Validation of the 24,25-dihydroxyvitamin D to 25-hydroxyvitamin D ratio as a biomarker of 25-hydroxyvitamin D clearance.

The formation of 24,25-dihydroxyvitamin D (24,25(OH)D) from 25-hydroxyvitamin D (25(OH)D) is the primary mechanism for the metabolic clearance of 25(OH)D, and is regulated by tissue-level vitamin D activity. The ratio of 24,25(OH)D to 25(OH)D in blood (vitamin D metabolite ratio, VDMR) is postulated to be a marker of 25(OH)D clearance, however this has never been tested. We measured baseline 24,25(OH)D and 25(OH)D concentrations in 87 participants by liquid chromatography-tandem mass spectrometry. Following an infusion of deuterated 25(OH)D, blood samples for each participant were collected over 56 days and analyzed for deuterated vitamin D metabolites. 25(OH)D clearance and the deuterated metabolite-to-parent AUC ratio (ratio of the AUC of deuterated 24,25(OH)D to that of deuterated 25(OH)D) were calculated. We compared the VDMR with these two measures using correlation coefficients and linear regression. Participants had a mean age of 64 ± 11years, 41 % were female, 30 % were self-described Black, 28 % had non-dialysis chronic kidney disease (CKD) and 23 % had kidney failure treated with hemodialysis. The VDMR was strongly correlated with 25(OH)D clearance and the deuterated metabolite-to-parent AUC ratio (r = 0.51 and 0.76, respectively). Adjusting for 25(OH)D clearance or the deuterated metabolite-to-parent AUC ratio in addition to clinical covariates, lower VDMR was observed in participants with CKD and kidney failure than in healthy controls; in Black than White participants; and in those with lower serum albumin. Our findings validate the VDMR as a measure of 25(OH)D clearance. This relationship was biased by characteristics including race and kidney disease, which warrant consideration in studies assessing the VDMR.