Selenium: What It Does, How Much You Need, and What It Means for Athletes

Key Takeaways: Selenium

Selenium works through selenoproteins. Humans have 25 of them, and they are involved in antioxidant defense, inflammation control and the production of active thyroid hormone.[1][2]
The US recommended intake for adults is 55 µg/day.[3] The European Food Safety Authority (EFSA) uses an adequate intake of 70 µg/day[4] and a tolerable upper intake level of 255 µg/day.[5]
The relationship between selenium and health is U-shaped: extra selenium may help people with low status, while people whose status is already adequate or high may be harmed by supplements.[1]
In a systematic review of six randomized trials, selenium supplements raised glutathione peroxidase and lowered lipid hydroperoxides, but did not improve aerobic or anaerobic performance.[6]
Finland is a special case: soil selenium is low, so selenium has been added to fertilizers since the 1980s, but organically grown plant foods have stayed close to pre-program selenium levels.[7][8] At least one in five Finns gets less selenium than the average requirement,[9] yet in my clinical view low intake rarely shows up as recognisable symptoms, and without symptoms medical intervention is not necessarily needed, depending on diet and individual risk factors.
In my clinical practice, selenium is rarely measured and deficiency is seldom, if ever, recognised. For athletes, I generally recommend a varied diet first, testing before supplementing, and keeping stand-alone selenium supplements for a clear indication, such as a confirmed deficiency.

Introduction: Why Selenium Deserves an Athlete’s Attention

In the 1970s, the average Finn took in only 0.025 mg (25 µg) of selenium per day. Intake was so low that in 1984 Finland officially decided to add selenium, as sodium selenate, to multinutrient fertilizers, and almost every fertilizer used in the country since 1985 has contained it.[7] At the time of a 2015 review, Finland was still the only country to have taken this step nationwide.[7]

That story captures the two things worth knowing about selenium. First, selenium concentrations in food depend heavily on the soil they are grown in.[8] Second, the range between deficiency and toxicity is narrow.[10] From a sports medicine perspective, selenium is a textbook example of a nutrient where “more” is not “better.”

This article covers what selenium does in the body, how much you need, what happens with deficiency and excess, how selenium affects the thyroid, how selenium status is tested, and what the research actually shows for athletes.

What Is Selenium? Selenoproteins Explained

Selenium is a trace element whose biological work is carried out by selenoproteins. A Lancet review describes their effects as wide-ranging, from antioxidant and anti-inflammatory actions to the production of active thyroid hormone.[1] Selenium is built into these proteins in the form of selenocysteine, and humans have 25 selenoproteins.[2]

A key selenoprotein in exercise research is glutathione peroxidase (GPx), a highly efficient antioxidant enzyme whose activity depends closely on selenium being available.[6]

Low selenium status has been associated with a higher risk of mortality, poorer immune function and cognitive decline.[1] These are associations, not proof that low selenium causes these outcomes.

How Much Selenium Do You Need?

Selenium recommendations differ between countries, mainly because experts have not agreed on what “optimal” selenium status is.[10]

United States: the recommended dietary allowance is 55 µg/day for people aged 14 years and older, with a tolerable upper intake level of 400 µg/day.[3]
United Kingdom: a 2010 UK trial reports reference nutrient intakes of 75 µg/day for men and 60 µg/day for women.[10]
European Union (EFSA): an adequate intake of 70 µg/day for adults[4] and, since 2023, a tolerable upper intake level of 255 µg/day for adult men and women.[5]
Nordic countries and Finland: the Nordic Nutrition Recommendations 2023 set an adequate intake of 75 µg/day for women and 90 µg/day for men,[8] and the Finnish national nutrition recommendations from 2024 are based on these Nordic recommendations.[9]

EFSA based its upper limit on hair loss (alopecia), which it considers an early and well-established sign of selenium excess. EFSA took a lowest-observed-adverse-effect level of 330 µg/day from the SELECT trial and applied an uncertainty factor of 1.3.[5]

What about “optimal” rather than “adequate”? In a randomized, placebo-controlled trial of 119 UK adults aged 50–64, whose mean baseline plasma selenium was 95.7 ng/mL, selenoprotein P (a selenium transport protein) reached its maximum with 50 µg/day of selenium on top of a habitual intake of about 55 µg/day.[10] Selenoprotein P levelled off at a plasma selenium concentration of 124 ng/mL.[10]

Food Sources of Selenium

Dietary sources include seafood, peas and lentils, beans, whole grains, organ meats, dairy products and vegetables.[6] In a US context, a varied diet that includes meat, grains, vegetables and nuts is described as typically supplying enough selenium that supplements are unnecessary.[3]

Selenium enters the food chain from soil, where it is taken up by plants, and soils in some regions contain much more selenium than others.[3] The Nordic and Baltic countries have low soil selenium, which leads to low selenium concentrations in locally grown foods. Finland has addressed this by adding selenium to fertilizers, whereas the other Nordic countries add it to animal feed.[8] At the time of the 2015 review, all crop fertilizers in Finland contained 15 mg of selenium per kilogram, and national monitoring has shown the program to be effective and safe in raising selenium intake across the whole population.[7] Organic production is different: the same 2015 review reported that in Finland, organically grown plant foods had selenium levels generally comparable to foods produced before the fertilizer program began, and milk from organically fed cows contained 50% less selenium than ordinary milk.[7]

Finns are therefore in a special position when it comes to selenium. From a clinical perspective, this is one more reason why a varied diet matters. In my view, a very one-sided diet, or relying heavily on organic food, is not necessarily justified as far as selenium is concerned.

In my understanding, healthy adults in Finland who eat a normal diet and have no absorption problems, underlying disease or pregnancy rarely need supplements, with vitamin D as the most common exception. The national recommendations note that supplements are often unnecessary but matter for specific groups, for example vitamin D for children and young people, and vitamin B12 and iodine for people following a vegan diet.[9] A multivitamin can be taken, and I do not recall seeing recommendations to supplement selenium specifically.

Brazil Nuts: Powerful but Unpredictable

Brazil nuts are a rich natural source of selenium.[11] In a 12-week randomized trial in 59 New Zealand adults, two Brazil nuts a day raised plasma selenium by 64.2%, compared with 61.0% for 100 µg/day of selenomethionine and 7.6% for placebo.[11] The nuts were expected to deliver about 100 µg per day, but measured intake averaged 53 µg/day, with a possible range of 20–84 µg.[11] That variability is the main practical caveat: you cannot know exactly how much selenium a given nut contains.

Selenium Deficiency

The classic deficiency disease is Keshan disease, an endemic cardiomyopathy seen in China among populations with low selenium status.[6]

Interestingly, even the Finnish program illustrates how hard selenium’s health effects are to measure: the researchers noted that because there were no selenium-deficiency diseases and no reference population, it was difficult to draw conclusions about the program’s impact on human health.[7]

Milder Selenium Deficiency: Vague Symptoms

Beyond Keshan disease, a 2026 Japanese study of 39 neurosurgery patients receiving enteral and/or parenteral nutrition summarises the previously reported features of selenium deficiency as cardiomyopathy leading to arrhythmia and tachycardia, muscle pain and weakness, hemolysis, macrocytic anemia, and damage to nails and hair.[12] According to the same authors, situations that may result in selenium deficiency include anorexia nervosa and prolonged parenteral or enteral nutrition.[12] They also note that the symptoms vary and may be caused by factors unrelated to selenium, and that in earlier cross-sectional studies most people with low selenium levels had no clinical symptoms.[12]

From a clinical perspective, these symptoms are easily attributed to something else. Muscle pain and weakness can have so many other causes that, in practice, selenium deficiency often ends up below other diagnostic options. Arrhythmias and tachycardia are assessed separately in cardiology, and while I would assume the link is known there too, it is probably not the first option on the list. Nail and hair problems are also multifactorial: trace element deficiencies are known to contribute, but they are rare, and other diagnoses usually come first. Hemolysis and anemia are likewise investigated separately, and in my view selenium deficiency can easily be missed in the work-up of macrocytic anemia or hemolysis, although in my experience a sufficiently broad investigation, for example in hematology, can take it into account.

In my clinical experience, I do not think I have ever seen a selenium deficiency, and I do not recall seeing it on a patient’s list of diagnoses. Part of the reason may be that selenium is not tested routinely and its symptoms are not well known among clinicians. In my experience, and as far as I can tell among colleagues, selenium deficiency or a need for selenium supplementation is not common, and in everyday practice selenium rarely comes to mind.

According to the Finnish national nutrition recommendations from 2024, at least one in five Finns gets less selenium than the average requirement.[9] This refers to intake, not to clinical deficiency, and it illustrates how intake and clinical symptoms do not necessarily go hand in hand. In my view, low selenium status is probably more widespread than current clinical medicine, or clinicians, are able to recognise. That does not necessarily mean that people develop clinical symptoms, and even if they do, recognising them would be incomplete and difficult. With this in mind, in my view low intake without symptoms does not necessarily call for medical intervention; the need for further assessment depends on the overall diet, individual risk factors and the clinical context.

Because selenium has a narrow safe range and toxicity can develop relatively easily compared with many other supplements, I would be very cautious with selenium supplementation, and for the same reason I find it problematic to give general advice on supplementing it. In my view, a sensible way to correct low selenium status, assuming there is no absorption disorder, starts with the diet: making sure it is varied enough while avoiding excesses. I would not make the diet one-sided in pursuit of selenium; the priority is simply a varied diet. The situation rarely needs separate follow-up testing, although it can be rechecked, and in my view improving the diet is often enough.

Too Much Selenium: Toxicity and the U-Shaped Curve

The central message from selenium research is the U-shaped relationship between status and health. Extra selenium may benefit people whose status is low, whereas people with adequate-to-high status may be harmed and should not take selenium supplements.[1]

Acute Selenium Toxicity

The clearest warning comes from a 2008 US outbreak caused by a liquid dietary supplement that contained 200 times the selenium stated on its label. Investigators identified 201 cases in 10 states, and the median estimated selenium dose was 41,749 µg/day.[3] Frequently reported symptoms were diarrhea (78%), fatigue (75%), hair loss (72%), joint pain (70%), nail discoloration or brittleness (61%) and nausea (58%).[3] Some symptoms persisted for 90 days or longer, including fingernail discoloration and loss (52%), fatigue (35%) and hair loss (29%).[3]

That outbreak involved a manufacturing error and extreme doses. The more relevant question for most people is what happens with long-term use of ordinary supplement doses.

Long-Term Supplementation: Diabetes, Cancer and Mortality

Type 2 diabetes: In a secondary analysis of the Nutritional Prevention of Cancer trial, 1,202 people from low-selenium areas of the eastern United States without diabetes at baseline took 200 µg/day of selenium or placebo. Over an average 7.7 years, type 2 diabetes developed in 58 people in the selenium group and 39 in the placebo group (hazard ratio 1.55, 95% CI 1.03–2.33).[13] The risk was highest in those with the highest baseline plasma selenium (hazard ratio 2.70 in the top tertile).[13] Diabetes was a secondary outcome of the original trial, so the authors called for cautious interpretation.[13]
Prostate cancer: The SELECT trial randomized 35,533 men to selenium (200 µg/day as L-selenomethionine), vitamin E, both, or placebo. Selenium did not prevent prostate cancer (hazard ratio 1.04, 99% CI 0.87–1.24), and there was a statistically non-significant increase in type 2 diabetes in the selenium-only group (relative risk 1.07, 99% CI 0.94–1.22).[14]
Cancer overall: A 2018 Cochrane review including 10 randomized trials with 27,232 participants concluded, with high-certainty evidence, that well-designed trials show no benefit of selenium supplements for reducing cancer risk. In SELECT, selenium also increased the risk of alopecia and dermatitis.[15]
Mortality: In the Danish PRECISE trial, 491 adults aged 60–74 took 100, 200 or 300 µg/day of selenium (as selenium-enriched yeast) or placebo for five years and were then followed for another ten. Over the full follow-up, the 300 µg/day group had higher all-cause mortality than placebo (hazard ratio 1.59, 95% CI 1.02–2.46), whereas the non-significant mortality reductions seen with 100 and 200 µg/day during treatment disappeared after it stopped.[2] Mortality was not an original endpoint and the sample was small, but the authors concluded that total selenium intakes above 300 µg/day and high-dose supplements should be avoided.[2]

Based on available intake data, EFSA considers adults unlikely to exceed the upper limit, except for regular users of high-dose selenium supplements and regular consumers of Brazil nuts.[5]

Selenium and the Thyroid

Selenoproteins are involved in producing active thyroid hormone, and a 2012 Lancet review reports that higher selenium status or supplementation reduces the risk of autoimmune thyroid disease.[1]

A large meta-analysis published in Thyroid in 2024 included 35 studies of people with Hashimoto thyroiditis. In patients not taking thyroid hormone replacement, selenium lowered TSH (standardized mean difference −0.21; 7 cohorts, 869 participants).[16] Thyroid peroxidase antibodies (TPOAb) fell in patients with and without hormone replacement (standardized mean difference −0.96; 29 cohorts, 2,358 participants), although heterogeneity was high (I² = 90%) and the certainty of evidence for this outcome was rated low.[16] Adverse effects were comparable to control groups, free T4 and free T3 did not change significantly, and the overall certainty of the evidence was rated moderate.[16]

In practice, selenium can shift thyroid laboratory markers in diagnosed Hashimoto thyroiditis. These trials were done in people with the disease, so they do not show a benefit for athletes with a healthy thyroid.

Selenium Testing: Plasma Selenium and Selenoprotein P

In the studies discussed in this article, selenium status was assessed mainly from plasma or serum selenium. In research settings, selenoprotein P is considered a useful marker of status in populations with relatively low selenium intakes because it responds to different dietary forms of selenium.[10]

Some practical points help with interpretation:

Where selenoproteins saturate: Selenoproteins are likely replete at a plasma selenium concentration of about 125 µg/L, where selenoprotein P reaches a plateau.[2] In the UK trial, that plateau was at 124 ng/mL.[10] (ng/mL and µg/L are the same unit.)
Why GPx is a weak status marker: The UK trial authors note, citing earlier studies, that plasma GPx activity plateaus at a plasma selenium of 70–90 ng/mL, so it stops reflecting status above that range.[10]
The selenomethionine effect: Selenomethionine, the main selenium form in selenium-enriched yeast, can be incorporated non-specifically into plasma proteins such as albumin. Plasma selenium can therefore keep rising with supplementation even when selenoproteins are already saturated.[10][2] A high plasma selenium on supplements does not mean “more function.”
When supplements add nothing: The UK trial authors suggest that people who are already selenium-replete (intake above 100 µg/day and plasma selenium above 130 ng/mL) should avoid supplements because they probably provide no additional health benefit.[10]

If your laboratory reports selenium in µmol/L, multiply by roughly 79 (selenium’s atomic mass) to convert to µg/L.

In everyday clinical work, selenium rarely plays a role in decision-making, and it is seldom measured at all. Selenium is missing entirely from most of the laboratory panels I have available, which in my view suggests it is not seen as being as clinically central as a trace element such as iron. In my experience, clinicians are aware of its importance, but in most situations they may not see a specific reason to test it. For athletes, I would generally use the same testing indications as for anyone else: in my view there is no particular reason to test selenium in an athlete straight away, and ordinary periodic check-ups are usually enough. When a patient has unusual symptoms, I start with basic investigations and widen the work-up step by step as other causes are ruled out. In my experience, trace elements are usually considered when a patient has a condition that clearly impairs absorption, such as celiac disease or Crohn’s disease, or when there is malnutrition or anorexia. In these situations, clinicians are in my experience more likely to suspect trace element deficiencies, and testing is often broadened to include selenium.

What Selenium Means for Athletes

Do Athletes Need More Selenium?

Exercise can increase the production of reactive oxygen species, and GPx is part of the defense system that handles them, so it is reasonable to ask whether training raises selenium needs.[6]

Some of the most direct data come from a French cross-sectional study of 118 well-trained athletes who kept 7-day food and activity records. Selenium intakes were below two-thirds of the French recommended allowance in 23% of the men and 66% of the women. Even so, only 2.6% of the athletes had low plasma selenium (below 0.75 µmol/L).[17] Athletes with the highest daily energy expenditure also had the highest selenium intakes and plasma selenium, and the authors interpreted this as showing that selenium requirements increase in athletes, although not in a linear relationship with energy expenditure. A cross-sectional design like this cannot establish requirements directly.[17]

Two lessons follow. Estimated dietary intake and measured blood status can tell very different stories, and in this study low intakes were more common in women than in men. A systematic review also lists possible routes of selenium loss in athletes, including gastrointestinal losses, increased losses in sweat and urine, malabsorption and malnutrition, but these are proposed mechanisms rather than outcomes measured in the trials it reviewed.[6]

Do Selenium Supplements Improve Performance?

The most complete answer comes from a 2020 systematic review in Nutrients. Of 150 articles identified, 6 randomized, placebo-controlled trials met the inclusion criteria.[6] Participants took 180 or 240 µg/day of selenomethionine or 200 µg/day of sodium selenite for 4 to 14 weeks.[6]

What changed: Supplementation lowered lipid hydroperoxide levels and increased GPx in plasma, red blood cells and muscle.[6]
What did not change: There were no significant effects on athletic performance, testosterone, creatine kinase, or training-induced adaptations in oxidative enzyme activity and muscle fiber type (myosin heavy chain expression).[6]
Mitochondria: The review also reported that selenium dampened exercise-related changes in muscle mitochondria.[6]
Bottom line from the authors: Selenium supplementation provides no benefit for aerobic or anaerobic performance, although it may help prevent deficiency in athletes doing high-intensity, high-volume training.[6]

The evidence base is thin. The review authors acknowledge that the included studies were few, had small samples made up mostly of men, and were conducted a long time ago.[6]

Clinical synthesis (not a direct research finding): Better antioxidant markers did not translate into better performance in these trials. Combined with the dampening of mitochondrial changes, this argues against using high-dose selenium as a training aid in an athlete who is not deficient.

Conclusion: Selenium Is a Precision Nutrient

Selenium is essential: it powers 25 selenoproteins involved in antioxidant defense, inflammation control and thyroid hormone activation.[1][2] But its benefits follow a U-shaped curve.[1] Long-term supplementation has been linked to type 2 diabetes in one trial, did not prevent cancer, and at 300 µg/day was associated with higher mortality in another.[13][15][2] In the trials reviewed in physically active people, supplements improved antioxidant markers but not performance.[6]

The athlete’s selenium strategy is simple: eat a varied diet, count every source, and if there is a real reason to suspect low status, test before you supplement.

In my clinical experience, selenium rarely comes up in everyday decision-making, and I do not think I have ever seen a selenium deficiency. When athletes or other patients ask about it, I usually recommend a varied diet first and testing before any supplement. A multivitamin with a small amount of selenium can usually be taken safely, but I would not start taking selenium on its own straight from a jar. In my view, it is sensible to avoid stand-alone selenium supplements without a clear indication, such as a documented deficiency. Low selenium intake may well be more common than clinicians can recognise, but it does not necessarily cause symptoms, and the symptoms it can cause are vague and easily attributed to something else. Because the safe range is so narrow, I find general supplementation advice problematic, and in Finland too, a varied diet rather than a one-sided or heavily organic one remains, in my view, the best approach.

Selenium: What It Does, How Much You Need, and What It Means for Athletes

Table of Contents

Key Takeaways: Selenium

Introduction: Why Selenium Deserves an Athlete’s Attention

What Is Selenium? Selenoproteins Explained