In so-called alternative medicine (SCAM), vitamin D is often recommended for a range of indications, including cancer prevention. Observational studies did indeed suggest that low vitamin D status may be a risk factor for cancer. On the basis of such evidence, a team of researchers sought to determine if vitamin D supplementation lowers the risk of cancer and precancers.
The Vitamin D and type 2 diabetes (D2d) cancer outcomes study (D2dCA) is an ancillary study to the D2d study, which was conducted at 22 academic medical centers in the United States. Participants had prediabetes and overweight/obesity and were free of cancer for the previous 5 years. Participants were randomized to receive vitamin D3 4000 IU daily or placebo. At scheduled study visits (4 times/year), cancer and precancer events were identified by questionnaires. Clinical data were collected and adjudicated for all reported events. Cox proportional hazard models compared the hazard ratio (HR) of incident cancers and precancers between groups.
Over a median follow-up period of 2.9 years, among 2385 participants (mean age 60 years and 25-hydroxyvitamin D 28 ng/mL), there were 89 cases of cancer. The hazard ratio (HR) of incident cancer for vitamin D vs placebo was 1.07 (95% CI 0.70, 1.62). Of 241 participants with incident precancers, 239 had colorectal adenomatous polyps. The HR for colorectal polyps for vitamin D vs placebo was 0.83 (95% CI 0.64, 1.07).
The authors concluded that, in the D2d population of participants with prediabetes and overweight/obesity, not selected for vitamin D insufficiency, vitamin D supplementation did not have a significant effect on risk of incident cancer or colorectal polyps.
For some reason, the discussions about the value of vitamin D for any condition tend to be dominated by emotion rather than evidence. This is particularly true in the realm of SCAM. The trial is a significant step forward. It is rigorous and generates clear findings. Let’s hope it will contribute to a more rational debate about the value of vitamin D in cancer prevention.