The effect of ultraviolet B radiation treatments on calcium excretion and vitamin D metabolites in kidney stone formers

Several factors could explain the effect of warm, sunny weather on kidney stone formation. To determine the role of sun exposure, we used a light box to administer artificial ultraviolet B radiation during the winter months in New York City. Eleven male stone formers and 7 age- and sex-matched controls received 10 UVB light exposures over a two-week period while maintaining a 400 mg calcium diet. 25-OH vitamin D levels increased significantly (p < 0.001) in both patients (25.9 ± 9.8 to 51.6 ± 14.1 ng/ml) and controls (21.3 ± 7.1 to 49.6 ± 3.1 ng/ml). However, 1,25 dihydroxyvitamin D levels rose in the patients (50.8 ± 14.8 to 55.9 ± 13.1 pg/ml) but fell (60.1 ± 6.5 to 49.4 ± 3.1 pg/ml) in the controls. Only 2 of the 11 patients but all of the controls demonstrated this down regulation of 1,25 dihydroxyvitamin D levels (p < 0.002). 24,25 dihydroxyvitamin D levels tended to rise in both groups but parathyroid hormone levels were unchanged. There was a trend, which did not reach statistical significance, for parameters of calcium excretion to increase after UVB radiation. 24-hour urinary calcium excretion rose 24% from 140 to 173 mg in the patients and 31% from 113 to 148 mg in the controls. The ratio of calcium/creatinine following a one gram calcium load showed a small increase after UVB radiation from 0.17 to 0.20 in patients and 0.118 to 0.124 in the controls. However, no correlation could be discerned between changes in vitamin D metabolite concentrations and changes in urinary calcium. Our data support the notion that regulation of 1,25 dihydroxyvitamin D is different in stone formers but that changes in this metabolite cannot alone account for the phenomenon of absorptive hypercalciuria.