Selenium
A trace mineral with outsized impact on thyroid and immune health.
Selenium is critical for thyroid hormone metabolism, antioxidant defense via glutathione peroxidase, and DNA repair. Low selenium levels are linked to increased cancer risk, impaired immune function, and cognitive decline. The SU.VI.MAX trial found that selenium supplementation reduced cancer incidence in men by 31%.
How Selenium affects aging
Nearly everything selenium does, it does as part of selenoproteins — roughly 25 enzymes with selenium built into their active sites. The glutathione peroxidases neutralize hydrogen peroxide and lipid peroxides before they can damage membranes and DNA; thioredoxin reductases maintain the redox balance every cell depends on. Because oxidative damage accumulates across the lifespan, these enzymes sit directly on one of the classic aging pathways. When selenium supply falls short, the body triages — protecting the brain and endocrine glands first while other tissues run their antioxidant defenses below capacity.
The thyroid holds more selenium per gram than any other organ. Deiodinase enzymes — themselves selenoproteins — convert the storage hormone T4 into active T3, so selenium status shapes metabolic rate, energy, and temperature regulation, all of which tend to drift with age. Selenium matters equally for the aging immune system: low status in older adults is associated with weaker immune responses and, in observational studies, with higher mortality and faster cognitive decline. Adequate selenium supports the T-cell proliferation and natural killer cell activity that immunosenescence steadily erodes.
Selenium's relationship with longevity is U-shaped, and that shape is the most useful thing to know about it. Benefits appear when low status is corrected — antioxidant selenoenzymes reach full activity at moderate blood levels and gain nothing beyond that. Above the optimal range, extra selenium is not neutral: supplementing people who are already replete has been linked to adverse outcomes, including a signal for increased type 2 diabetes risk. And because soil selenium varies enormously by region, the same diet can be fully sufficient in one country and marginal in another.
Dosage
RDA: 55mcg for adults
Optimal for longevity: Researchers suggest 100-200mcg daily, though exceeding 400mcg can be toxic.
Selenomethionine — the form found in food and in better supplements — is absorbed very efficiently and incorporated into body proteins; inorganic selenite is less well retained. One to two Brazil nuts a day is the simplest food-first strategy, though their selenium content varies widely with the soil they grew in. Fish, shellfish, eggs, and organ meats are more consistent sources, and most mixed diets that include them land in a good range.
The window between enough and too much is narrower than for most nutrients. The tolerable upper limit is 400mcg per day, and chronic excess causes selenosis — brittle hair and nails, garlic breath, fatigue. If you already eat fish and nuts regularly, a high-dose selenium supplement is more likely to overshoot than to help; 100–200mcg of total daily intake covers the optimal range.
Deficiency signs
~15% of the global population is selenium-deficient; rates vary by region due to soil content
- Weakened immune function
- Thyroid problems or fatigue
- Hair thinning or loss
- Mental fog or difficulty concentrating
Top food sources
What the research shows
The definitive review of selenium biology, and the source of the field's central caution. Low selenium status is associated with higher mortality, weaker immune function, and cognitive decline, and correcting it brings clear benefits. But the relationship is U-shaped: people with adequate-to-high status may be adversely affected by supplements, including a possible increase in type 2 diabetes risk. Rayman's conclusion — supplement the deficient, not the replete — remains the best one-line summary of selenium science.
The famous null result. In 35,533 generally well-nourished men, 200mcg of selenium daily — alone or with vitamin E — did not prevent prostate cancer or any other cancer over a median 5.5 years, and the trial was stopped early for futility. A clear demonstration that adding selenium to an already-replete population delivers no anticancer benefit.
The trial that launched selenium's anticancer reputation. In 1,312 patients with prior skin cancer, 200mcg of selenium daily did not reduce the primary endpoint of new skin cancers — but secondary analyses showed significant reductions in total cancer incidence and cancer mortality. Read together with SELECT's later null result, the most coherent interpretation is that benefits concentrated in participants who started with low selenium status.
Frequently asked questions
How many Brazil nuts should I eat per day?
One to two is plenty — a single nut can carry around 96mcg of selenium, roughly twice the RDA. Their content varies a lot with the soil they grew in, so treat them as a generous average rather than a precise dose. Eating handfuls daily risks creeping past the safe range over time.
Should I take a selenium supplement?
Only if your intake is genuinely low — for example, if you rarely eat fish, seafood, eggs, or nuts, or live in a low-selenium soil region. The large SELECT trial found no benefit from supplementing already well-nourished people. Food-first is the safer default for this mineral.
Does selenium help with thyroid problems?
Selenium is essential for activating thyroid hormone — the enzymes converting T4 to active T3 are selenoproteins — so a deficiency can impair thyroid function. Research also suggests adequate selenium reduces the risk of autoimmune thyroid disease. If you have a diagnosed thyroid condition, discuss selenium with your doctor rather than self-dosing.
Can too much selenium be harmful?
Yes, and the margin is smaller than for most nutrients. Chronic intake above the 400mcg daily upper limit can cause selenosis — brittle hair and nails, garlic-scented breath, fatigue — and supplementing people with already-high status has been linked to a possible increase in type 2 diabetes risk.
Does selenium prevent cancer?
The honest answer is: only maybe, and only if you start deficient. An early trial in 1996 found fewer total cancers among supplemented patients, but the much larger SELECT trial found no protection in well-nourished men. Correcting a low level is worthwhile; adding more on top of adequacy is not.
Track your selenium intake
Biohack logs selenium alongside 22 other longevity nutrients in every meal.