Do vitamin D supplements prevent cancer? Two large randomised trials say no

Vitamin D supplements do not appear to prevent cancer. Two of the largest randomised, placebo-controlled trials ever run on the question — one in the United States, one in Australia — tested daily or monthly vitamin D against a dummy pill in tens of thousands of adults, and neither found a reduction in cancer incidence or cancer death. This is the highest-quality evidence available, and it points clearly away from the popular idea that topping up vitamin D lowers cancer risk.

Why randomised trials matter here

Observational studies have long linked low blood levels of vitamin D to higher rates of several cancers. But blood vitamin D is entangled with everything from sunlight exposure and outdoor activity to body weight and general health, so an association tells you little about whether swallowing a supplement changes anything. The only way to isolate the supplement's effect is to randomly assign people to take it or not, and then wait. Two trials did exactly that.

VITAL: 25,871 adults, no reduction in cancer

The VITAL trial, published in the New England Journal of Medicine, randomised 25,871 US adults — men aged 50 or older and women aged 55 or older, including 5,106 Black participants — to vitamin D3 at 2000 IU per day or placebo, in a design that also tested omega-3 fish oil [s1].

Over a median follow-up of 5.3 years, invasive cancer of any type was diagnosed in 1,617 participants — 793 in the vitamin D group and 824 in the placebo group [s1]. The hazard ratio was 0.96, with a 95% confidence interval of 0.88 to 1.06 (P=0.47) — a result statistically indistinguishable from no effect [s1]. Death from cancer, a secondary endpoint, gave a hazard ratio of 0.83 (95% confidence interval 0.67 to 1.02), a suggestion of possible benefit that did not reach statistical significance [s1]. For individual sites the numbers were flat: breast cancer 1.02, prostate 0.88, colorectal 1.09, all with intervals crossing 1 [s1]. The trial's own conclusion was blunt: vitamin D did not lower the incidence of invasive cancer [s1].

D-Health: 21,315 adults, and a signal pointing the wrong way

The Australian D-Health trial used a different regimen — a large monthly dose of 60,000 IU — in 21,315 people aged 60 or older, randomised to vitamin D3 or placebo for five years [s2]. Blood testing confirmed the supplement worked biochemically: mean serum 25-hydroxy-vitamin D reached 115 nmol/L in the vitamin D group versus 77 in the placebo group [s2].

Its primary endpoint was all-cause mortality, and there was no benefit: hazard ratio 1.04 (95% confidence interval 0.93 to 1.18) [s2]. For cancer mortality specifically, the hazard ratio was 1.15 (95% confidence interval 0.96 to 1.39, P=0.13) — not significant, but numerically above 1 [s2]. In an exploratory analysis that excluded the first two years of follow-up, participants on vitamin D had a numerically higher hazard of cancer death (hazard ratio 1.24, 95% confidence interval 1.01 to 1.54, P=0.05) [s2]. The investigators were careful about what this does and does not mean: it is an exploratory finding, not a primary result, but it is enough that they invoked the precautionary principle, suggesting this high monthly dose "might not be appropriate in people who are vitamin D-replete" [s2].

Reading the two trials together

The consistent message is that supplementing largely vitamin-D-sufficient older adults does not prevent cancer [s1][s2]. VITAL found no effect on incidence and a non-significant hint of lower cancer death; D-Health found no effect on mortality and, if anything, a hint in the harmful direction for cancer death [s1][s2]. Those two mortality signals do not cancel to a clean "no effect" so much as bracket it — the honest reading is that any true effect is small and its direction is not established.

Several caveats bound the conclusion. Both trials enrolled populations that were mostly vitamin-D-sufficient at baseline, so they cannot say whether correcting genuine deficiency changes cancer risk [s1][s2]. Both followed participants for roughly five to six years, and cancer takes longer than that to develop, so a delayed benefit cannot be fully excluded [s1][s2]. And neither studied children or younger adults.

What the trials do settle is the everyday version of the question. For an older adult taking vitamin D in the hope of preventing cancer, the best available randomised evidence does not support that hope. This article is informational and is not medical advice; decisions about supplementation, especially where deficiency or bone health is involved, are clinical ones.