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Abstract: FR-PO568

BMI and Calcium Kidney Stone Formation

Session Information

Category: Fluid‚ Electrolyte‚ and Acid-Base Disorders

  • 1002 Fluid‚ Electrolyte‚ and Acid-Base Disorders: Clinical


  • Yu, Tammy, Lifespan Health System, Providence, Rhode Island, United States
  • Sheth, Himesh, Lifespan Health System, Providence, Rhode Island, United States
  • Tang, Jie, Lifespan Health System, Providence, Rhode Island, United States

Population studies show a clear association between obesity and kidney stone incidence. Here, we examined the relationship between body mass index (BMI) and laboratory markers of stone risk in calcium stone formers (CSF).


Patients were recruited from a kidney stone clinic for this retrospective study. All had either confirmed prior calcium (Ca) stones or a 24-hour urine stone risk index predictive of Ca stones. Participants had a 24-hour urine stone risk profile and a renal ultrasound to evaluate stone burden. Medical history and BMI were obtained within 3 months of laboratory testing.


A total of 187 CSF were included in this study. Mean age was 53, 44% were male, 80% were Caucasian, 19% had diabetes, 42% had hypertension, and 32% had dyslipidemia. BMI was positively correlated with calculated 24-hour urine creatinine clearance (CrCl), both before (p=0.00003) and after (p=0.00002) adjustment for demographics (age, race, and gender). However, in subgroup analysis this association became nonsignificant in women (p=0.09), perhaps due to lower muscle mass.

Higher BMI was correlated with increased stone burden both before and after demographic adjustment (p=0.02, 0.02, respectively). BMI was inversely associated with serum Ca (p=0.04) and 25-hydroxy vitamin D (p=0.01) levels. No significant association was observed with other serum markers of bone-mineral metabolism, including phosphorus (P), 1,25-dihydroxy-vitamin D, and PTH.

Table 1 shows the association between BMI and several urinary markers of increased Ca stone risk. No association was observed with other urinary risk factors (not shown).


Higher BMI was associated with higher CrCl, lower serum Ca and vitamin D, and higher stone burden in our cohort of CSF. As in prior studies of uric acid (UA) stone formers, obesity likely contributes to an increased Ca stone risk by lowering urine pH and increasing urine UA and P. Hyperuricosuria promotes the crystallization of Ca oxalate, while hyperphosphaturia may play a role in the formation of Randall plaque.

Table 1
Urine Marker Without Adjustment for Demographics Adjusted for Demographics
pH-0.02 (p=0.004)-0.02 (p=0.002)
Ammonia0.5 (p=0.0003)0.5 (p=0.0008)
Uric Acid10.6 (p=0.00002)10.2 (p=0.0003)
Uric Acid Supersaturation0.04 (p=0.002)0.03 (p=0.004)
Phosphorus14.9 (p=0.00003)13.6 (p=0.00007)