The Free Hormone Hypothesis: Is Free Serum 25-Hydroxyvitamin D a Better Marker for Bone Mineral Density in Older Women?

It is presently unclear whether free serum 25-hydroxyvitamin D (S-25(OH)D) better reflects bone health than total S-25(OH)D. We have previously shown that summer total S-25(OH)D values are more useful to predict bone mineral density (BMD) than winter values. Our objective was therefore to compare the relative importance of free and total S-25(OH)D for BMD by season. BMD was measured by dual-energy X-ray absorptiometry (DXA) in 5002 Swedish women (mean age 68 years) randomly selected from a large population-based longitudinal cohort study. Free S-25(OH)D was analyzed by a commercial ELISA and total S-25(OH)D by HPLC-tandem mass spectrometry (MS/MS). Free and total S-25(OH)D co-varied with season, with 26% and 29% higher values in August compared with those in January-March (nadir). There were no differences in mean BMD between categories of free or total S-25(OH)D in samples collected during winter. Women with higher total S-25(OH)D measured during summer had higher BMD at the total hip. Compared with women who had total S-25(OH)D values above 80 nmol/L during summer, adjusted BMD at the total hip was 6% (95% CI, 1% to 11%) lower for S-25(OH)D concentrations between 30 and 40 mmol/L, and 11% (95% CI, 3% to 19%) lower for those with total S-25(OH)D <30 nmol/L. In contrast, free S-25(OH)D measured during summer was not associated with BMD. Compared with women who had highest free S-25(OH)D measured during summer (>8.8 pmol/L), those with intermediate (2.4-3.5 pmol/L) and lowest (<2.4 pmol/L) free S-25(OH)D during summer did not have lower total hip BMD values (3% [95% CI, -2% to 7%] and -2% [95% CI, -8% to 4%]). In addition, we found no added value for the prediction of BMD with the combined measurement of total and free S-25(OH)D during summer or winter. We conclude that vitamin D status assessed by direct measurements of free S-25(OH)D does not reflect BMD better than total S-25(OH)D. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.