Prevalence of Vitamin D Inadequacy Among Postmenopausal North American Women Receiving Osteoporosis Therapy

Abstract

An adequate intake of vitamin D and calcium is viewed as a necessary part of treating osteoporosis. Recommended daily intake in the United States is 400 IU (10 μg) in persons aged 51–70 years and 600 IU (15 μg) for those over age 70. Vitamin D status is evaluated by measuring the serum level of 25-hydroxyvitamin D (25[OH]D), the major circulating metabolite of vitamin D. Many epidemiologic studies have confirmed that vitamin D inadequacy is a common problem throughout the world. This study estimated serum 25(OH)D levels in 1536 postmenopausal Norh American women who were receiving medication to prevent or treat osteoporosis. All of them had been postmenopausal for at least 2 years and were 55 years of age or older. More than 90% of the study population was white and the mean age was 71 years. Nearly 60% of participants reported taking 400 IU or more of vitamin D daily. Nearly one fifth (18.7%) had had a fracture caused by falling within the past 5 years. The mean serum 25(OH)D level was 30.4 ng/mL, and the 25th and 75th percentile values were 22 and 37 ng/mL, respectively. Several cutoff points were used in this study. Regardless of the level used, vitamin D inadequacy was significantly greater in women who reported taking less than 400 IU of vitamin D daily. At a cutoff point of 30 ng/mL, the prevalence of inadequacy was 63% in women taking less than 400 IU daily and 45% in those taking 400 IU or more. Multivariate analysis yielded 8 variables that were significantly associated with vitamin D inadequacy: age over 80, being nonwhite, a body mass index exceeding 30 kg/m2, taking medications that affect vitamin D metabolism, supplementation of less than 400 IU daily, a lack of exercise, failure to discuss the importance of vitamin D with a physician, and an educational level below grade 12. The number of risk factors correlated with the risk of vitamin D inadequacy but, even in 203 women who lacked any risk factors, the prevalence of vitamin D inadequacy was 32%. Women with relatively low serum 25(OH)D levels had a significantly higher prevalence of secondary hyperparathyroidism. The prevalence of vitamin D inadequacy in postmenopausal women in North America who are receiving medication to forestall or treat osteoporosis is unacceptably high. Both physicians and the public need to be better educated about how to optimize vitamin D supplementation in these women.