Selenium
Also called: selenomethionine.
Last reviewed 2026-09-16. How this works.
Prevention Grade C
Human evidence exists and does not settle it.
Selenium did nothing for people who were short of it, and the signal ran the wrong way in those who were not.
The effect appears to depend on how much you had to begin with.
The claim
The claim is that selenium supplementation prevents cancer, prostate cancer in particular.
What it does
Selenium is required for several antioxidant enzymes, and the deficiency argument is the one usually made. What the trial found is more interesting than the mechanism.
The evidence
A case-cohort analysis inside a large randomised prevention trial split people by how much selenium they already had, measured in toenail clippings. Among those with lower baseline status, supplementation had no effect.1
Among those who already had more, the signal ran in the opposite direction. Supplementing someone who was not short of it did not merely fail to help.1
One further detail undercuts the premise. Toenail selenium in people who were not supplementing was not itself associated with prostate cancer risk, so having less of it was not a risk in the first place.1
This is the first of four places in this chapter where the same pattern appears, and the book names it: what a supplement does depends on the baseline status of the person taking it.1
The studies this rests on
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Baseline selenium status within the SELECT prevention trial1
An observational study, double blind, measured a clinical outcome.
no effect in men with lower baseline status; the signal ran the other way in men with higher baseline status.
What is absent
Even in the group the deficiency argument is about, people with lower baseline selenium, supplementation showed no benefit.
Scope: people with low baseline selenium.
Established by trial scope, checked 2026-09-16. Result: the case-cohort analysis found no effect in the lower baseline status group.1
What to ask
Is there any reason to think I am short of a particular nutrient, and would you test for it?
The dish-to-person gap
The register holds no comparison between a laboratory concentration and a blood level for selenium.
When to call your team — no warning signs are recorded
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The register records no warning signs for selenium. That is not a statement that nothing can go wrong.
Interactions
The register holds no measured interaction between selenium and a cancer treatment class. Chapter 16 of the book covers supplement interactions in general, and an empty list here means nobody has measured this one.
Nobody having looked is not the same as nothing being there. That is what this line says and all it says.
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
The question this entry raises is about baseline status, which is a clinical question rather than a trial question.
References
Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk. A case-cohort analysis within the SELECT trial.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 5.