Intra-trial mean 25(OH)D and PTH levels and risk of falling in older men and women in the Boston STOP IT trial

Context Supplementation with vitamin D has the potential to both reduce and increase risk of falling, and parathyroid hormone (PTH) may contribute to fall risk. Objective To assess the associations of intra-trial mean circulating levels of 25-hydroxyvitamin D [25(OH)D] and PTH on incident falls in healthy older adults. Design Observational within a clinical trial. Setting The Bone Metabolism Laboratory at the USDA Nutrition Center at Tufts University. Participants 410 men and women age 65 years and older who participated in the 3-year Boston STOP IT trial to determine the effect of supplementation with 700 IU of vitamin D3 plus calcium on incident falls (secondary endpoint). Intra-trial exposures of 25(OH)D and PTH were calculated as the mean of biannual measures up to and including the first fall. Main outcome measures: incidence of first fall Results Intra-trial mean 25(OH)D was significantly associated with risk of falling in a U-shaped pattern; the range associated with minimal risk of falling was approximately 20-40 ng/ml. PTH was not significantly associated with risk of falling. Conclusions The findings highlight the importance of maintaining the circulating 25(OH)D level between 20 and 40 ng/ml, the range that is also recommended for bone health. At PTH levels within the normal range, there was no detectible independent association of PTH with fall risk.