Vitamin D status in patients with idiopathic calcium nephrolithiasis

Background – Classical physiopathologic models claim that high 25-OH vitamin-D can influence calcium kidney stone onset promoting urinary calcium excretion. Clinical practice instead shows that many patients with idiopathic calcium nephrolithiasis (ICN) are vitamin-D-defective. Aims – Our aim was to compare serum 25-OH-vitamin-D levels in a large cohort of kidney stone formers and healthy controls and, secondly, to evaluate the effects of vitamin-D deficiency on phosphocalcic metabolism in stone formers (SF). Methods – At the Stone Clinic of Parma University Hospital, Italy, we retrospectively evaluated 887 patients (364M, mean age 51±14 years) with ICN and 1031 matched healthy controls (CTRL) from the same area. Serum 25-OH-vitamin-D levels were collected for all subjects. Season of 25-OH-vitamin-D test, serum PTH, calcium, phosphorus and 24-h urinary calcium, phosphorus, citrate, ammonium and pH were also collected for stone formers. Data were analyzed with Mann-Whitney’s U test, Spearman’s rho, Kruskal-Wallis and χ2 test. Results – After correction for age and sex, 25-OH-vitamin-D levels were significantly lower in stone formers (median 18, IQR 12-24 ng/ml) than in controls (median 22, IQR 14-30 ng/ml, p<0.0001). The prevalence of vitamin-D insufficiency (20-30 ng/ml) was 31% (278/887) in SF and 32% (333/1031) in CTRL (p=0.654). The prevalence of deficiency (<20 ng/ml) was 56% (497/887) in SF and 42% (433/1031) in CTRL (p<0.001). Normal values (>30 ng/ml) were found only in 13% (112/887) of SF and in 26% (265/1031) of CTRL (p<0.001). 25-OH-vitamin D levels in SF were anyway significantly higher in summer (median 22 ng/ml, IQR 17-29) than in winter (median 16 ng/ml, IQR 10-23, p<0.005). In SF, PTH levels were higher in women than in men (median 52 IQR 41-65 vs 48 IQR 38-61 pg/ml, p<0.001) and in subjects aged >45 than in subjects aged ≤45 (median 52 IQR 42-67 vs 46 IQR 34-58 pg/ml, p<0.001) and they were inversely correlated with 25-OH-vitamin-D (Spearman’s rho -1.7, p<0.0001). Serum Ca++, but not calciuria, was significantly lower in vitamin-D-deficient than in vitamin-D-sufficient patients. Urinary phosphorus, citrate, ammonium and pH were unrelated to vitamin D levels. Conclusions – ICN patients have lower serum 25-OH-vitamin-D levels than healthy controls, thus suggesting a possible pathogenic role of this deficiency in promoting the onset of kidney stones.