Calcium isotope ratio in kidney stones: preliminary exploration of mechanism from the geochemical perspective.

Stable calcium (Ca) isotope ratios are sensitive and radiation-free biomarkers in monitoring biological processes in human bodies. Recently, the Ca isotope ratios of bone, blood, and urine have been widely reported to study bone mineral balance. However, as a pure Ca crystallization product, there is no report on the Ca isotope ratios of kidney stones, even though the prevalence of kidney stones is currently on the rise. Here, we measured Ca isotope data of 21 kidney stone samples collected in Beijing, China. The δ44/42CaNIST 915a values ranged from 0.25‰ to 2.85‰ for calcium oxalate, and from 0.38‰ to 3.00‰ and 0.61‰ to 0.69‰ for carbonate apatite and uric acid, respectively. Kidney stones have heavier Ca isotope ratios than bone or blood, which is probably because complexed Ca contains more heavy Ca isotopes than free Ca2+. Ca isotope evidence suggests that magnesium (Mg) affects kidney stone formation, as the δ44/42CaNIST 915a value is inversely correlated with the Ca/Mg ratio. This study provides important preliminary reference values on the Ca isotopic composition of kidney stones and proposes a factor influencing Ca isotope fractionation in biological processes for future research.