Can Obese Stone Formers Follow Dietary Recommendations?

Abstract

Introduction:

Obese patients have an increased risk of kidney stones, and several studies have identified specific urinary derangements on 24-hour collections. The objective of this study was to assess obese and super-obese stone formers, and their compliance with dietary modifications over time, based on 24-hour urine outcomes.

Patients and Methods:

A retrospective review was performed searching for all stone formers who completed a 24-hour urine collection before and after dietary counseling for stone prevention. Patients were excluded if placed on medical therapy in addition to dietary therapy. Patients were divided in three main groups according to their body mass index (BMI): 30, 30–40, and >40 kg/m². Demographic data and 24-hour urine stone risk parameters (volume, sodium, uric acid, citrate, and oxalate) were assessed. Initial 24-hour urine results were compared to follow-up results after dietary counseling. Then, the outcomes from each group were compared to each other.

Results:

Two hundred and fourteen stone formers (67% male) were identified with a mean age of 49.5±15.0 years. One hundred twenty-eight (59.8%) patients had BMI <30 kg/m², 61 (28.5%) between 30–40 kg/m², and 25 (11.7%) >40 kg/m². Among patients with BMI <30 kg/m², there were significant improvements in all urinary parameters (p<0.001) as well as in the group with BMI between 30 to 40 kg/m² (p=0.02 for oxalate, p<0.001 for other parameters). Among super-obese patients, there were significant improvements in the urinary volume (p=0.03), sodium (p<0.001), uric acid (p=0.001), and oxalate (p<0.001). There were no significant differences in the improvements observed in the urinary volume (p=0.69), sodium (p=0.08), uric acid (p=0.17), and citrate levels (p=0.97) between the groups.

Conclusion:

Dietary recommendations can be an equally effective strategy in decreasing the risk or stone recurrence in obese and super-obese kidney stone formers as it is in those who are not obese.

Introduction

With the rising prevalence of obesity globally, there has been a marked increase in healthcare spending and resources dedicated to these patients.^1,2 Obesity is a well-known risk factor for chronic conditions such as hypertension, diabetes, and dyslipidemia, and it has also been associated with nephrolithiasis.³ A recent population-based analysis has shown that as the incidence of kidney stones has increased from 5.2% to 8.8% in 2010, so too has the risk of stone disease in patients with diabetes (OR 1.59) and obesity (OR 1.55).⁴ Indeed, obese patients have a higher chance of developing kidney stones and are more likely to present with stone recurrence.¹

Given this heightened risk of incident stone and stone recurrence, identifying effective dietary and medical approaches to stone prevention is imperative.¹ Compared to nonobese patients, increased levels of urinary sodium, calcium, and uric acid, and decreased urinary pH and citrate have been reported in obese patients as well as a preponderance of uric acid stones.^5,6 Most of these urinary derangements can be addressed with oral hydration and dietary modifications, such as decreasing sodium and protein intake, while increasing fruits and vegetables.⁷ Whereas dietary compliance can be difficult for any patient, we hypothesized that it may be even harder for obese patients as diet management is one of the challenges that commonly leads to obesity.

The aim of our study was to assess the outcomes of dietary counseling in obese and super-obese stone former patients with respect to their 24-hour urine changes over time.

Patients and Methods

Study design

After Institutional Review Board approval, we retrospectively identified kidney stone patients with a 24-hour urine analyses before and after a dietary counseling session. Patients were divided in three groups according to their body mass index (BMI): BMI <30 kg/m² (normal and overweight patients), 30 to 40 kg/m² (obese patients), and >40 kg/m² (super-obese patients). Baseline 24-hour urine collections were used to identify patients with low volume (<2000 mL), hypernatriuria (>200 mmol/24 hour), hyperoxaluria (>50 mg/24 hour), hyperuricosuria (>700 mg/24 hour), and hypocitraturia (<320 mg/24 hour). Baseline 24-hour urine parameters were compared to those obtained at last follow-up. Patients were followed with repeat 24-hour urine collections every 3 months. When multiple 24-hour urine collections were performed, the initial sample was compared to the test providing the longest follow-up period. A single-day collection was obtained at each time point.

To evaluate nonstone-related outcomes of dietary recommendations, the serum glucose level (GLU), systolic blood pressure (SBP), and BMI before and after the consultation were also recorded and compared. Children (under 18 years), patients with incomplete 24-hour urine analysis or with less than 4 weeks between urine collections, and those who were on medications related to stone management (i.e., allopurinol, thiazides, citrates, vitamin B6) were excluded from analysis.

All stone patients were given tailored dietary recommendations directed by their baseline 24-hour urine stone risk parameters, as well as weight loss and exercise recommendations when indicated. Patients with low urinary volume were counseled to increase their oral intake of fluids (at least 2 L per day); patients with hypernatriuria were counseled to decrease salt on diet to less than 1500 mg/day; patients with hyperuricosuria were counseled to decrease the animal protein intake, with a target of 0.8–1 g/kg/day; and patients with hypocitraturia were counseled to increase fruits rich in citric acid and/or potassium citrate (lemons, limes, oranges, and melons). Patients with hyperoxaluria underwent a detailed inventory of items they may be eating that were high in bioavailable oxalates, including spinach, rhubarb, nuts, and chocolate. They were also counseled on optimal intake (1200 mg/day) and timing of dietary calcium to counteract oxalate absorption. Counseling was provided in our multidisciplinary stone clinic by a registered dietician.

Statistical analysis

Results are expressed in proportion, mean, and standard deviation. The Mann–Whitney test or the Student's t-test were used to compare continuous variables, while the Fisher's exact test was used to compare categorical variables. One-way analysis of variance (ANOVA) was used to perform the comparison between the three main groups (BMI <30, 30 to 40, and >40 kg/m²). Statistical analysis was performed with SPSS version 20.0 (SPSS, Inc., Chicago, IL) and the significance level was set up at p<0.05.

Results

Two hundred and fourteen stone former patients (67% male) were identified with a mean age of 49.5±15.0 years. The average BMI was 30.1±7.0 kg/m² and length of follow-up between the 24-hour urine analyses was 259.6±458.9 days (range from 30 to 4000 days). One hundred twenty-eight (59.8%) patients had BMI lower than 30 kg/m², 61 (28.5%) between 30 and 40 kg/m², and 25 (11.7%) higher than 40 kg/m². There were no differences between age (p=0.711) and gender (p=0.350) among the groups.

Among patients with BMI <30 kg/m², there were significant improvements in all urinary parameters (p<0.001) (Table 1). The most remarkable change was in the urinary citrate level (326%), while the urinary uric acid level had the lowest relative alteration. Normal levels were reached by 41%, 75%, 72%, 83%, and 67% of patients with low volume, hypernatriuria, hyperuricosuria, hypocitraturia, and hyperoxaluria, respectively.

Table 1.

Demographic Data and 24-Hour Urine Analysis from Normal/Overweight Stone Formers (<30 Kgm ²)

Abnormality	N^a	Gender Male (%)	Age (years)	BMI (kg/m²)	Baseline	After diet	% Difference	p-Value
Low urine volume (<2000 mL/day)	73	63%	47.7±17.3	25.7±2.9	1296.9±460.5	1961.0±814.8	51	<0.001
Hypernatriuria (>200 mmol/day)	45	82%	47.2±14.3	26.3±2.5	265.7±56.6	162.1±66.5	38	<0.001
Hyperuricosuria (>700 mg/day)	25	92%	46.2±11.0	27.0±2.4	855.9±170.1	609.6±229.0	28	<0.001
Hypocitraturia (<320 mg/day)	24	58%	46.8±22.1	25.1±3.2	168.0±89.3	716.2±549.0	326	<0.001
Hyperoxaluria (>50 mg/day)	52	84%	49.7±13.4	26.3±2.8	69.7±23.5	48.6±23.5	30	<0.001

One hundred twenty-eight patients with a mean follow-up of 62±40 days.

BMI=body mass index.

In the obese group with BMI between 30 and 40 kg/m², there were also significant improvements in all urinary parameters (p<0.001 for all, expect for urinary oxalate, p=0.02) (Table 2). The urinary citrate level had the highest relative improvement, increasing more than four times (426%). Normal levels were reached by 55%, 80%, 81%, 100%, and 61% of patients with low volume, hypernatriuria, hyperuriccosuria, hypocitraturia, and hyperoxaluria, respectively.

Table 2.

Demographic Data and 24-Hour Urine Analysis from Obese Stone Formers (30–40 Kgm ²)

Abnormality	N^a	Gender Male (%)	Age (years)	BMI (kg/m²)	Baseline	After diet	% Difference	p-Value
Low urine volume (<2000 mL/day)	36	67%	50.0±13.0	33.2±2.4	1381.2±276.8	2075.6±912.0	50	<0.001
Hypernatriuria (>200 mmol/day)	30	73%	47.9±14.3	33.7±2.8	270.1±65.9	169.1±77.0	37	<0.001
Hyperuricosuria (>700 mg/day)	21	90%	44.3±14.2	33.0±2.7	929.3±172.2	549.2±246.6	41	<0.001
Hypocitraturia (<320 mg/day)	10	80%	43.0±13.9	33.4±3.0	138.4±117.5	728.3±309.4	427	<0.001
Hyperoxaluria (>50 mg/day)	23	87%	47.8±14.0	33.1±3.0	64.5±12.3	50.3±25.8	21	0.022

Sixty-one patients with a mean follow-up of 271±99 days.

Among super-obese patients, there were significant improvements in the urinary volume (p=0.03), sodium (p<0.001), uric acid (p=0.001), and oxalate (p<0.001) (Table 3). Again, the urinary citrate level had the highest improvement (343%, p=0.08), while the urinary sodium level had the most modest change (30%). Normal levels were reached by 55%, 73%, 72%, 50%, and 58% of patients with low volume, hypernatriuria, hyperuriccosuria, hypocitraturia, and hyperoxaluria, respectively.

Table 3.

Demographic Data and 24-Hour Urine Analysis from Super-Obese Stone Formers (>40 Kgm ²)

Abnormality	N^a	Gender Male (%)	Age (years)	BMI (kg/m²)	Baseline	After diet	% Difference	p-Value
Low urine volume (<2000 mL/day)	9	55%	46.7±11.1	42.0±5.0	1589.8±382.0	2381.2±1010.0	50	0.030
Hypernatriuria (>200 mmol/day)	15	76%	52.0±9.2	46.2±5.4	300.5±106.3	211.3±162.8	30	<0.001
Hyperuricosuria (>700 mg/day)	11	82%	50.0±10.2	44.0±4.7	1095.0±237.2	667.2±242.7	39	0.001
Hypocitraturia (<320 mg/day)	4	75%	47.7±17.9	49.3±5.2	153.2±56.4	679.0±605.2	343	0.083
Hyperoxaluria (>50 mg/day)	17	71%	50.3±8.3	43.5±5.45	110.5±117.5	49.7±17.7	55	<0.001

Twenty-five patients with a mean follow-up of 1240±792 days.

There were no significant differences in the improvements observed in the urinary volume (p=0.69), sodium (p=0.08), uric acid (p=0.17), and citrate levels (p=0.97) between the groups. Only the urinary oxalate level had a significantly better improvement in the super-obese patients compared to the other groups (55% vs 21%—obese, vs 30%—normal/overweight; p=0.04).

Obese and super-obese patients had a significant improvement in their BMI after dietary recommendations compared to their baseline (36.6±6.4 vs 34.6±6.4 kg/m²; p=0.04). However, systolic blood pressure (130.0±13.8 vs 128.5±18.2 mm Hg; p=0.49) and serum GLU (99.6±28.9 vs 100.1±31.4 mm Hg; p=0.94) did not change. When patients with normal baseline SBP and GLU were excluded, there was still no significant difference in SBP (p=0.49) or GLU (p=0.41) on follow-up.

Discussion

Obese patients have an increased risk of developing kidney stones.¹ The lifetime stone risk is higher and the stone size is also larger in patients with higher BMI.⁸ The association between obesity and stones may, in part, be related to increased intake of lithogenic substances, such as refined sugars, calcium, oxalate, and purines, as well as low fluid intake.^9,10 This study demonstrates that obese and super-obese patients can follow dietary recommendations aimed at decreasing the risk of stone recurrence, and have a similar improvement in urinary parameters as seen in nonobese patients.

Nephrolithiasis has been associated with a higher prevalence of chronic diseases and adverse cardiovascular outcomes.¹¹ Rule et al. reported that stone formers had a 38% increased risk of myocardial infarction when compared to nonstone formers, and that increased risk remained at 31% after adjustment for chronic kidney disease (CKD) and other comorbidities. There is also a consensus that renal stone disease is associated with an increased risk of CKD.¹² El-Zoghby et al. evaluated 6926 stone formers and 24,620 matched controls followed for a mean of 9 years, and reported an increased risk of end-stage renal disease in stone formers after adjusting for diabetes, hypertension, dyslipidemia, and gout (hazard ratio: 2.09; 95% confidence interval: 1.45–3.01).¹³ Stone prevention may be of particular importance in obese patients, who are already at risk for several chronic conditions, including coronary artery disease and CKD.^14,15 In addition, the morbidity associated with surgical management of stone disease in the morbidly obese can be significant.

Several studies have characterized urinary abnormalities in obese stone formers that could be addressed with dietary interventions,^16
–18 with approximately half of the patients being identified with hypocitraturia and hyperuricosuria.¹⁶ Using a national database, Powell et al. analyzed 5942 subjects with stones and found obesity was associated with an increased excretion of sodium, calcium, magnesium, citrate, sulfate, phosphate, oxalate, uric acid, and cystine.¹⁷ Taylor et al. performed a multivariate analysis of The Health Professionals Follow-up Study prospective cohort (51,000 subjects, with 1473 stone cases), and reported that excretion of urinary oxalate, uric acid, sodium, and phosphate increased with increasing BMI.¹⁸

Dietary modification is a cost-effective initial step in kidney stone prevention, but can be fraught with compliance issues. We hypothesized that it may be even more difficult for obese or super-obese patients, as dietary discretions likely contribute in varying degrees to their underlying obesity. Our study demonstrates, however, that obese and super-obese stone formers were able to significantly increase the urine volume and citrate levels, while decreasing the sodium, calcium, and uric acid levels.

Our study has some limitations. It is retrospective in nature, thus subject to the shortcomings of a nonprospective study design. The groups and subgroups of patients are different in number and they were not followed up for the same period of time. Our follow-up period of just under 1 year is too short to comment on radiographic or clinical stone recurrence, therefore changes in 24-hour urinary parameters are used as surrogate endpoints.

Conclusion

Obese and super-obese stone former patients can effectively follow dietary recommendations, and improve their urinary stone risk parameters with dietary modification.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

References

Ahmed

, Ahmed

, Khalil

. Renal stone disease and obesity: What is important for urologists and nephrologists?. Ren Fail, 2012; 34:1348–1354.

Slack

, Myers

, Martin

, Heymsfield

. The geographic concentration of U.S adult obesity prevalence and associated social, economic, and environmental factors. Obesity, 2013 [Epub ahead of print]; DOI: 10.1002/oby.20502.

Taylor

, Stampfer

, Curhan

. Obesity, weight gain, and the risk of kidney stones. JAMA, 2005; 293:455–462.

Scales

Jr ., Smith

, Hanley

, Saigal

; Urologic diseases in America Project. Prevalence of kidney stones in the United States. Eur Urol, 2012; 62:160–165.

Lee

, Kim

, Yun

, Lee

, Kim

. Impact of obesity in patients with urolithiasis and its prognostic usefulness in stone recurrence. J Urol, 2008; 179:570–574.

Al-Hayek

, Schwen

, Jackman

, Averch

. The impact of obesity on urine composition and nephrolithiasis management. J Endourol/Endourol Soc, 2013; 27:379–383.

Heilberg

, Goldfarb

. Optimum nutrition for kidney stone disease. Adv Chronic Kidney Dis, 2013; 20:165–174.

Mosli

, Mosli

. Increased body mass index is associated with larger renal calculi. Urology, 2012; 80:974–977.

Curhan

, Willett

, Rimm

, Stampfer

. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med, 1993; 328:833–838.

10.

Kaul

, Sidhu

, Sharma

, Nath

. Calculogenic potential of galactose and fructose in relation to urinary excretion of lithogenic substances in vitamin B6 deficient and control rats. J Am Coll Nutr, 1996; 15:295–302.

11.

Domingos

, Serra

. Nephrolithiasis is associated with an increased prevalence of cardiovascular disease. Nephrol Dial Transplant, 2011; 26:864–868.

12.

Rule

, Roger

, Melton

3rd , et al. Kidney stones associate with increased risk for myocardial infarction. J Am Soc Nephrol, 2010; 21:1641–1644.

13.

El-Zoghby

, Lieske

, Foley

, et al. Urolithiasis and the risk of ESRD. Clin J Am Soc Nephrol, 2012; 7:1409–1415.

14.

Borgeson

, Sharma

. Obesity, immunomodulation and chronic kidney disease. Curr Opin Pharmacol, 2013; 13:618–624.

15.

Schmidt

, Johannesdottir

, Lemeshow

, et al. Obesity in young men, and individual and combined risks of type 2 diabetes, cardiovascular morbidity and death before 55 years of age: A Danish 33-year follow-up study. BMJ Open, 2013; 3:pii:e002698.

16.

Ekeruo

, Tan

, Young

, 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.

17.

Powell

, Stoller

, Schwartz

, et al. Impact of body weight on urinary electrolytes in urinary stone formers. Urology, 2000; 55:825–830.

18.

Taylor

, Curhan

. Demographic, dietary, and urinary factors and 24-h urinary calcium excretion. Clin J Am Soc Nephrol, 2009; 4:1980–1987.