Selenium and cancer: how a promising trial was overturned

Selenium supplements do not prevent cancer. A small trial in the 1990s produced a tantalising hint that they might, which drove years of supplement sales, but the much larger trial built to confirm it found no benefit and some signals of harm, including a slightly raised risk of type 2 diabetes [s1][s3]. It is a textbook example of why a promising secondary finding has to be tested head-on before it becomes advice.

The trace element, and the hope

Selenium is an essential trace element the body needs in tiny amounts, and severe deficiency causes real disease. The optimism about cancer came from a specific place: the Nutritional Prevention of Cancer (NPC) trial, which randomised 1,312 people with a history of skin cancer to 200 micrograms of selenium a day or placebo [s4]. Selenium did nothing for the trial's actual target — it did not reduce basal or squamous cell skin cancers [s4]. But its secondary analyses were striking: compared with placebo, the selenium group had significantly lower total cancer mortality (relative risk 0.50, 95 percent confidence interval 0.31 to 0.80) and lower total cancer incidence (relative risk 0.63, 95 percent confidence interval 0.47 to 0.85), with apparent reductions in lung, colorectal and prostate cancers [s4]. The authors were explicit that these secondary results needed confirmation in a purpose-built trial before any public-health recommendation [s4]. That caution was well placed.

SELECT: the confirmation that wasn't

The Selenium and Vitamin E Cancer Prevention Trial (SELECT) was that purpose-built trial, and it was large. It randomised 35,533 relatively healthy men in the United States, Canada and Puerto Rico to selenium (200 micrograms a day from L-selenomethionine), vitamin E (400 IU a day), both, or placebo [s1]. The first report, in 2009, found no reduction in prostate cancer from selenium: the hazard ratio was 1.04 (99 percent confidence interval 0.87 to 1.24), essentially flat [s1]. There was no benefit for any other prespecified cancer, and two uncomfortable signals: a statistically non-significant increase in prostate cancer in the vitamin E group, and a non-significant rise in type 2 diabetes in the selenium group (relative risk 1.07, 99 percent confidence interval 0.94 to 1.22) [s1].

Longer follow-up made one of those signals worse. The 2011 analysis, with more cases, found that vitamin E significantly increased prostate cancer risk — a hazard ratio of 1.17 (99 percent confidence interval 1.004 to 1.36) — while selenium alone sat at 1.09 (99 percent confidence interval 0.93 to 1.27) and the combination at 1.05 (99 percent confidence interval 0.89 to 1.22) [s2]. In absolute terms the increase per 1,000 person-years was 1.6 cases for vitamin E, 0.8 for selenium and 0.4 for the combination [s2]. Neither supplement, alone or together, prevented anything; one of them caused harm [s2].

What the whole evidence base says

Pooling everything sharpens the verdict. A Cochrane review gathered 83 studies, including randomised trials of 27,232 participants, and found no protective effect: the summary risk ratio for developing any cancer was 1.01 (95 percent confidence interval 0.93 to 1.10), and for prostate cancer 1.01 (95 percent confidence interval 0.90 to 1.14), both graded high-certainty [s3]. The review also flagged the recurring harms — a slightly increased risk of type 2 diabetes, and, in the largest trial, more high-grade prostate cancer among men who already had high selenium status [s3]. Observational studies still show lower cancer rates in people with higher selenium exposure, but the reviewers judged that evidence very low certainty, prone to confounding, and note that the NPC trial's original hypothesis — that people with low selenium could cut their cancer risk by topping up — has not been confirmed [s3].

The thyroid caveat, and the food angle

Selenium is genuinely important for the thyroid gland, which is the strongest reason not to overstate this as "selenium is useless." Its role there sits alongside another trace element the thyroid depends on, iodine, whose deficiency story we cover in iodine and public health. The pattern here — a nutrient the body clearly needs, whose supplement fails to deliver the hoped-for prevention — is the same one seen with beta-carotene and vitamin A and with vitamin D in healthy adults. For most people, selenium comes adequately from food — Brazil nuts, seafood, meat and grains — and the gap that supplements are sold to fill largely is not there, a theme we return to in plant-based diets and nutrient gaps.

How to read this

The evidence that selenium supplements prevent cancer is not thin or mixed — it is clear and negative, resting on a large randomised trial and a high-certainty review [s2][s3]. The honest lesson is about method as much as selenium: the exciting NPC secondary finding was a hypothesis, not a result, and the trial built to test it overturned it [s1][s4]. This article is informational and is not medical or dietary advice; anyone considering supplements for a diagnosed deficiency should discuss it with a qualified clinician.