nutrition preceded each investigation day. On days 1-4 creatinine was measured to examine the compliance of participated subjects. Day 5 of each phase was chosen as control (neutral mineral water), respectively loading day with plum-, cranberry- and blackcurrant juice (for composition see Table 2). The study protocol is summarized in Table 3.

The urine was collected in polyethylene containers and mixed with 5 ml/l urine of a 5% solution of thymol in isopropanol to preserve the urine. During the collection period, the containers and their contents were maintained at 5°C. Each morning after completion of urine collec-tion the urine samples were brought to our laboratory by the study subjects. Immediately volume, specific gravity (urinometer) and pH (potentiometer) were recorded. Urine samples were tested for the presence of blood and infection. Nitrite-positive and haematuria samples were discarded. The methods used in the analysis of urine samples, with the relative CV for each method were as follows: Na, K and Ca by flame photometry, 1.3; Mg by xylidyl-blue reaction, 0.3; NH4+ by ion-selective electrode, 1.5; chloride by coulomb metric titration, 2.0; inorganic phosphate by phosphate molybdate reaction, <5; inorganic sulphate by nephelometry, <5; creatinine by Jaffé reaction, 2.0; uric acid by uricase method, <5; citric acid by citrate lyase method, 1.6; oxalic acid by HPLC enzyme reactor method (Hönow et al, 1997), 0.5 (Hesse & Bach, 1982; Hesse et al, 1997).

The relative supersaturations (RS) values for calcium oxalate, uric acid, brushite and struvite were calculated using EQUIL 2 (Finlayson, 1977; Werness et al, 1985). The Wilcoxon matched-pairs signed-rank test, as a non-parametric test of significance, was used testing two matched samples (control and loading day). The significance level was at 5%, and the P-values (two tailed) were *0.05P>0.01; **0.01P>0.001; ***P£0.001.


Several urinary parameters were altered following the consumption of each juice. The main results are reported in Table 4. The urinary volume in the control and the three loading phases were nearly equal, which represents the standardization of the investigated subjects.

In the dosage of 330 ml blackcurrant juice alkalized whereas cranberry juice acidified the 24 h urine statistically significantly. The citric acid excretion was increased statistically significantly after consumption of blackcurrant juice, but it was just slightly and not significantly decreased by cranberry juice. The oxalic acid excretion were increased statistically significantly after the exposure to both juices. The excretions of calcium, magnesium and uric acid were not significantly changed. The relative supersaturation for calcium oxalate, uric acid, brushite and struvite were not significantly affected by blackcurrant juice, however cranberry juice increased the relative supersaturation for uric acid significantly, and decreased, but statistically not significantly the relative supersaturation for struvite and brushite.

Plum juice had no observed effect of any of the urinary biochemical or physicochemical parameters in a statistically significant way.


The significant increase of the urinary pH value and the increased citric acid excretion after ingestion of blackcurrant juice did not decrease the relative supersaturation for calcium oxalate and uric acid, as might have been expected regarding the favourable changes in urinary composition for these stone compounds. On the other hand the oxalic acid excretion increased significantly as well which in our opinion is responsible for the lack of change in relative supersaturation. The increased oxalic acid excretion was caused be the oxalic acid content in blackcurrant juice (204 µmol/l) and especially the high content of ascorbic acid (177 mg/100 g), which is intermediately metabolized to oxalic acid and excreted in the urine. These findings conform with those of Hughes et al (1981) and Levine et al (1996), who found a statistically significant increase in the urinary excretion of oxalic acid after ingestion of megadoses of ascorbic acid. The alkalizing effect of the blackcurrant juice and the consecutively increased citric acid excretion is explained in the content of citric acid (2.88 g/100 g), the intake of about 9 g per day, and its metabolism to bicarbonate (Simpson, 1983).

The acidifying effect of cranberry juice was expected, but it is not known which compound in the cranberries is responsible for the decrease in urinary pH value. The significant increase in risk of uric acid stone formation is a consequence of the decreased pH value, however the relative supersaturation for struvite and brushite were slightly decreased. Probably the effect of cranberry juice on urinary composition would be bigger after ingesting a higher amount of juice, but because of the energy content of fruit juices the intake should not exceed 500 ml per day, and we also wanted to maintain physiological conditions. The explanation of the increased oxalic acid excretion is the same as that given for blackcurrant juice.

In contrast to Blatherwick and Long (1923) the ingestion of plum juice did not change the urinary pH-value.

Although the study was carried out on healthy subjects rather than in stone-forming patients, the findings suggests that blackcurrant juice could support the metaphylaxis and treatment of uric acid urolithiasis because of its alkalizing effect. Cranberry juice may be useful in supporting acidification of urine, as is indicated in the case of apatite, brushite and struvite stones and on urinary tract infection. We note that the results were obtained in healthy subjects and that further research is needed to evaluate the effects of cranberry and blackcurrant juice on urinary parameters in stone-forming patients.

The anti-infectious properties, not the subject of the present study, provide an additional beneficial effect for the patient.


The authors wishes to thank Mrs B Bär for the excellent technical assistance.


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Table 1 Standardized daily diet

Table 2 Composition of the fruit juices

Table 3 Study protocol

Table 4 Mean (s.e.m.) urinary biochemical and physicochemical parameters in 24 h urine before and after consumption of plum-, cranberry- and blackcurrant juice

October 2002, Volume 56, Number 10, Pages 1020-1023
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