Biochemical effects in patients with calcium oxalate stone disease during combined treatment with bendroflumethiazide and magnesium oxide

Rethinking the role of urinary magnesium in calcium urolithiasis

BACKGROUND AND PURPOSE

The role of magnesium in urinary stone formation remains undefined. In vivo, magnesium inhibits stone formation in hyperoxaluric rats, and small clinical studies suggest a protective effect of magnesium supplementation in calcium oxalate stone formers. We performed a retrospective review of more than 7,000 stone patients to see if there is a relation between urinary magnesium and other stone risk variable constituents.

MATERIALS AND METHODS

A national database of stone formers categorized by residential ZIP code was queried, and, using strict inclusion criteria, 2,147 patients having pure calcium oxalate stones were identified. There were 1,912 (89%) eumagnesuric (43-246 mg/24 hours) and 235 (11%) hypomagnesuric (<43 mg/24 hours) patients.

RESULTS

Patients with decreased urinary magnesium excretion had significantly less daily urine excretion of citrate, calcium, oxalate, uric acid, and sodium than the eumagnesuric group (p < 0.0001). Stone recurrence was slightly more common in the hypomagnesuric group, although the difference was not statistically significant. The percentage of patients voiding <1 L of urine per day was significantly higher in the hypomagnesuric group. In the eumagnesuric group, males outnumbered females 2:1, whereas hypomagnesuric patients showed a female predominance of 1.4:1.

CONCLUSION

The beneficial effects of urinary magnesium on stone formation may be less than previously reported. The role of oral magnesium supplementation and the subsequent increase in urinary magnesium in calcium urinary stone formation remains unknown. Our data suggest that its effect on or interaction with citrate may be influential on urinary citrate concentrations. If magnesium has a protective effect, it may work through pathways that enhance citrate excretion.

Effect of combined supplementation of magnesium oxide and pyridoxine in calcium-oxalate stone formers

A combined supplement of magnesium oxide (300 mg/day) and pyridoxine.HCl (10 mg/day) was given p.o. to 16 recurrent calcium oxalate (CaOx) stone formers, and its therapeutic efficacy was biochemically evaluated by measuring various parameters of blood (Na, K, Mg, urea, creatinine, calcium, phosphate, uric acid, alanine transaminase, aspartate transaminase and alkaline phosphatase) and urine (volume, pH, creatinine, Na, K, Mg, uric acid, calcium, phosphate, oxalate and citrate) at 0, 30, 60, 90 and 120 days of treatment. Serum Mg significantly (P < 0.01) increased after 30 days of treatment and remained constant thereafter while other blood parameters were unaltered. Combined treatment led to a significant increase in the urinary excretion of Mg and citrate over pretreatment values while oxalate excretion showed a gradual and significant decline during the therapy. The results confirmed the efficacy of MgO-pyridoxine supplementation in terms of changes in urinary excretion of lithogenic and inhibitory components, leading to a significant (P < 0.01) decrease in CaOx risk index from 0.09 +/- 0.04 at 0 day to 0.05 +/- 0.02 after 120 days of treatment.

Effects of magnesium salts in preventing experimental oxalate urolithiasis in rats

Magnesium oxide, magnesium hydroxide, magnesium sulfate, magnesium trisilicate, and magnesium citrate were added to a calcium-oxalate lithogenic diet in order to determine their effects in preventing lithogenesis. Male Wistar-strain rats which had been fed the glycolic-acid diet developed marked urinary calculi within four weeks. Rats in the magnesium-hydroxide, magnesium-citrate, and magnesium-trisilicate groups, however, had almost no stones in the urinary system. Rats in the magnesium-oxide and magnesium-sulfate groups showed significantly less effect than those in the former three groups. During the experimental period, the 24-hour urinary oxalate excretion and concentration were higher in the glycolic-acid group than in the other groups. The urinary citrate excretion and concentration were the highest in the magnesium-hydroxide and magnesium-citrate groups and higher in the magnesium-trisilicate and magnesium-oxide groups than in the magnesium-sulfate and glycolic-acid groups. Similar trends were observed in the urinary magnesium excretion and in its concentration. The urinary calcium excretion and concentration were higher in the experimental groups than in the glycolic-acid group. The urinary calcium/magnesium ratio remained mostly unchanged. Therefore, it can be concluded that alkaline magnesium salts increase the urinary calcium and magnesium concentrations, without changing the calcium/magnesium ratio, and inhibit urinary calculi formation, most likely by increasing the urinary citrate concentration.

Effect of magnesium citrate and magnesium oxide on the crystallization of calcium salts in urine: changes produced by food-magnesium interaction

The effect of magnesium citrate and magnesium oxide on urinary biochemistry and on the crystallization of calcium salts was examined in 7 normal subjects and 4 patients with recurrent calcium oxalate nephrolithiasis. When magnesium citrate or magnesium oxide was administered on an empty stomach (10 mEq. 4 times per day or 486 mg. magnesium per day for 2 weeks) urinary magnesium increased by only 77 to 79 mg. per day and urinary citrate increased by 98 to 142 mg. per day. However, urinary calcium increased by 21 to 25 mg. per day. No significant changes were noted in urinary saturation of calcium oxalate or brushite or in the limit of metastability (formation product) of these salts. However, when magnesium salts were provided with meals there were more prominent increases in urinary magnesium (by 92 to 96 mg. per day) and in citrate (by 218 to 226 mg. per day). Moreover, urinary oxalate decreased. Owing to these changes the urinary saturation of calcium oxalate decreased and the formation product increased. If magnesium citrate and magnesium oxide are to be used in the management of recurrent calcium oxalate nephrolithiasis, they should be administered with meals.

Magnesium bioavailability from magnesium citrate and magnesium oxide

This study compared magnesium oxide and magnesium citrate with respect to in vitro solubility and in vivo gastrointestinal absorbability. The solubility of 25 mmol magnesium citrate and magnesium oxide was examined in vitro in solutions containing varying amounts of hydrochloric acid (0-24.2 mEq) in 300 ml distilled water intended to mimic achlorhydric to peak acid secretory states. Magnesium oxide was virtually insoluble in water and only 43% soluble in simulated peak acid secretion (24.2 mEq hydrochloric acid/300 ml). Magnesium citrate had high solubility even in water (55%) and was substantially more soluble than magnesium oxide in all states of acid secretion. Reprecipitation of magnesium citrate and magnesium oxide did not occur when the filtrates from the solubility studies were titrated to pH 6 and 7 to stimulate pancreatic bicarbonate secretion. Approximately 65% of magnesium citrate was complexed as soluble magnesium citrate, whereas magnesium complexation was not present in the magnesium oxide system. Magnesium absorption from the two magnesium salts was measured in vivo in normal volunteers by assessing the rise in urinary magnesium following oral magnesium load. The increment in urinary magnesium following magnesium citrate load (25 mmol) was significantly higher than that obtained from magnesium oxide load (during 4 hours post-load, 0.22 vs 0.006 mg/mg creatinine, p less than 0.05; during second 2 hours post-load, 0.035 vs 0.008 mg/mg creatinine, p less than 0.05). Thus, magnesium citrate was more soluble and bioavailable than magnesium oxide.