Stone risk unaffected by calcium/oxalate imbalance

Atlanta-Imbalances in meal content of calcium and oxalate throughout the day appears to have no impact on calcium oxalate stone risk in persons with high oxalate intake, at least in healthy non-stone-forming adults who maintain a normal daily calcium intake, according to a recent study.

"Typically, calcium and oxalate form complexes in the GI tract and are eliminated together in the feces. However, in periods of reduced calcium consumption, there is more unbound oxalate available for absorption, and this can result in increased urinary excretion of oxalate that can increase stone risk," said first author Jessica Lange, MD, urology resident at Wake Forest University School of Medicine, Winston-Salem, NC.

"The latter scenario has real-world relevance, as many people consume moderate amounts of calcium at breakfast and lunch with a low amount of oxalate but ingest a high amount of oxalate at dinner relative to the calcium intake. In our study mimicking this situation, we found that if an individual consumes at least the recommended amount of dietary calcium daily, there is not an increased impact on stone risk from a high dietary oxalate intake. However, it would also be interesting to repeat this study in a calcium oxalate stone-forming population," said Dr. Lange, who worked on the study with Dean G. Assimos, MD, and colleagues.

During the study, subjects consumed controlled meals prepared in a metabolic kitchen. For the balanced phase, all meals contained 333 mg calcium and 250 mg oxalate, while subjects consumed 400 mg calcium and 20 mg oxalate at breakfast and lunch and 200 mg calcium and 710 mg oxalate at dinner during the imbalanced phase. Each phase lasted 1 week, and there was a 1-week washout period between phases. Both diets were indexed to 2,500 calories and were similar in micronutrients, macronutrients, and fluid intake.

Urine specimens were collected on the last 4 days of each phase. Urinary oxalate and calcium and the Tiselius index for calcium-oxalate supersaturation were analyzed for the 24-hour collection and for subdivided collection periods: 8 a.m. to 1 p.m., 1 p.m. to 6 p.m., 6 p.m. to 11 p.m., and 11 p.m. to 8 a.m.

No significant differences over 24 hours

There were no significant differences between the two study phases in total 24-hour excretion of calcium or oxalate or in the Tiselius index. Tiselius index also remained similar for the two phases in all analyses of the subdivided urine collections, although there were some statistically significant differences in calcium and oxalate excretion between the balanced and imbalanced phases at some time periods. Specifically, calcium excretion was significantly higher during the imbalanced study phase than during the balanced phase during the 1 p.m. to 6 p.m. and 6 p.m. to 11 p.m. collection periods and was significantly lower during the imbalanced phase for the 11 p.m. to 8 a.m. period. For oxalate, mean excretion during the 1 p.m. to 6 p.m. time period was significantly lower during the imbalanced phase compared with the balanced phase.

Dr. Lange noted that the lack of any effects of the dietary imbalance between calcium and oxalate on findings from the 24-hour urinary excretion might be because calcium remains in the gut longer than previously thought.

"Perhaps the calcium is in the GI tract long enough to bind the bolus of oxalate from dinner in the imbalanced diet phase," she said.