The lower a woman's vitamin D levels as she transitions to menopause, the greater her risk of sustaining a non-traumatic fracture.
"Women who have levels less than 20 nanograms per milliliter would need some type of vitamin D supplementation," Dr. Jane A. Cauley of the University of Pittsburgh, who worked on the study, told Reuters Health. The findings back up the Institute of Medicine's conclusion that levels lower than 20 ng/ml are insufficient for skeletal health, she added.
Bone loss is known to accelerate during the menopausal transition, and postmenopausal women require significantly more calcium to reach neutral calcium balance than perimenopausal women, Dr. Cauley and her team note in their report, online February 23 in the Journal of Clinical Endocrinology and Metabolism.
To determine if serum 25(OH)D levels would be associated with changes in bone mineral density (BMD) and fracture risk, the researchers looked at data from the Study of Women's Health Across the Nation (SWAN) on 124 women with an incident traumatic fracture, 88 with an incident non-traumatic fracture, and 1,532 who did not sustain fractures during the study's average follow-up period of 9.5 years.
Each woman had her serum 25(OH)D level checked at her third annual visit, while her BMD was assessed annually. The women's mean age at the study's outset was 48.5.
Women with levels of 20 ng/ml or higher had a 46% lower risk of fracture during follow-up than women with lower levels of 25(OH)D. However, there was no association between baseline 25(OH)D and changes in BMD at the lumbar spine or femoral neck across menopause.
Roughly 22% of the women in the study had vitamin D levels greater than 30 ng/ml, Dr. Cauley said, so the study lacked power to determine if these women's non-traumatic fracture risk was further reduced.
"We didn't see any relationship between 25(OH)D and these bone changes, but we were somewhat limited in that we didn't have any information on how vitamin D changed across menopause," she added.