Vitamin D lowers bone-fracture risk only in adequate doses

According to a new review, taking more than 800 international units of vitamin D daily could reduce the risk of hip fractures in older women by 30 percent.

To evaluate the efficacy of vitamin D supplementation in reducing fracture risk, the meta-analysis included data from 11 double-blind, randomized, controlled trials of oral vitamin D supplementation (taken daily, weekly, or every 4 months). Supplements were taken with or without calcium and compared with the effects of a placebo or calcium alone in 31,022 people aged 65 or over (1). The analysis showed that a lower risk of fracture was evident only at the highest vitamin D intake level (median 800 IU daily), with a 30% lower risk of hip fracture and a 14% lower risk of any non-vertebral fracture. A subgroup analysis showed that at the highest actual-intake level, the risk of hip fracture was reduced among all participants, whether they lived in the community or in an institution. The results also suggest that groups who were most vulnerable to vitamin D deficiency – people aged 85 or over and those with very low baseline blood concentrations of 25-hydroxy-vitamin D – benefited from vitamin D supplementation at least as much as others did.

The researchers commented that these findings support the most recent recommendation from the Institute of Medicine that people aged 65 or over receive 800 IU of vitamin D per day (2), but would also suggest that a 25-hydroxyvitamin D level of more than 60 nmol per liter may be most beneficial for reducing the risk of fractures. Furthermore, although the data did not allow the researchers to determine whether the actual-intake level of a calcium supplement influenced the effect of vitamin D at the highest actual-intake level, it is still important for future studies to consider the possibility that a calcium-supplement intake of 1000 mg per day or more, combined with high-dose vitamin D (above 800 IU per day) may be harmful. Calcium supple-ments taken without vitamin D have been reported to increase the risk of hip fracture (3).

The results of meta-analyses examining the relationship between vitamin D supplementation and fracture reduction have been inconsistent. Previous meta-analyses suggested that the benefits of vitamin D may be limited to older people living in institutions, and that the dose of vitamin D is irrelevant when vitamin D is combined with calcium.

1. Bischoff-Ferrari H. A. et al. A pooled analysis of vitamin D dose requirements for fracture prevention. N Engl J Med. 2012; 367:40–49.
   
   
2. Ross A. C. et al. The 2011 Dietary Reference Intakes for Calcium and Vitamin D: what dietetics practitioners need to know. J Am Diet Assoc. 2011; 111:524–527.
   
   
3. Bischoff-Ferrari H. A. et al. Calcium intake and hip fracture risk in men and women: a metaanalysis of prospective cohort studies and randomized controlled trials. Am J Clin Nutr. 2007; 86:1780–1790.