SAN DIEGO – Postmenopausal women who are being treated for low bone mineral density have nearly a fivefold greater response rate to bisphosphonate therapy if they maintain a serum vitamin D level of 33 ng/mL or higher than they do with a level below that threshold, according to an award-winning study.
Moreover, a multivariate analysis demonstrated that for each 1-ng/mL decrease in serum vitamin D level, the likelihood of a favorable response to bisphosphonate therapy dropped by 5%, Dr. Amanda Carmel reported at the annual meeting of the American Society for Bone and Mineral Research.
This is the first study to identify a threshold level of serum vitamin D that defines improved therapeutic outcomes with bisphosphonates. The identified threshold of 33 ng/mL is higher than recommended as adequate for the general population in the 2010 Institute of Medicine report. The discrepancy suggests that higher vitamin D levels may be required for specific therapeutic outcomes, said Dr. Carmel, whose study earned the 2011 ASBMR Shun-Ichi Harada Young Investigator Award.
The study was based upon a chart review of 210 women (mean age, 66 years) who had been on bisphosphonate therapy for low bone mass or osteoporosis for an average of 5 years. Half were on alendronate (Fosamax), 27% were on risedronate (Actonel), and the rest were on ibandronate (Boniva) or zoledronic acid (Reclast).
In all, 52% of subjects were categorized as bisphosphonate nonresponders on the basis of a T score less than –3 that persisted on dual-energy x-ray absorptiometry scans taken at least 18 months apart, or a greater-than-3% decrease in bone mineral density, or an incident fracture on therapy.
The average serum vitamin D level was 42.1 ng/mL in responders and 32.3% in nonresponders. A level of 33 ng/mL or less was present in 21% of responders and 58% of nonresponders, said Dr. Carmel of Cornell University, New York.
Several audience members expressed skepticism regarding her conclusions. They argued that Dr. Carmel may have mistaken cause and effect. In their view, the most likely explanation for nonresponse to bisphosphonates is poor adherence to medication, and women who are noncompliant with their bisphosphonate are also probably going to be nonadherent to their vitamin D supplementation.
Dr. Carmel replied that she and her colleagues screened for poor compliance by asking women if they were taking their bisphosphonate regularly and excluding those who said they weren’t.
She reported having no financial conflicts.