Long-Term Change in Vitamin D Status and its Association With Change in Total Hip Bone Mineral Density in Older Women: A Population-Based Cohort Study

Abstract

Objective

To examine if long-term constant low vitamin D status in the sunny season has a greater impact on bone mineral density (BMD) over time than long-term constant low vitamin D status in the dark season.

Patients and Methods

In a longitudinal cohort study conducted from November 3, 2003 to May 22, 2019, 1802 Swedish women living in Uppsala County (latitude 58^oN) (mean baseline age of 65 years and average follow-up of 12 years) had vitamin D status measured by serum 25-hydroxyvitamin D concentration (S-25OHD). Participants were stratified by season of blood draw (dark [November-April] vs sunny [May-October]). We examined the association of long-term stable season-specific S-25OHD with 12-year changes in total hip BMD, measured by dual-energy x-ray absorptiometry, and investigated if increasing S-25OHD during follow-up influenced changes in BMD by baseline S-25OHD levels and season.

Results

Compared with longitudinally sunny season constant S-25OHD>70 nmol/L, women with sunny season constant S-25OHD<40 nmol/L displayed 10.0% (95% CI,3.8%-16.1%) lower total hip BMD at follow-up. No difference in BMD was observed by dark season S-25OHD. Among women with baseline sunny season S-25OHD<45 nmol/L, each 20 nmol/L increase in S-25OHD during follow-up was associated with a 2.5% increase in hip BMD (95% CI,0.5-4.6). This estimate was attenuated when increasing the low S-25OHD cut-off and was not observed with dark season samples.

Conclusion

Women with sunny season S-25OHD<40-50 nmol/L are a likely target group for vitamin D interventions to improve BMD. Blood samples taken during the dark season are less informative for determining future bone health.