SELECT: What the Prostate Cancer Trial Found

A prevention trial that found the opposite of what it hoped

SELECT set out to show that two cheap supplements could stop prostate cancer before it started. Earlier studies had hinted that both selenium and vitamin E might help. This was the large, careful test of that idea.

It found no protection. It found harm.

The scale of the test

35,533 men were randomly assigned between 2001 and 2004, across 427 study sites in the United States, Canada and Puerto Rico. 34,887 of them were in the main analysis.

They were split into four groups of roughly equal size. 8,752 took selenium. 8,737 took vitamin E. 8,702 took both. 8,696 took neither, receiving matching dummy pills instead.

To join, a man needed a PSA of 4.0 ng/mL or less and a rectal exam that raised no suspicion. Black men could join from age 50, everyone else from 55.

The doses were specific: 400 IU a day of all rac-alpha-tocopheryl acetate, and 200 micrograms a day of L-selenomethionine.

More prostate cancers in the vitamin E group

In the placebo group, 529 men developed prostate cancer.

In the vitamin E group, 620 did. That is a hazard ratio of 1.17 (99% CI 1.004 to 1.36; p=0.008) — a 17% increase in risk.

Selenium alone gave 575 cases (hazard ratio 1.09; 99% CI 0.93 to 1.27; p=0.18). The two together gave 555 cases (hazard ratio 1.05; 99% CI 0.89 to 1.22; p=0.46). Neither of those reached statistical significance.

How large the extra risk was in practice

A 17% increase sounds alarming on its own. The absolute figures are steadier.

Compared with placebo, the extra risk per 1,000 person-years was 1.6 cases for vitamin E, 0.8 for selenium and 0.4 for the combination.

So for vitamin E, roughly one to two extra prostate cancers per 1,000 men per year. Small per person, and not small at all across a population that buys these pills by the million.

Why this trial is still cited

SELECT is the standard example of why supplement claims need randomised evidence. A plausible idea, supported by observational data and taken by millions of people, turned out to increase the very disease it was meant to prevent.

That is not an argument against nutrition. It is an argument against assuming that a nutrient in a pill behaves like the same nutrient in food.

What this finding cannot tell you

• It tested one dose and one chemical form of each supplement. Other forms, other doses and vitamin E from food were not studied.
• Supplements were stopped in 2008 while follow-up continued, so some of the cancers counted were diagnosed after the men stopped taking the pills.
• Participants were relatively healthy men being screened with PSA. How hard you look affects how many cancers you find.
• It measured prostate cancer specifically. It is not a verdict on vitamin E for anything else.

Questions about supplements

• Am I taking any supplement at a dose above what food would give me?
• Does anything I take interact with my treatment or my other medicines?
• Is there randomised evidence behind a supplement I have been recommended?
• Would my team like a full list of what I take, including vitamins?

Sources

This article was written from the sources below, which were checked on the source-check date shown above.

• JAMA: Vitamin E and the Risk of Prostate Cancer — Selenium and Vitamin E Cancer Prevention Trial (SELECT) (primary)

How this article was prepared

Prepared by Cancer Explained's AI-assisted editorial system and checked against the sources listed below. This article has not been reviewed by a healthcare professional unless a named reviewer is specifically shown.

Cancer Explained is published by the National Cancer Information Foundation as a nonprofit-oriented public-interest education project. It is not a diagnostic service, does not recommend treatments, and is not for emergencies.

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