Login to Access Video or Poster Abstract: MP90-12
Sources of Funding: AUA Northeastern Section Young Investigator Grant

Introduction

Urinary magnesium has been shown previously to inhibit kidney stone formation in chemical models however, when applied to in vivo human models the results have been conflicting. The purpose of this study is to investigate the timing of magnesium supplementation on the inhibitory effect on nephrolithiasis. We hypothesize that if magnesium is taken with meals, more will be absorbed in the small intestine and excreted in the kidney to allow for better inhibitory effect, specifically by reducing oxalate excretion.

Methods

We prospectively enrolled known calcium oxalate stone formers with isolated hyperoxaluria identified on 24 hour stone risk testing. Patients were then randomized to take magnesium supplementation either fasting or with food. An initial 24 hour urine collection was obtained on enrollment and then repeated after 7 days of magnesium supplementation to determine the effect on urinary excretion of oxalate. Participants were given a controlled diet during the 7 days of intervention which included adequate fluid intake, low oxalate, low salt, moderate animal protein, and normal calcium intake – the standard dietary treatment for hyperoxaluric kidney stone patients.

Results

Seven patients were enrolled with 3 patients randomized to each arm of magnesium supplementation. Those taking it with food experienced 41.2mg/d increase in urinary magnesium and a 25.2mg/d decrease in their urinary oxalate over the course of 7 days as compared to a 14.3mg/d increase in magnesium and 13.7mg/d decrease in citrate for those taking magnesium while fasting. There were only modest decreases in calcium oxalate supersaturation and calcium but profound increases in stone protective factors like citrate (Table). Secondary endpoints including sodium (decrease 53mg/d with food vs 84mg/d fasting) also showed improvement with little difference between groups.

Conclusions

Those taking magnesium supplementation with food experienced a more than 3 fold increase in urinary magnesium and twice the reduction in urinary oxalate as those who took it while fasting. Additionally, secondary endpoints like citrate and sodium showed improvement with modest differences between groups. Our pilot study supports the need for further investigation with a larger sample to establish the significance of these trends.

Funding

AUA Northeastern Section Young Investigator Grant