Effects of lemon–tomato juice consumption on crystal formation in the urine of patients with calcium oxalate stones: A randomized crossover clinical trial

Abstract

Background

Dietary supplementation with citrate-containing juices may serve as an effective alternative to potassium citrate therapy for preventing calcium oxalate stone recurrence. This study was performed to evaluate whether consumption of lemon–tomato juice can decrease the tendency for stone formation in the urine of calcium oxalate stone formers.

Materials and methods

The study was conducted as a prospective interventional randomized crossover clinical trial with a repeated-measures design. Twenty-two patients with calcium oxalate stones and no metabolic abnormalities in the urine treated with lithotripsy at a tertiary care center from August 2017 to July 2018 were recruited. After a 14-hour overnight fasting, urine samples were collected after the patients consumed either milk only or milk and lemon–tomato juice. Their urine was tested for multiple parameters, including urine pH, specific gravity, calcium–creatinine ratio, and supersaturation with sodium oxalate, followed by optical density measurement via spectrophotometry.

Results

There were no significant differences in the background characteristics between the 2 groups. The optical density of the urine samples obtained after consumption of milk only was significantly higher than that after consumption of milk and lemon–tomato juice (mean = 0.131 for milk only vs. 0.053 for milk and lemon–tomato juice, p < 0.001). The urine calcium–creatinine ratio was similar between the groups (mean = 0.141 for milk only vs. 0.076 for milk and lemon–tomato juice, p = 0.019).

Conclusions

The addition of lemon–tomato juice as a source of citrate in the diet significantly decreases the established risk factors for calcium oxalate stone formation in patients. This study was prospectively registered at CTRI under number CTRI/2017/04/008312 on April 7, 2017.

1. Introduction

Calcium oxalate (CaOx) stones are the most common kidney stones, followed by calcium phosphate and uric acid stones. Supersaturation of urine with crystal components—calcium and oxalate in CaOx stone formers—is a typical event that occurs during stone formation.^[1,2] As the supersaturation of a substance increases, the risk of particle interaction and crystal formation increases.^[3] CaOx stone formation is a multistep process that involves nucleation and crystal growth, aggregation, and retention in a microenvironment of supersaturation and urine pH alteration.^[4,5]

Potassium citrate is commonly prescribed to recurrent CaOx stone formers to maintain urine alkalinity and reduce the urinary calcium level.^[6] However, long-term patient compliance is generally low owing to the ulcerogenic potential of potassium citrate leading to gastrointestinal symptoms, in addition to other factors such as cost of the therapy.^[7] Dietary supplementation with citrate-containing juices may serve as an effective alternative to potassium citrate therapy.^[8,9] Both lemon and tomato juices have been shown to contain a high concentration of citric acid and low concentration of oxalate.^[9–11] This study was conceptualized to determine whether consumption of lemon–tomato juice can decrease the tendency for crystal formation in the urine of CaOx stone formers.

2. Materials and methods

2.1. Patient population

Thirty-four patients aged between 13 and 50 years with CaOx stones in the kidney or upper ureter who were evaluated for their stone and treated with extracorporeal shock wave lithotripsy in the urology department of our tertiary care hospital from August 2017 to July 2018 were initially recruited; however, only 22 met the inclusion criteria (Fig. 1).

Figure 1.

CONSORT flow diagram.

Patients were excluded from the study when they had urinary stones that were not based on calcium (uric acid, cystine, or struvite); had obstructive uropathy, chronic urosepsis, positive urine culture, gross hematuria, renal failure (serum creatinine level of >2.0 mg/dL), renal tubular acidosis, medullary sponge kidney, anatomical urinary tract abnormalities, or active cancer; were pregnant; or were unable to provide informed consent. This study was approved by the ethics committee of our hospital, and all patients provided written informed consent to participate in the study. The study was registered prospectively at a clinical trials registry.

2.2. Randomization procedure

This prospective interventional randomized crossover clinical trial had a repeated-measures design in which each patient was randomly assigned to a sequence of interventions in the form of diet. The sample size was calculated to be 22 patients, who were then randomly allocated to 2 groups at an allocation ratio of 1:1 following simple randomization using a computer-generated list of random numbers, with 11 patients in each group. The first group (group 1) was provided with 200 mL of milk only, and the second group (group 2) with 200 mL of milk followed by 200 mL of lemon–tomato juice. Three days later, the patients were asked to return, and the dietary intervention was reversed (200 mL of milk followed by 200 mL of lemon–tomato juice for group 1 and 200 mL of milk only for group 2). The random allocation sequence was generated by the second author, and individual sealed envelopes for each patient numbered from 1 to 22 containing the group to which that patient belonged were handed over to a laboratory technician. The first author enrolled and sent the participants to the laboratory technician who opened the sealed covers and assigned them to the interventions.

2.3. Intervention methodology

All patients were instructed to fast overnight for 14 hours from 6:00 pm to 8:00 am, with an allowed intake of 500 mL of mineral water only to avoid dehydration, and an early morning urine sample (EMU) was collected. The patients were then provided with 200 mL of milk only and were adequately hydrated with mineral water of known calcium content until urine samples were collected 4 hours later for analysis. The second batch of tests was conducted after 3 days of normal diet. The procedure followed was the same as above, except that the patients were provided with 200 mL of milk followed by 200 mL of lemon–tomato juice prepared by mixing in a blender one whole lemon removed of its seeds and four average-sized tomatoes removed of their seeds. Neither water, sugar, nor salt was added to the juice obtained. Milk was provided as a source of calcium and lemon–tomato juice as a source of citrate.

2.4. Urine analysis

The collected urine samples were assayed to evaluate the urine pH, specific gravity, and calcium–creatinine ratio (UCCR). They were then subjected to crystal aggregation assay for spectrophotometric estimation of the optical density (OD) during supersaturation at 620 nm wavelength.^[1] Two milliliters of the centrifuged urine samples were poured into the cuvette, and further analysis was performed in the cuvette. Thereafter, 10 μL of sodium oxalate was added per minute to the urine samples with continuous stirring to a maximum of 150 μL. The OD was measured every minute for 61 minutes.

2.5. Statistical analysis

Continuous variables were expressed as means and standard deviations (SDs), and statistical significance for comparisons was determined using unpaired and paired t tests. A p value of <0.05 was considered statistically significant. Data were analyzed using Statistical Product and Service Solutions version 24. The mean and SD values obtained were first compared between the 2 groups using an unpaired t test to assess whether there were any significant differences depending on which diet was administered first. Thereafter, we compared the values of the EMU samples collected on day 1 and day 4 using a paired t test to evaluate whether there was any discordance in the values of the samples obtained from the same participants 3 days apart. Finally, we grouped all values obtained after consumption of milk alone and compared them with the values obtained after consumption of milk and lemon–tomato juice using a paired t test to determine whether there was any significant difference depending on the nature of the diet that was administered.

3. Results

Twenty-two patients, including 16 men and 6 women, participated in the study. They were divided into 2 groups of 11 patients each, comprising 8 men and 3 women. The 2 groups showed no significant differences in the mean age, pH, specific gravity, and UCCR as shown in Table 1.

Table 1.

Comparison of the mean values between the two groups.

	Age	Early morning urine—Day 1			Early morning urine—Day 4
		pH	Specific gravity	UCCR	pH	Specific gravity	UCCR
Group 1	41.2 ± 7.76	5.87 ± 0.508	1.017 ± 0.006	0.081 ± 0.056	5.85 ± 0.689	1.014 ± 0.007	0.072 ± 0.039
Group 2	40.9 ± 9.62	5.74 ± 0.468	1.020 ± 0.006	0.061 ± 0.023	5.62 ± 0.352	1.020 ± 0.008	0.051 ± 0.016
p	0.942	0.554	0.234	0.287	0.336	0.069	0.120

Values are presented as means ± SDs. SD = standard deviation; UCCR = urine calcium–creatinine ratio.

Among the EMU samples obtained from the same participants 3 days apart, no significant differences were found in the mean pH, specific gravity, and UCCR as displayed in Table 2.

Table 2.

Comparison of the mean values of the EMU samples obtained 3 days apart.

	pH			Specific gravity			UCCR
	EMU—Day 1	EMU—Day 4	p	EMU—Day 1	EMU—Day 4	p	EMU—Day 1	EMU—Day 4	p
Group 1	5.87 ± 0.508	5.85 ± 0.689	0.943	1.017 ± 0.006	1.014 ± 0.007	0.172	0.081 ± 0.056	0.072 ± 0.039	0.602
Group 2	5.74 ± 0.468	5.62 ± 0.352	0.464	1.020 ± 0.006	1.020 ± 0.008	0.756	0.061 ± 0.023	0.051 ± 0.016	0.161
All 22 patients	5.81 ± 0.481	5.74 ± 0.547	0.605	1.019 ± 0.006	1.017 ± 0.008	0.176	0.071 ± 0.043	0.061 ± 0.031	0.292

Values are presented as means ± SDs. EMU = early morning urine; SD = standard deviation; UCCR = urine calcium–creatinine ratio.

In the comparison between the groups, we found that the mean UCCR after consumption of milk only was 0.141 (SD = 0.144), whereas that after consumption of milk and lemon–tomato juice was 0.076 (SD = 0.068). This difference was significant (p = 0.019) as shown in Table 3.

Table 3.

Effects of consumption of milk only and milk and lemon–tomato juice on the UCCR and OD.

	UCCR			OD
	Milk only	Milk + lemon–tomato juice	p	Milk only	Milk + lemon–tomato juice	p
Group 1	0.205 ± 0.182	0.106 ± 0.083	0.017	0.092 ± 0.061	0.034 ± 0.062	<0.001
Group 2	0.077 ± 0.038	0.046 ± 0.029	0.024	0.170 ± 0.120	0.071 ± 0.101	0.003
All 22 patients	0.141 ± 0.144	0.076 ± 0.068	0.019	0.131 ± 0.101	0.053 ± 0.085	<0.001

Values are presented as means ± SDs. OD = optical density; SD = standard deviation; UCCR = urine calcium–creatinine ratio.

The mean OD (maximum OD obtained from the urine samples) in the crystal aggregation study of the urine samples after consumption of milk only was 0.131 (SD = 0.101), whereas that after consumption of milk and lemon–tomato juice was 0.053 (SD = 0.085). This difference was also significant (p < 0.001).

4. Discussion

The UCCR after consumption of milk only was significantly higher than that after consumption of milk and lemon–tomato juice. This clearly shows that consumption of lemon–tomato juice significantly decreases the amount of calcium excreted in the urine. This is in agreement with the statement by Rimer et al. that citrate plays a crucial role in lowering urine calcium levels via sequestration of calcium ions through the formation of soluble citrate–calcium complexes, which are more soluble in the urine at a physiological urine pH.^[12]

The crystal aggregation assay via supersaturation showed that the OD after consumption of milk only was significantly higher than that after consumption of milk and lemon–tomato juice. This finding implies that the tendency for crystal formation in patients treated for CaOx stones can significantly decrease if lemon–tomato juice is added to the diet as a source of citrate. This is consistent with the report by Barghouthy and Somani that in the urine, citrate from citrus and noncitrus fruit juices inhibits the spontaneous nucleation and agglomeration of CaOx crystals^[13]; however, such finding is in contrast to that of Ho et al. that consumption of citrate-containing juices fails to reduce CaOx crystal formation.^[14]

The administration of alkaline citrate salts is commonly recommended for the medical treatment of renal stones, although compliance to this treatment is limited by the gastrointestinal side effects and cost.^[7,15] As a natural source of dietary citrate, citrus fruits and juices have been used as alternatives to potassium citrate. Owing to its high concentration of citrate, lemon juice is the most commonly recommended citrus fruit juice to patients.^[16] An important but largely underestimated factor is the high sugar content of the juice consumed; lemon juice is usually consumed with added sugar or salt, both of which are established risk factors for urolithiasis.^[13,17] Hence, lemon juice should be consumed without added salt or sugar, but this makes it unpalatable and may be associated with gastrointestinal disturbances in many patients. Such recommendation can contribute to decreased patient adherence to long-term lemon juice supplementation.^[18] This may explain the fact that many studies do not show the expected clear protective role of lemon juice against stone formation.^[13,19] The high citrate and magnesium contents and the low sodium and oxalate contents of tomato juice have been established by various studies, making this juice an ideal candidate for the prevention of stone formation. However, poor patient compliance has been reported even with long-term tomato juice supplementation.^[10,20] In contrast, lemon–tomato juice is palatable to the majority of patients even if consumed without added sugar and salt, and its consumption is not associated with severe gastrointestinal disturbances; hence, lemon–tomato juice may be a more acceptable option for long-term juice supplementation. The results of this study clearly indicate that the addition of lemon–tomato juice as a source of citrate in the diet significantly decreases the established risk factors for CaOx stone formation. Accordingly, lemon–tomato juice supplementation may significantly reduce the tendency for CaOx stone formation in patients. This potential can be attributed to various mechanisms, including increase in the urine volume owing to increased consumption of liquid, increase in the excretion of citrate in the urine, and decrease in the excretion of sodium and oxalate, all of which may contribute to the reduction in CaOx stone formation. A limitation of the present study is that although the patients were randomly allocated to 2 groups, blinding was not possible. Future long-term prospective randomized trials comparing potassium citrate therapy with lemon–tomato juice supplementation in a larger patient population are needed before dietary recommendations can be made.

5. Conclusions

The addition of lemon–tomato juice as a source of citrate in the diet significantly decreases the established risk factors for CaOx stone formation. This finding indicates that lemon–tomato juice supplementation may reduce the tendency for recurrent CaOx stone formation in patients. More long-term prospective randomized trials comparing potassium citrate therapy with lemon–tomato juice supplementation in a larger patient population are needed before dietary recommendations can be made.

Acknowledgments

The authors wish to acknowledge Mr Jayakumar, a statistician, for providing advice regarding proper statistical methods and Mrs Rejitha, a laboratory technician, for helping in the analysis of the urine samples.

Statement of ethics

This study was approved by the human ethics committee of Medical College, Thiruvananthapuram, under IEC number 05/12/2017/MCT on March 24, 2017. All participants provided their written informed consent for this study. This study was also prospectively registered at Clinical Trials Registry–India (CTRI) under number CTRI/2017/04/008312 on April 7, 2017 (http://www.ctri.nic.in/Clinicaltrials/pmaindet2.php?trialid=18402). All procedures were performed in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Conflict of interest statement

The authors declare no conflicts of interest.

Funding source

Sathish K. Gopala received a grant from the State Board for Medical Research, Government Medical College, Thiruvananthapuram, Kerala, India, a government funding agency, under grant number A2/SBMR(2016–2017)/27717/2016/MCT.

Author contributions

SKG: Study concept and research design, data acquisition, and critical manuscript revision;

JJ: Study concept and research design, data analysis, and original manuscript draft writing;

JC: Data analysis and original manuscript draft writing.