TL;DR: Selenium is an essential trace element with the narrowest gap between "not enough" and "too much" of any common nutrient. It is built into selenoproteins, including the glutathione peroxidases that handle oxidative stress and the enzymes that activate thyroid hormone, so deficiency has real consequences. Keshan disease, a cardiomyopathy occurring in selenium-poor regions of China, is the clearest demonstration of that. But selenium is the textbook case of a U-shaped exposure curve: mortality risk appears elevated at both low and high intake. The trial history is instructive and slightly humbling. Observational data suggested selenium might protect against cancer. The large SELECT trial tested that directly and found no cancer prevention, alongside signals of harm including increased type 2 diabetes risk. Cochrane reviews of selenium supplementation have concluded there is no convincing evidence of benefit for cancer prevention. The practical position is simple: if you eat a varied diet in most Western countries, you almost certainly get enough. Supplementing beyond adequacy is not supported and may cause harm. Brazil nuts are so selenium-dense that a few a day can approach the upper limit.
What does selenium do in the body?
Selenium works almost entirely through selenoproteins, of which humans make around 25. Unlike most minerals, which act as loosely bound cofactors, selenium is incorporated directly into the protein chain as the amino acid selenocysteine, using a dedicated and rather remarkable piece of biological machinery that reassigns a stop codon to insert it. That level of evolutionary investment tells you selenium is not optional.
The two selenoprotein families that matter most for this discussion. First, the glutathione peroxidases, which reduce hydrogen peroxide and lipid peroxides, and are a core part of the cell's antioxidant defence. Second, the iodothyronine deiodinases, which convert thyroxine to the active thyroid hormone triiodothyronine. Selenium deficiency therefore impairs thyroid hormone activation, which is why selenium and iodine deficiency interact and why selenium appears in discussions of thyroid disease.
Selenoprotein P, made in the liver, is the main transport form and the best available marker of selenium status.
Because glutathione peroxidase is an antioxidant enzyme, selenium got swept into the antioxidant enthusiasm of the 1990s and 2000s, and much of the supplement interest traces to that. The reasoning was that oxidative damage drives ageing and cancer, selenium supports antioxidant enzymes, therefore more selenium should mean less damage. It is a clean chain of logic and it did not survive testing, for reasons the trial section covers. Notably, selenoprotein synthesis saturates: once you have enough selenium to fully express your selenoproteins, additional selenium does not produce more of them. It simply accumulates in other, less regulated forms. That single fact explains most of why the supplement story failed.
What happens if you do not get enough selenium?
Severe deficiency is rare and geographically clustered, and it produces two named diseases that established the essentiality of the element.
Keshan disease is a cardiomyopathy that occurred in selenium-poor regions of China, named after the county where it was described. It affected children and women of childbearing age particularly, and selenium supplementation programmes largely eliminated it. There is good evidence that a coxsackievirus infection interacts with selenium deficiency to produce the disease, meaning selenium deficiency alone is necessary but not sufficient. It remains one of the more compelling demonstrations that a trace element deficiency can kill.
Kashin-Beck disease is an osteoarthropathy affecting cartilage, also occurring in selenium-deficient regions of China, Tibet, and Siberia, though its causation is more contested and likely multifactorial.
Milder insufficiency is harder to pin down. It has been associated with impaired immune function, thyroid dysfunction, and in observational data with higher mortality, though the usual reverse-causation concerns apply since illness affects selenium status.
The critical determinant of selenium intake is soil, which makes this nutrient unusually geographical. Selenium content in food reflects the selenium content of the soil where the food was grown, and that varies enormously. Parts of China and Finland historically had very low soil selenium, and Finland responded by adding selenium to fertilisers nationally, which raised population intake measurably. The United States generally has selenium-adequate soils. Parts of Europe, including the UK, have lower soil selenium, and UK intakes have declined over decades, partly attributed to a shift from importing North American wheat with high selenium content to using European wheat with less. Whether this decline has health consequences is genuinely uncertain and has been debated in the nutrition literature without resolution.
Frank deficiency in the UK or US population is nonetheless uncommon. Risk concentrates in people on long-term total parenteral nutrition without adequate supplementation, people with severe malabsorptive conditions, and people undergoing dialysis.
Does selenium prevent cancer?
No, and the history of how the field arrived at that answer is worth telling because it is one of the better cautionary tales in nutrition science.
The hypothesis was reasonable. Observational studies had found that people with lower selenium status had higher rates of some cancers. Then the Nutritional Prevention of Cancer trial, run in the eastern United States where soil selenium is relatively low, tested selenium supplementation for skin cancer prevention. It failed on its primary endpoint but reported, in secondary analyses, reductions in prostate, lung, and colorectal cancer. Secondary endpoints in a trial that missed its primary endpoint are hypothesis-generating at best, but the result generated considerable excitement.
So the hypothesis was tested properly. SELECT, the Selenium and Vitamin E Cancer Prevention Trial, was a very large randomised trial run by the National Cancer Institute testing selenium and vitamin E, alone and combined, for prostate cancer prevention. It was stopped early. There was no prevention of prostate cancer. There were signals of harm: the vitamin E arm showed an increase in prostate cancer in later follow-up, and the selenium arm showed a signal of increased type 2 diabetes risk. Neither finding was what anyone expected.
Cochrane has reviewed the selenium and cancer literature and concluded that there is no convincing evidence that selenium supplementation prevents cancer, and has noted the evidence of increased type 2 diabetes risk. That is about as clear as evidence synthesis gets on a supplement question.
Why did it fail? The saturation point is the likely explanation. The NPC trial recruited in a population with relatively low baseline selenium, where supplementation might plausibly have corrected an insufficiency. SELECT recruited men who were largely selenium-replete already, so the supplement pushed them from adequate to more-than-adequate, which does nothing useful and may do harm. This is the same structural story as vitamin D and mortality trials: the deficiency-correction effect and the supplementation-of-the-replete effect are different questions, and trials that conflate them return negative results. The lesson is not that selenium is irrelevant. It is that nutrient supplementation works on deficiency and not on adequacy.
For cancer risk, the dietary evidence remains more promising than the supplement evidence, which is the pattern across nearly all of nutrition. Our post on cruciferous vegetables and cancer covers a case where the food-level evidence is comparatively encouraging.
What is the U-shaped curve and why does it matter?
Selenium has one of the narrowest therapeutic windows in nutrition, and the shape of the exposure-response relationship is the single most important thing to understand about it.
At the low end, deficiency causes cardiomyopathy, impaired immunity, and thyroid dysfunction. At the high end, selenosis causes hair loss, brittle nails, a garlic odour on the breath, gastrointestinal upset, skin lesions, and neurological symptoms, and at extreme doses it is lethal. In between, there is a range where selenoprotein expression is fully saturated and everything is fine.
The problem is the width of that range. The RDA for adults is 55 micrograms per day. The tolerable upper intake level is 400 micrograms per day. That is a ratio of roughly seven to one, which sounds generous until you compare it with nutrients where the ratio is fifty or a hundred to one, and until you notice how easily a few foods or supplements can move you across it.
Analyses of cohort data, including work in NHANES, have described U-shaped or non-linear relationships between selenium status and all-cause mortality, with risk elevated at both the low and high ends. These are observational and carry the usual caveats, but they align with what the mechanistic and trial evidence suggests. The diabetes signal from SELECT and from other trials in selenium-replete populations is the most concrete evidence of harm at the upper end, and there are hypotheses about selenoproteins interfering with insulin signalling, though the mechanism is not settled.
The practical upshot is that with selenium, unlike with something like vitamin C where excess is simply excreted, more is not neutral. There is a real cost to overshooting, and the margin is not large. Anyone promoting high-dose selenium as an antioxidant or longevity intervention is operating against both the trial evidence and the toxicology.
Where does selenium come from and how much do you need?
The RDA is 55 micrograms per day for adults in the US, with the UK reference nutrient intake at around 75 micrograms for men and 60 for women. Higher amounts apply in pregnancy and lactation. These are small quantities and most varied diets supply them.
Food sources: Brazil nuts are in a category of their own and deserve a specific warning below. Seafood is a good source, particularly tuna, sardines, and shellfish. Organ meats, especially kidney and liver, are very high. Muscle meat, poultry, and eggs contribute reliably. Cereals and grains contribute according to soil selenium, which is why they are a good source in North America and a less reliable one in parts of Europe. Dairy contributes modestly.
Now the Brazil nut point, because it is the single most practical thing in this article. Brazil nuts concentrate selenium to an extraordinary degree, and the content varies wildly depending on where the tree grew. A single Brazil nut can contain anywhere from roughly 50 to over 90 micrograms of selenium, and some analyses have reported considerably higher values. That means one nut can supply a full day's requirement, and a small handful can approach or exceed the 400 microgram upper limit. There are documented case reports of selenium toxicity from habitual heavy Brazil nut consumption.
This is genuinely unusual. Almost no other whole food can take you into toxic territory for a micronutrient through casual overconsumption. If you like Brazil nuts, a couple a day is a reasonable ceiling, and treating them as a snack food you eat by the handful is not advisable. Our nuts and longevity post covers the broader nut evidence, which is quite favourable, and Brazil nuts are the one exception to eating nuts freely.
On supplements, the honest recommendation for most people in most Western countries is not to bother. If you eat a varied diet including any combination of seafood, meat, eggs, and grains, you are almost certainly adequate, and additional selenium has no demonstrated benefit and a documented diabetes signal. Multivitamins typically contain modest amounts, in the region of 55 micrograms, which is unlikely to cause a problem but is also unlikely to add anything. Standalone high-dose selenium supplements, often sold at 200 micrograms, are where the risk sits, particularly if combined with Brazil nuts and a multivitamin. The doses stack, and people rarely account for that.
The exceptions where supplementation may be considered under medical supervision include diagnosed deficiency, certain malabsorptive conditions, and specific clinical contexts such as some thyroid conditions, where selenium has been studied in autoimmune thyroiditis with results that remain inconclusive. These are conversations to have with a clinician, not decisions to make from a blog. The general principle, set out in our how not to die: the evidence-based basics, is that the interventions worth your attention are the ones with large, replicated effects, and selenium supplementation is not among them.
Frequently Asked Questions
How many Brazil nuts should I eat per day? One or two is a sensible ceiling if you eat them regularly. Brazil nuts are the most selenium-dense food known, and a single nut can contain roughly 50 to 90 micrograms or more, which meets or exceeds the entire daily requirement. Content varies enormously depending on soil selenium where the tree grew, so you cannot know what any given nut contains. Regularly eating handfuls can approach or exceed the 400 microgram upper limit, and selenium toxicity from heavy Brazil nut consumption has been reported.
Should I take a selenium supplement? For most people in Western countries, no. Varied diets containing seafood, meat, eggs, or grains reliably supply enough, and the trial evidence does not support supplementation in replete people. SELECT found no cancer prevention and a signal of increased type 2 diabetes risk, and Cochrane reviews have concluded there is no convincing evidence of cancer prevention benefit. Supplementation makes sense for diagnosed deficiency or specific medical conditions, under clinical supervision.
What are the symptoms of too much selenium? Selenosis presents with hair loss, brittle or discoloured nails, a distinctive garlic-like odour on the breath, a metallic taste, nausea and gastrointestinal upset, skin rashes, irritability, and in more severe cases neurological symptoms. It usually results from chronic intake well above the 400 microgram upper limit, most often from supplements, from heavy Brazil nut consumption, or from mislabelled products. If you suspect it, stop the source and see a clinician.
Does selenium help with thyroid problems? It is genuinely relevant to thyroid biology, since selenoproteins activate thyroid hormone and protect the thyroid gland from oxidative damage, and selenium has been studied in autoimmune thyroiditis. Some trials have reported reductions in thyroid antibody levels, but whether this translates into clinical benefit for patients has not been established, and the evidence remains inconclusive. This is a decision for an endocrinologist rather than self-treatment, particularly given the narrow safe range.
Is low soil selenium in the UK a problem? It is a real phenomenon with uncertain consequences. UK soils are relatively low in selenium and population intakes have declined over decades, partly attributed to a shift away from selenium-rich imported North American wheat. Average UK intakes sit below some recommendations. Whether this constitutes a public health problem is genuinely debated in the nutrition literature and has not been resolved, because the health effects of mild insufficiency, as distinct from frank deficiency, are hard to measure. Frank deficiency remains uncommon.
Medical Disclaimer
This article is for general information only and does not constitute medical advice, diagnosis, or treatment. It is not a substitute for consultation with a qualified healthcare professional who knows your medical history. Do not start, stop, or change any supplement on the basis of this content, and do not act on a biomarker result without discussing it with a clinician who can interpret it in context. This applies with particular force to selenium, which has a narrow margin between adequate and toxic intake, and where supplements can stack with dietary sources such as Brazil nuts and with multivitamins in ways people do not anticipate. Seek professional advice before supplementing, especially if you have thyroid disease, diabetes, kidney disease, a malabsorptive condition, or if you are pregnant or breastfeeding. If you have symptoms that concern you, seek medical attention rather than relying on information found online.