One study found no significant differences between groups in the risk of fracture, but did find an increased risk for kidney stone formation in the supplementation group (N. Engl. J. Med. 2006;354:669–83). The results surprised medical experts. “This is contradictory to what we all expected and what we all believe is true,” Dr. Lim said. The second study found no difference between groups in the incidence of colorectal cancer or in tumor characteristics (N. Engl. J. Med. 2006;354:684–96).
Editorials in the same issue pointed out the studies' limitations. The 400 IU/day dose of vitamin D in both studies may have been suboptimal. There is a growing consensus to revise recommended doses upward.
In the study of fracture risk, subjects were not selected based on low bone mineral density, and 75% in both groups were on hormone replacement therapy. All subjects were allowed to take personal supplements. In the placebo group, 64% of women took calcium supplements, and 42% took 400 IU/day of vitamin D (N. Engl. J. Med. 2006;354:750–2).
In the other study, the mean age at entry was 62 years, “which is just reaching the high-risk age for development of colorectal cancer,” Dr. Lim said, and the 7-year follow-up was too early to detect significant differences between groups for cancer incidence (N. Engl. J. Med. 2006;354:752–4).