Vitamin D Levels Predict Bisphosphonate Response
Carmel A, et al. The 25(OH)D level associated with a favorable bisphosphonate response is >33 ng/mL. Annual meeting of the American Society for Bone and Mineral Research (ASBMR 2011); Abstract 1137.
The study included 210 women who had been on bisphosphonates for at least 18 months. Almost all were white (83%) with a mean age of 66. The mean femoral neck T score was -2.
They were considered nonresponders if their T score remained below -3 with treatment, if they had a decrease in BMD of more than 3%, or if they had an incident fracture while on therapy.
They were classified as responders if their T scores were maintained above -3, their BMD was stable, and they had no fractures.
About half were being treated with alendronate (Fosamax), one quarter with risedronate (Actonel), and the remainder with ibandronate (Boniva) or zoledronate/zoledronic acid (Zometa).
Slightly more than half were nonresponders (111 versus 99), with the most common reason for nonresponse being a loss of 3% or more in BMD while on treatment.
Factors that were associated with nonresponse included older age, longer bisphosphonate use, and low vitamin D levels.
The researchers then looked at response according to specific 25(OH)D levels, with 33 ng/mL being a level thought to be associated with decreased fracture risk.
After adjusting for age, body mass index, type of bisphosphonate, and duration of treatment, they confirmed that a level of at least 33 ng/mL had the highest odds ratio for response to bisphosphonates, as well as the strongest P-value (4.5-fold greater odds of bisphosphonate response, estimated OR 4.5, P=0.0001).
"This level [33 ng/mL] is higher than that recommended by the Institute of Medicine as adequate for the general population, and many patients have levels well below this, so vitamin D supplementation may need to be higher for this therapeutic outcome," Carmel said.