Effect of Being Overweight on Urinary Metabolic Risk Factors for Kidney Stone Formation

Linda Shavit; Pietro Manuel Ferraro; Nikhil Johri; William Robertson; Steven B. Walsh; Shabbir Moochhala; Robert Unwin


Nephrol Dial Transplant. 2015;30(4):607-613. 

In This Article

Abstract and Introduction


Background The prevalence and incidence of kidney stone disease have increased markedly during the past several decades, and studies have demonstrated that inappropriate dietary habits are leading to more obesity and overweight (OW) in children and adults, which may be important in stone formation. Obese and OW patients share most of the same risk factors for cardiovascular morbidity, while the impact of being OW, rather than obese, on urinary metabolic parameters of kidney stone formers (KSF) is less well known. The aims of this study were to investigate urinary metabolic parameters, stone composition and probability of stone formation (Psf) in OW KSF when compared with normal weight (NW) and obese KSF.

Methods The kidney stone database for KSF attending a large metabolic stone clinic was investigated. Patients with a recorded BMI, confirmed diagnosis of kidney stone disease and full metabolic evaluation were divided into three categories: BMI ≤25.0 kg/m2 (NW group), BMI 25–30 kg/m2 (OW group) and BMI >30.0 kg/m2 (obese group). Twenty-four hour urinary volume (U.Vol), pH (U.pH), calcium (U.Ca), oxalate (U.Ox), citrate (U.Cit), uric acid (U.UA), magnesium (U.Mg), sodium (U.Na) and potassium (U.K) excretions, along with stone composition and Psf, were then compared among the groups.

Results A total of 2132 patients were studied, of whom 833 (39%) were NW, 863 (40.5%) were OW and 436 (20.5%) were obese. OW and obese KSF were older (mean age 43 ± 15 in NW, 48 ± 13 in OW and 50 ± 12 years in obese; P for trend <0.001), demonstrated increased female predominance and higher prevalence of diabetes, hypertension and gout. There were no statistically significant differences in U.Vol and U.Mg among the groups. However, significantly higher levels of U.Ca, U.Ox, U.Cit, by crude analysis, and U.UA (3.3 ± 1.1 versus 3.8 ± 1.2 versus 4.0 ± 1.2 mmol/L; P < 0.001 for trend), U.Na (151 ± 57 versus 165 ± 60 versus 184 ± 63 mmol/L; P < 0.001 for trend), and lower U.pH (6.3 ± 0.5 versus 6.1 ± 0.5 versus 6.0 ± 0.6; P < 0.001 for trend) by both crude and multivariate adjusted analysis models were demonstrated in OW and obese KSF. Stone composition data (N = 640) showed a significantly higher incidence of uric acid stones in OW and obese groups (P for trend < 0.001). In addition, higher Psf for CaOx, UA and CaOx/UA stone types were detected in OW and obese compared with NW KSF.

Conclusions Similar to obese KSF, OW KSF show clear alterations in metabolic urinary profiles that are associated with increased overall risk of stone formation. This greater risk is primarily due to raised U.UA and U.Na, lower U.pH and higher prevalence of hypercalciuria, along with unchanged levels of the commonly measured urinary lithogenesis inhibitors. Moreover, our study established a higher incidence of uric acid, but not calcium, stones in OW KSF. Thus, appropriate evaluation and follow-up may be warranted even in OW patients who are at risk of increased stone formation. Whether modest weight loss in OW KSF will have a favourable impact on their metabolic urinary profiles and thereby diminish the risk of further stone formation needs exploring.


Several epidemiological studies have shown a positive association between incident stone risk and body mass index (BMI). Multiple risk factors have been proposed to explain this association and they include diet-dependent changes in urinary metabolic profile, altered renal acid–base metabolism and deficient ammonia production and excretion, which may all be linked to insulin resistance and impaired glucose metabolism[1] Various urinary biochemical abnormalities have been recognized to increase the propensity for kidney stone formation in obese patients.[2–6] Lower urinary pH, together with greater urinary calcium, uric acid and oxalate excretions have been linked to obesity in kidney stone formers (KSF). Moreover, the impact of body fat distribution, along with total fat mass, appears to influence stone risk.[7] Both total body fat and trunk fat show an association with lower 24-h urinary pH, higher urinary uric acid and impaired urinary NH4+ excretion, whereas leg fat mass is not associated with urinary pH. Furthermore, a recent study demonstrated the independent association of low physical activity, higher caloric intake and BMI with increased risk of incident kidney stone development in a large cohort of post-menopausal women.[8] Although the association between overt obesity and a risk of kidney stone disease is clear, the impact of being overweight (OW), rather than obese, on urinary metabolic profiles has not been explored to the same extent. Moreover, the contribution of dietary factors and the potential effect of modest weight loss on the metabolic urinary profile of OW KSF are still unclear.

In the current study, we performed an analysis of prospectively collected demographic, clinical and dietary, and biochemical data of 2123 idiopathic calcium and uric acid KSF followed from 1995 to 2012 at the University College London Stone Clinic (University College and Royal Free Hospitals). The aims of the study were to investigate urinary metabolic parameters and stone composition, and to estimate the probability of stone formation (Psf) in OW KSF compared with normal weight (NW) and obese KSF. In addition, a contribution of dietary factors on the above-mentioned parameters was studied.