Does taking vitamin D help against cancer?

Low blood levels of vitamin D are associated in observational research with a higher risk of various cancers, including colorectal, breast, bladder and ovarian cancer. But an association is not proof that supplementation actually helps: people with a deficiency may be less healthy for other reasons, and those confounding factors are difficult to rule out.

Large clinical trials in humans give a clearer answer, and it is a disappointing one for supplementation. A Finnish study in nearly 2,500 older adults over five years, in which participants received up to 3,200 IU per day, found no significant reduction in cancer incidence. The Women's Health Initiative, one of the largest trials ever conducted in women, likewise found no evidence that calcium plus vitamin D prevents breast or colorectal cancer in postmenopausal women.

There is, however, an important caveat to those negative results. In both large trials, participants already had, on average, sufficient vitamin D in their blood at the start. The idea is that supplementation adds little if you do not have a deficiency. One systematic review concludes that studies focusing specifically on people with a demonstrable deficiency (blood levels below roughly 40 ng/mL) do show a reduction in cancer risk and cancer mortality. But that conclusion comes from a single author and rests on a selection of studies, so the strength of that evidence is limited.

In short: if your vitamin D level is normal, extra supplementation probably offers no protection against cancer. Whether supplementation does anything in the case of a genuine deficiency has not yet been established with certainty. Have your level tested if you are unsure; if you are well below the norm, it is certainly worthwhile to address your deficiency, even though the cancer-protective effect of doing so has not yet been confirmed.