Abstract
Objectives
To compare body mass index (BMI); serum parameters; and urine parameters between patients with and without urolithiasis.
Methods
Data from 1164 patients admitted to our Department of Urology from January 2011 to July 2013 were retrospectively reviewed; 714 patients (age, 5–87 years; male:female ratio, 1.8:1) exhibited urolithiasis, and 450 patients (age, 12–94 years; male:female ratio, 3.8:1) did not. Blood and urine were collected from patients the morning after hospital admission. Serum and urine parameters were checked by an automatic biochemistry analyzer. Statistical analysis included the Mann–Whitney U test and binary logistic regression.
Results
Serum sodium, potassium, chloride, calcium, phosphorus, and carbon dioxide combining power significantly differed between groups. In male patients, serum sodium, calcium, and phosphorus levels were higher in the urolithiasis group, whereas serum potassium and urine pH levels were lower. In female patients, serum sodium was higher in the urolithiasis group. BMI was higher in the urolithiasis group in all patients, male and female. Respective β-values of serum sodium and BMI in male patients were 0.077 and 0.084; in female patients, these values were 0.119 and 0.102.
Conclusions
Changes in serum sodium and BMI may be involved in the pathogenesis and treatment of urolithiasis.
Keywords: Urinary calculus, electrolyte, serum sodium, body mass index, serum chloride, serum phosphorus, serum carbon dioxide, serum potassium, serum calcium, serum pH
Introduction
Urolithiasis is a frequently occurring global disease. The incidence of urolithiasis is increasing in Europe and the United States1–4 and ranges from 0.12% to 6.02% in China.5 In China, there is a significant regional difference in the incidence of urolithiasis, such that it is higher in the south than in the north.6 China is one of three regions with a high incidence of urolithiasis (the other regions comprise the United States and Europe); moreover, urolithiasis is one of the most common diseases in China. In addition, there is a growing epidemic of overweight worldwide. Several studies have analyzed the relationship between body weight and abnormal levels of various urinary constituents of stone promoters and inhibitors (e.g., urinary calcium, citrate, sulfate, phosphate, oxalate, uric acid, and pH).7–9 Taylor et al.10 first demonstrated the elevated risk of incident urolithiasis in both men and women with increasing body mass index (BMI). However, little is known regarding the association of serum electrolyte constituents with urinary stone formation among individuals with urolithiasis. Serum electrolyte ions (e.g., calcium, phosphorus, sodium, chloride, and potassium) may contribute to formation of urinary stones.11,12 The present study was performed to compare serum calcium, phosphorus, potassium, sodium, chloride, and carbon dioxide combining power (CO2CP), as well as urine pH, urine specific gravity (SG), and BMI between patients with and without urolithiasis.
Patients and methods
Study population and selection criteria
This retrospective study was approved by the institutional review board of The Affiliated Hospital of Qingdao University, which waived the requirement for informed consent. This study constituted a retrospective review of a series of consecutive patients admitted to the Department of Urology from January 2011 to July 2013. Inclusion criteria for the urolithiasis group consisted of a diagnosis of “calculus of kidney,” “calculus of ureter,” or “calculus of bladder and urethra.” These diagnoses were confirmed by the presence of ultrasonographic or radiographic evidence (e.g., X-rays of kidney, ureters, and bladder), as well as intravenous pyelography. A search for “uric acid calculus” diagnoses was performed among ultrasonography or computed tomography records. Patients with renal colic, abnormal pain, hematuria, interrupted micturition, and urinary sediment were also included. The non-urolithiasis group included all other patients admitted to the Department of Urology during the same time period. Patients with a confirmed personal or family history of urolithiasis, metabolic abnormality, endocrine abnormality, rheumatological abnormality, gastrointestinal abnormality, surgical genitourinary reconstruction, or traumatic disease were excluded to avoid confounding bias.
Methods
All study participants were evaluated with the same standard protocol during normal dietary intake. Blood and urine were collected from patients on the morning after hospital admission. During the collection period, the urine was kept in sterile, closed containers at 4°C without addition of preservatives; all measurements were conducted within 4 hours after the end of the collection period. Following urine sample collection, blood sample collection was performed. For each blood sample, the following serum parameters were measured by standard laboratory technique using a Beckman Synchron CX5 automatic biochemistry analyzer (Beckman Instruments, Brea, CA, USA): calcium, phosphorus, potassium, sodium, chloride, and CO2CP. Urine samples were analyzed to determine pH and specific gravity using a commercial kit (Boehringer Mannheim, Mannheim, Germany). BMI was calculated as weight in kilograms divided by the square of height in meters (kg/m2) from self-reported patient height and weight values at admission.
Statistical analysis
Statistical data were analyzed with SPSS Statistics, version 19.0 (IBM Corp., Armonk, NY, USA). Results were expressed as mean ± standard deviation; p values <0.05 were considered to be statistically significant. Student’s t-test and the non-parametric Mann–Whitney U test were used for comparisons between two groups. Binary logistic regression was used to further analyze the effects of serum electrolyte levels, urine pH, urine SG, and BMI on urolithiasis, compared between men and women. Multiple linear regression was used to analyze associations of BMI with serum electrolyte level, urine pH, and urine SG in patients with urolithiasis.
Results
The final study population included 1164 patients; 714 patients exhibited urolithiasis (463 male patients and 251 female patients). The remaining 450 patients did not exhibit urolithiasis (358 male patients and 92 female patients). Patient age in the urolithiasis group ranged from 5 to 87 years (mean age, 50.6 ± 14.7 years); the male:female ratio was 1.8:1. Patient age in the non-urolithiasis group ranged from 12 to 94 years (mean age, 61.8 ± 16.4 years); the male:female ratio was 3.8:1.
Among 714 patients with urolithiasis, 405 had kidney stones (145 patients had bilateral kidney stones). Moreover, two patients had a single kidney, six patients had sponge kidney, and 307 patients had ureteral stones, including 16 patients who had bilateral ureteral stones; 29 patients had both kidney and ureteral stones; 33 patients had bladder stones, including eight patients who had benign prostatic hyperplasia, one patient who had ureteral stones, and three patients who had kidney stones. There were significant differences in serum sodium, potassium, chloride, calcium, phosphorus, and CO2CP levels between the urolithiasis and non-urolithiasis groups (P < 0.05). In male patients, serum sodium, calcium, and phosphorus levels were significantly higher in the urolithiasis group than in the non-urolithiasis group (P < 0.05); serum potassium and urine pH levels were significantly lower in the urolithiasis group than in the non-urolithiasis group (P < 0.05). In female patients, the serum sodium level was higher in the urolithiasis group than in the non-urolithiasis group (P = 0.083). BMI was significantly higher in the urolithiasis group than in the non-urolithiasis group in all patients, male patients, and female patients (P < 0.05). Characteristics of serum electrolytes, urine pH, urine specific gravity, and BMI in patients with and without urolithiasis are shown in Tables 1, 2, and 3.
Table 1.
Serum parameters, urine parameters, and BMI in patients with and without urolithiasis.
| Variable | Urolithiasis group (n = 714) | Non-urolithiasis group (n = 450) | P value |
|---|---|---|---|
| Serum sodium (mmol/L) | 140.91 ± 2.62 | 140.37 ± 2.65 | 0.018 |
| Serum potassium (mmol/L) | 3.96 ± 0.41 | 4.05 ± 0.45 | 0.001 |
| Serum phosphorus (mmol/L) | 1.20 ± 0.35 | 1.13 ± 0.24 | <0.001 |
| Serum calcium (mmol/L) | 2.31 ± 0.17 | 2.29 ± 0.17 | 0.025 |
| Serum chloride (mmol/L) | 105.12 ± 3.32 | 104.60 ± 3.26 | 0.025 |
| Serum CO2CP (mmol/L) | 25.88 ± 2.60 | 26.29 ± 2.58 | 0.018 |
| Urine pH | 6.12 ± 0.78 | 6.18 ± 0.88 | 0.283 |
| Urine SG | 1.02 ± 0.01 | 1.02 ± 0.01 | 0.972 |
| BMI | 24.61 ± 3.86 | 23.61 ± 3.63 | <0.001 |
Abbreviations: CO2CP, carbon dioxide combining power; BMI, body mass index.
Table 2.
Serum parameters, urine parameters, and BMI in male patients with and without urolithiasis.
| Variable | Urolithiasis group (n = 463) | Non-urolithiasis group (n = 358) | P value |
|---|---|---|---|
| Serum sodium (mmol/L) | 140.96 ± 2.70 | 140.46 ± 2.54 | 0.040 |
| Serum potassium (mmol/L) | 3.97 ± 0.39 | 4.08 ± 0.45 | <0.001 |
| Serum phosphorus (mmol/L) | 1.18 ± 0.26 | 1.12 ± 0.24 | <0.001 |
| Serum calcium (mmol/L) | 2.31 ± 0.16 | 2.30 ± 0.16 | 0.031 |
| Serum chloride (mmol/L) | 104.88 ± 3.11 | 104.52 ± 3.22 | 0.204 |
| Serum CO2CP (mmol/L) | 26.11 ± 2.60 | 26.40 ± 2.56 | 0.152 |
| Urine pH | 6.04 ± 0.76 | 6.18 ± 0.87 | 0.023 |
| Urine specific gravity | 1.02 ± 0.01 | 1.02 ± 0.01 | 0.529 |
| BMI | 25.00 ± 3.69 | 23.84 ± 3.62 | <0.001 |
Abbreviations: CO2CP, carbon dioxide combining power; BMI, body mass index.
Table 3.
Serum parameters, urine parameters, and BMI in female patients with and without urolithiasis.
| Variable | Urolithiasis group (n = 251) | Non-urolithiasis group (n = 92) | P value |
|---|---|---|---|
| Serum sodium (mmol/L) | 140.81 ± 2.47 | 140.03 ± 3.06 | 0.083 |
| Serum potassium (mmol/L) | 3.95 ± 0.45 | 3.94 ± 0.45 | 0.898 |
| Serum phosphorus (mmol/L) | 1.25 ± 0.46 | 1.19 ± 0.21 | 0.154 |
| Serum calcium (mmol/L) | 2.31 ± 0.19 | 2.27 ± 0.21 | 0.289 |
| Serum chloride (mmol/L) | 105.54 ± 3.65 | 104.90 ± 3.41 | 0.149 |
| Serum CO2CP (mmol/L) | 25.48 ± 2.55 | 25.87 ± 2.65 | 0.348 |
| Urine pH | 6.27 ± 0.79 | 6.19 ± 0.92 | 0.467 |
| Urine specific gravity | 1.02 ± 0.01 | 1.01 ± 0.01 | 0.780 |
| BMI | 23.94 ± 4.05 | 22.71 ± 3.55 | 0.020 |
Abbreviations: CO2CP, carbon dioxide combining power; BMI, body mass index.
A backward Wald method was used for binary logistic regression analysis. For male patients, variables kept in the model were serum sodium, serum potassium, pH, age, and BMI, with respective β-values of 0.077 (P = 0.045), −0.611 (P = 0.013), −0.221 (P = 0.081), −0.055 (P < 0.001), and 0.084 (P = 0.028). For female patients, variables kept in the model were serum sodium, serum CO2CP, age, and BMI, with respective β-values of 0.119 (P = 0.050), −0.114 (P = 0.081), −0.031 (P = 0.005), and 0.102 (P = 0.014). Notably, elevated serum sodium and BMI were positively associated with development of urolithiasis. Factors associated with urolithiasis in binary logistic regression are shown in Tables 4 and 5.
Table 4.
Binary logistic regression analysis of factors associated with urolithiasis in male patients.
| Factors | β | P value | EXP (B) | 95% CI |
|---|---|---|---|---|
| Serum sodium | 0.077 | 0.045 | 1.080 | 1.002–1.164 |
| Serum potassium | −0.611 | 0.013 | 0.543 | 0.336–0.878 |
| Urine pH | −0.221 | 0.081 | 0.802 | 0.626–1.027 |
| Age | −0.055 | <0.001 | 0.947 | 0.935–0.959 |
| BMI | 0.084 | 0.003 | 1.087 | 1.029–1.149 |
Abbreviations: BMI, body mass index; CI, confidence interval.
Table 5.
Binary logistic regression analysis of factors associated with urolithiasis in female patients.
| Factors | β | P value | EXP (B) | 95% CI |
|---|---|---|---|---|
| Serum sodium | 0.119 | 0.050 | 1.126 | 1.000–1.269 |
| CO2CP | −0.114 | 0.081 | 0.892 | 0.785–1.014 |
| Age | −0.031 | 0.005 | 0.969 | 0.948–0.991 |
| BMI | 0.102 | 0.014 | 1.107 | 1.021–1.201 |
Abbreviations: CO2CP, carbon dioxide combining power; BMI, body mass index; CI, confidence interval.
In multiple linear regression analysis, increasing BMI was positively associated with serum phosphorus and negatively associated with serum potassium and urine pH in male patients with urolithiasis (all P < 0.05). In female patients with urolithiasis, increasing BMI was negatively associated with urine pH and positively associated with age (both P < 0.05). Multiple linear regression results regarding BMI, serum electrolytes, urine pH, and urine SG in patients with urolithiasis are shown in Table 6.
Table 6.
Multiple linear regression analysis of factors associated with BMI in patients with urolithiasis.
| β | P value | |
|---|---|---|
| Male patients with urolithiasis | ||
| Serum potassium | −1.728 | 0.002 |
| Serum phosphorus | 2.259 | 0.006 |
| Urine pH | −0.773 | 0.006 |
| Female patients with urolithiasis | ||
| Age | 0.063 | 0.003 |
| Urine pH | −1.217 | 0.001 |
Discussion
Previously, numerous studies have focused on metabolic abnormalities, such as hyperparathyroidism and renal tubular acidosis, with respect to stone formation.13 The 24-hour urinary excretion of promoters and inhibitors of stone formation are of considerable interest in patients with urolithiasis.14 However, not all patients with urolithiasis have exactly defined metabolic abnormality syndromes.15 Several studies have investigated the possible inhibitory or promotive effects of trace elements in urolithiasis. Trace elements influence the external morphology of crystals, as well as hasten or slow the stone formation process.16,17 For instance, zinc and magnesium have inhibitory effects on calcium oxalate stone formation. However, iron and copper may promote calcium oxalate stone formation.11 To the best of our knowledge, little is known regarding the associations of serum electrolyte constituents with urinary stone formation. In the present study, we investigated serum parameters, urine parameters, and BMI in patients with and without urolithiasis. The reasons for different findings in male and female patients are unclear, although we presume that sex, hormones, and dietary factors contributed to these associations.
Because the incidence of urolithiasis differs according to sex and age, hierarchical analysis was performed in the present study: logistic regression was used to control for bias due to age and sex. Binary logistic regression analysis revealed that elevated serum sodium and BMI were positively associated with the onset of urolithiasis. There is limited research regarding the association of serum sodium level with stone formation. Moreira et al.12 reported that the predicted risk of calcium oxalate stones was positively associated with serum sodium and chloride levels. Although they did not identify mechanisms to link serum levels of sodium and chloride to a high risk of calcium oxalate stones, their results reinforced the role of sodium chloride in calcium oxalate stone formation. Several studies have shown a potential association between elevated risk of urolithiasis and elevated sodium consumption.18–20 A synergistic interaction has been reported among serum sodium, serum potassium, and serum calcium. In particular, because sodium and calcium transport are parallel processes in the proximal renal tubule, elevated urinary sodium levels may increase the risk of hypercalciuria, thus leading to calcium oxalate stone formation.21 A high-sodium diet has been shown to reduce renal tubular reabsorption of calcium and enhance intestinal reabsorption of calcium, leading to an overall increase in serum calcium and eventual increases in urinary calcium.22,23 In contrast, a low-salt diet can reduce calcium excretion in patients with known calcium stone disease.24 A high-sodium diet has been shown to increase calcium excretion and decrease citrate excretion. The reduction of citrate excretion is presumably due to a significant reduction in serum bicarbonate concentration.22 In our study, there were no significant differences in serum citrate levels between patients with and without urolithiasis, regardless of sex. Parivar et al.25 suggested that potassium intake was significantly associated with a low risk of stone formation. Our study found serum potassium levels were significantly lower in the urolithiasis group than in the non-urolithiasis group among male patients, but not among female patients. Tieder et al.26 proposed that an imbalanced phosphorus level is an important potential risk factor for urolithiasis. Dagnone and Norman27 reported that serum phosphorus level was not an independent risk factor for urinary stone recurrence or complications. In our study, there were no significant differences in serum phosphorus between the two groups.
Nutrition is regarded as a major environmental risk factor for calcium stone formation. Individuals residing in nations with western and westernized lifestyles are likely to have energy-rich and high-salt diets, which result in high BMI and elevated excretion of urinary lithogenic substances that promote idiopathic urolithiasis.10,28,29 High BMI is regarded as a cause of urolithiasis among both male and female patients.30 Patients with high BMI excrete more urinary sodium, oxalate, uric acid, and phosphorus; they also have lower urine pH. These factors are all presumed to promote calcium oxalate stone formation.28 Notably, a higher uric acid level was found to be more predictive of calcium oxalate stones, which emphasizes the importance of heterogeneous nucleation. Furthermore, a known core aspect of stone formation is the presence of acidic urine.31
Our multiple linear regression analysis suggested significant negative associations between BMI and urine pH in both male and female patients with urolithiasis. Consistent with our findings, Maalouf et al.7 demonstrated that urine pH was inversely associated with BMI in nearly 5000 patients with urolithiasis at two metabolic stone clinics in the United States. Furthermore, multivariate analyses by Taylor and Curhan8 and Seiner et al.28 demonstrated that BMI was significantly negatively associated with pH in both men and women. Daudon et al.32 previously indicated that greater body size was associated with greater risk of uric acid stone formation.
This study had the following limitations. First, data were collected retrospectively, which could have introduced unintentional bias. Second, stone analysis was not performed in this study. Certain stone types are often associated with specific metabolic abnormalities. Third, the study was not community-based. Participants in this study were patients who had been admitted to our hospital for urolithiasis or other urological symptoms; thus, their characteristics might not be generalizable to the overall Chinese population. Finally, many confounding factors that might have affected the results (e.g., dietary habits, sedentary lifestyle, hypertension, and weight change) were not considered.
Because the present study did not strictly use a case–control design, we could not conclude that increasing serum sodium level and BMI were independent risk factors for urolithiasis. However, our results showed that serum sodium level and BMI were significantly higher in patients with urolithiasis than in patients without urolithiasis; moreover, serum sodium and BMI were positively associated with the onset of urolithiasis in both sexes. Our findings suggest that changes in serum sodium level and BMI may be important in the pathogenesis and treatment of urolithiasis. Further research is needed to clearly define the role of serum sodium level in urolithiasis.
Declaration of conflicting interest
The authors declare that there is no conflict of interest.
Funding
The present study was supported by the Youth Foundation Project of the Affiliated Hospital of Qingdao University (QDFY-2017-50.
ORCID iD
Zaixian Zhang https://orcid.org/0000-0002-1331-4856
References
- 1.Hesse A, Brandle E, Wilbert D, et al. Study on the prevalence and incidence of urolithiasis in Germany comparing the years 1979 vs. 2000. Eur Urol 2003; 44: 709–713. [DOI] [PubMed] [Google Scholar]
- 2.Taylor EN, Stampfer MJ, Curhan GC. Diabetes mellitus and the risk of nephrolithiasis. Kidney Int 2005; 68: 1230–1235. [DOI] [PubMed] [Google Scholar]
- 3.Shekarriz B, Stoller ML. Uric acid nephrolithiasis: current concepts and controversies. J Urol 2002; 168: 1307–1314. [DOI] [PubMed] [Google Scholar]
- 4.Schwaderer AL, Oduguwa A, Kusumi K. Urinary stone disease in pediatric and adult metabolic bone clinic patients. Urolithiasis 2018; 46: 173–178. [DOI] [PubMed] [Google Scholar]
- 5.Tasian GE, Kabarriti AE, Kalmus A, et al. Kidney stone recurrence among children and adolescents. Urolithiasis 2017; 197: 246–252. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Zeng G, Mai Z, Xia S, et al. Prevalence of kidney stones in China: an ultrasonography based cross-sectional study. BJU Int. 2017; 120: 109–116. [DOI] [PubMed] [Google Scholar]
- 7.Maalouf NM, Sakhaee K, Parks JH, et al. Association of urinary pH with body weight in nephrolithiasis. Kidney Int 2004; 65: 1422–1425. [DOI] [PubMed] [Google Scholar]
- 8.Taylor EN, Curhan GC. Body size and 24-hour urine composition. Am J Kidney Dis 2006; 48: 905–915. [DOI] [PubMed] [Google Scholar]
- 9.Negri AL, Spivacow FR, Del Valle EE, et al. Role of overweight and obesity on the urinary excretion of promoters and inhibitors of stone formation in stone formers. Urol Res 2008; 36: 303–307. [DOI] [PubMed] [Google Scholar]
- 10.Taylor EN, Stampfer MJ, Curhan GC. Obesity, weight gain, and the risk of kidney stones. JAMA 2005; 293: 455–462. [DOI] [PubMed] [Google Scholar]
- 11.Atakan IH, Kaplan M, Seren G, et al. Serum, urinary and stone zinc, iron, magnesium and copper levels in idiopathic calcium oxalate stone patients. Int Urol Nephrol 2007; 39: 351–356. [DOI] [PubMed] [Google Scholar]
- 12.Moreira DM, Friedlander JI, Carons A, et al. Association of serum biochemical metabolic panel with stone composition. Int J Urol 2015; 22: 195–199. [DOI] [PubMed] [Google Scholar]
- 13.Goldfarb DS, Arowojolu O. Metabolic evaluation of first-time and recurrent stone formers. Urol Clin North Am 2013; 40: 13–20. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Coe FL, Parks JH, Asplin JR. The pathogenesis and treatment of kidney stones. N Engl J Med 1992; 327: 1141–1152. [DOI] [PubMed] [Google Scholar]
- 15.Pak CY, Poindexter JR, Adams-Huet B, et al. Predictive value of kidney stone composition in the detection of metabolic abnormalities. Am J Med 2003; 115: 26–32. [DOI] [PubMed] [Google Scholar]
- 16.Meyer JL, Angino EE. The role of trace metals in calcium urolithiasis. Invest Urol 1977; 14: 347–350. [PubMed] [Google Scholar]
- 17.Eusebio E, Elliot JS. Effect of trace metals on the crystallization of calcium oxalate. Invest Urol 1967; 4: 431–435. [PubMed] [Google Scholar]
- 18.Taylor EN, Fung TT, Curhan GC. DASH-style diet associates with reduced risk for kidney stones. J Am Soc Nephrol 2009; 20: 2253–2259. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Borghi L, Schianchi T, Meschi T, et al. Comparison of two diets for the prevention of recurrent stones in idiopathic hypercalciuria. N Engl J Med 2002; 346: 77–84. [DOI] [PubMed] [Google Scholar]
- 20.Fink HA, Akornor JW, Garimella PS, et al. Diet, fluid, or supplements for secondary prevention of nephrolithiasis: a systematic review and meta-analysis of randomized trials. Eur Urol 2009; 56: 72–80. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Silver J, Rubinger D, Friedlaender MM, et al. Sodium-dependent idiopathic hypercalciuria in renal-stone formers. Lancet 1983; 2: 484–486. [DOI] [PubMed] [Google Scholar]
- 22.Sakhaee K, Harvey JA, Padalino PK, et al. The potential role of salt abuse on the risk for kidney stone formation. J Urol 1993; 150: 310–312. [DOI] [PubMed] [Google Scholar]
- 23.Kok DJ, Iestra JA, Doorenbos CJ, et al. The effects of dietary excesses in animal protein and in sodium on the composition and the crystallization kinetics of calcium oxalate monohydrate in urines of healthy men. J Clin Endocrinol Metab 1990; 71: 861–867. [DOI] [PubMed] [Google Scholar]
- 24.Nouvenne A, Meschi T, Prati B, et al. Effects of a low-salt diet on idiopathic hypercalciuria in calcium-oxalate stone formers: a 3-mo randomized controlled trial. Am J Clin Nutr 2010; 91: 565–570. [DOI] [PubMed] [Google Scholar]
- 25.Parivar F, Low RK, Stoller ML. The influence of diet on urinary stone disease. J Urol 1996; 155: 432–440. [PubMed] [Google Scholar]
- 26.Tieder M, Modai D, Shaked U, et al. “Idiopathic” hypercalciuria and hereditary hypophosphatemic rickets. Two phenotypical expressions of a common genetic defect. N Engl J Med 1987; 316: 125–129. [DOI] [PubMed] [Google Scholar]
- 27.Dagnone AJ, Norman RW. Serum phosphate in stone formers: what does a low serum phosphate level mean? J Urol 2000; 163: 21–23. [DOI] [PubMed] [Google Scholar]
- 28.Siener R, Glatz S, Nicolay C, et al. The role of overweight and obesity in calcium oxalate stone formation. Obes Res 2004; 12: 106–113. [DOI] [PubMed] [Google Scholar]
- 29.Ekeruo WO, Tan YH, Young MD, et al. Metabolic risk factors and the impact of medical therapy on the management of nephrolithiasis in obese patients. J Urol 2004; 172: 159–163. [DOI] [PubMed] [Google Scholar]
- 30.Wang S, Zhang Y, Zhang X, et al. Upper urinary tract stone compositions: the role of age and gender. Int Braz J Urol 2020; 46: 70–80. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 31.Torricelli FC, De S, Liu X, et al. Can 24-hour urine stone risk profiles predict urinary stone composition? J Endourol 2014; 28: 735–738. [DOI] [PubMed] [Google Scholar]
- 32.Daudon M, Lacour B, Jungers P. Influence of body size on urinary stone composition in men and women. Urol Res 2006; 34: 193–199. [DOI] [PubMed] [Google Scholar]