An essential trace element incorporated into selenoproteins including the glutathione peroxidases. Supplements supply it as selenomethionine, high-selenium yeast or inorganic sodium selenite, which behave differently in the body.
Selenium has the narrowest gap between requirement and harm of any nutrient in this catalogue: the RDA is 55 µg/day and the upper limit is 400 µg/day, a margin of about sevenfold, and a single Brazil nut can contain over 90 µg. Its supplement market grew from an early trial suggesting less cancer in a selenium-supplemented group; the much larger SELECT trial that followed found no cancer benefit, and a re-analysis of the original trial found a significantly increased incidence of type 2 diabetes in the selenium arm. Where soil selenium is low — parts of China, Finland before fortification, some of New Zealand — supplementation matters. Where intake is already adequate, which includes most of North America, there is no demonstrated benefit and a documented risk.
Sold as
Tablet, Capsule, High-selenium yeast, Sodium selenite, Component of multivitamins and thyroid formulas
Also known as
Selenomethionine, Sodium selenite, Selenium yeast, Se
Regulation: The RDA is 55 µg/day for adults with a Tolerable Upper Intake Level of 400 µg/day, set on selenosis. Sold as a dietary supplement with no pre-market efficacy review; some multivitamins and "thyroid support" blends supply 100–200 µg, which stacks quickly with a Brazil nut habit.
What the research used
The amounts, forms and durations published trials of Selenium actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
55 µg
Strong
Set at the intake that maximises glutathione peroxidase activity. Typical North American intakes already exceed it because of selenium-rich soils in the grain-growing regions.
Studied for Tolerable Upper Intake Level, adults
400 µg
Strong
Set on selenosis — hair and nail brittleness and loss, garlic-like breath, gastrointestinal upset and neurological symptoms. The margin between requirement and harm is narrower than for almost any other nutrient.
Studied for Cancer prevention trials
200 µg
Strong
Selenium yeast or selenomethionine, daily · 5–7 years
The dose in both the Nutritional Prevention of Cancer trial and SELECT. SELECT found no cancer benefit, and a re-analysis of the earlier trial reported an increased incidence of type 2 diabetes in the selenium group.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🍽️With food
Not enough
Taken with food for tolerability. No trial has compared timings, and selenium status changes over weeks rather than hours.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Limited
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Selenium deficiency impairs immune function and correcting it in deficient populations improves several immune measures. In selenium-replete populations, supplementation has not been shown to reduce infections.
What may be going on: Selenocysteine is built into glutathione peroxidases and thioredoxin reductases, which limit oxidative damage in immune cells.
Limited evidence. Early human work, or mostly lab and animal research.
Selenium supplementation reduced thyroid autoantibody levels in several small trials in autoimmune thyroiditis. Whether that changes thyroid function or symptoms is unresolved, and guidelines do not recommend it routinely.
Limited evidence. Early human work, or mostly lab and animal research.
The SELECT trial randomised over 35,000 men to selenium, vitamin E, both or placebo and found no reduction in prostate cancer from selenium. The observational and early-trial data that motivated it did not survive the test.
Not enough evidence. Not enough human research to say anything useful yet.
No source is attached to this association yet, so it is rated not enough evidenceby default. It is listed because people commonly ask about it — not because there is evidence for it.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Selenosis — hair and nail loss at a modest multiple of the requirement
Chronic intake above the 400 µg/day upper limit causes brittle hair and nails progressing to hair loss and nail shedding, a garlic odour on the breath, rashes, gastrointestinal upset, irritability and peripheral neuropathy. A manufacturing error in one US supplement delivered around 200 times the labelled dose and produced hundreds of poisoning cases, which is a reminder of how narrow the margin is.
Too much
Increased type 2 diabetes risk signal
A re-analysis of the Nutritional Prevention of Cancer trial found significantly more new type 2 diabetes among people taking 200 µg/day, and SELECT showed a non-significant trend in the same direction. This is a reason not to take selenium routinely where intake is already adequate.
Too much
Brazil nuts count
A single Brazil nut can contain 68–91 µg of selenium and content varies severalfold with soil. Eating a handful daily alongside a selenium-containing supplement is an easy and entirely unintentional route to the upper limit.
Interaction
May interact with statins, chemotherapy and thyroid management
An antioxidant cocktail including selenium blunted the HDL response to a statin–niacin combination in one trial. High-dose antioxidants during chemotherapy or radiotherapy are generally discouraged pending better data, and selenium affects thyroid hormone metabolism, which matters for anyone on thyroid medication.
Worth knowing
Form matters
Selenomethionine is incorporated into body proteins non-specifically and accumulates over time; inorganic selenite does not. This affects how quickly a high intake builds up and how long it takes to clear.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.