Selenium is an essential trace mineral that supports your immune system, protects cells from damage, helps maintain brain function, and plays a role in heart health. Adults need just 55 micrograms (mcg) per day, a tiny amount easily obtained from food, yet many people around the world fall short depending on where their food is grown.
How Selenium Works in Your Body
Nearly all of selenium’s benefits come from specialized proteins called selenoproteins. Your body builds about 25 different selenoproteins, and most of them function as cleanup crews: they neutralize harmful byproducts of normal cell metabolism before those byproducts can damage DNA, cell membranes, and tissues. The most well-known of these proteins belongs to the glutathione peroxidase family, which breaks down peroxides that would otherwise trigger chain reactions of cellular damage.
This protective role is why selenium shows up in research on so many different health conditions. Oxidative damage contributes to heart disease, cognitive decline, immune dysfunction, and more. Selenium doesn’t cure any of these, but adequate levels give your body the raw material it needs to keep its defenses running properly.
Immune System Benefits
Selenium has a measurable effect on how your immune system responds to threats. It boosts the production of key signaling molecules that tell T cells (a critical type of white blood cell) to multiply and mature. At the same time, it dials down chronic inflammation by lowering levels of inflammatory markers like IL-6, TNF-alpha, and C-reactive protein, while raising levels of anti-inflammatory signals like IL-2.
When selenium levels are low, your immune cells struggle with one of their most basic jobs: generating the burst of reactive oxygen that kills invading bacteria and viruses. Adequate selenium restores this ability while also preventing that burst from damaging your own tissues. Selenium also shifts macrophages, another type of immune cell, from an inflammatory state to a repair-oriented state, which helps resolve infections without prolonged tissue damage.
Brain Health and Cognitive Function
Blood selenium levels tend to drop with age, and epidemiological surveys show a clear positive relationship between selenium status and cognitive ability, essentially a dose-response effect where higher levels correlate with sharper thinking. In people with Alzheimer’s disease, selenium levels in the blood are significantly lower than in healthy individuals. The same is true for carriers of the ApoE4 gene variant, the strongest genetic risk factor for Alzheimer’s.
The connection isn’t just correlation. Several selenoproteins are highly concentrated in the brain, particularly in regions affected by Alzheimer’s. One of these, GPX4, prevents a specific type of cell death called ferroptosis, which involves iron accumulation and destruction of cell membranes in the hippocampus, the brain’s memory center. In Alzheimer’s patients, GPX4 levels in the hippocampus are reduced, and iron accumulation and membrane damage are elevated. Another selenoprotein, SELENOK, is significantly decreased in the brains of Alzheimer’s patients, and its absence disrupts calcium signaling in neurons and destabilizes the receptors that brain cells use to communicate.
Heart Disease Risk
A meta-analysis published in The American Journal of Clinical Nutrition found that a 50% increase in blood selenium concentration was associated with a 24% reduction in coronary heart disease risk across observational studies. Several mechanisms likely contribute. Selenium neutralizes peroxide intermediates that would otherwise fuel the production of inflammatory compounds in blood vessel walls. It also appears to reduce platelet clumping and protect against the effects of toxic metals like mercury, cadmium, and arsenic, all of which have been implicated in artery damage.
The picture is less clear-cut when it comes to supplements. Randomized trials of selenium supplementation showed a modest, statistically insignificant reduction in heart disease risk. Studies looking at specific cardiovascular markers like blood pressure and cholesterol have produced mixed results. This suggests that having adequate selenium levels matters for heart health, but taking extra on top of an already sufficient diet may not add much benefit.
Best Food Sources of Selenium
Brazil nuts are the single richest source, with just one or two nuts often exceeding your daily needs. Beyond that standout, many common foods provide substantial amounts per serving:
- Sunflower seeds (1 cup, oil roasted): 106 mcg
- Yellowtail fish (half fillet, cooked): 68 mcg
- Pork, lean ham (1 cup, cooked): 67 mcg
- Blue mussels (1 cup, raw): 67 mcg
- Swordfish (3 oz, cooked): 58 mcg
- Whole-wheat pasta (1 cup, dry): 71 mcg
- Oysters (3 oz, farmed, raw): 54 mcg
- Canned white tuna (3 oz, drained): 51 mcg
- Turkey (1 cup, roasted): 53 mcg
- Tilapia (1 raw fillet): 49 mcg
Seafood, pork, poultry, whole grains, and seeds are consistently the best dietary sources. Eggs, beef, and dairy also contribute smaller amounts that add up over the course of a day. Most adults eating a varied diet with some animal protein or whole grains will meet the 55 mcg target without supplements.
Why Geography Affects Your Selenium Intake
The selenium content of plant foods depends almost entirely on the soil where they were grown. Some regions have naturally selenium-rich soil, while others, including parts of China, Eastern Europe, and New Zealand, have soils so depleted that crops grown there contain very little. This means the same serving of wheat bread could provide substantially different selenium levels depending on where the grain was farmed. In selenium-poor regions, farmers sometimes use selenium-enriched fertilizers to compensate, but people relying on locally grown food in these areas are at higher risk of deficiency.
Recommended Daily Amounts
The recommended dietary allowance for most adults (ages 14 and up) is 55 mcg per day. Pregnant women need 60 mcg, and breastfeeding women need 70 mcg. Children’s needs scale with age: 20 mcg for ages 1 to 3, 30 mcg for ages 4 to 8, and 40 mcg for ages 9 to 13.
These are small amounts. A single 3-ounce serving of swordfish or a cup of cooked pork covers your entire daily requirement. Even a cup of enriched pasta gets you there.
Too Much Selenium: Signs of Toxicity
The tolerable upper intake level for adults is 400 mcg per day. Consistently exceeding this amount can cause a condition called selenosis. Early signs include brittle nails, hair loss, and a garlic-like odor on the breath. More severe cases involve nail deformation and loss, tooth discoloration and decay, numbness in the extremities, and in rare cases, partial paralysis.
Toxicity from food alone is uncommon, with one exception: Brazil nuts. Because they concentrate selenium so efficiently, eating a handful daily could push you well past 400 mcg. A few per day is plenty. Supplement users face the greater risk, particularly if they’re also eating a selenium-rich diet or taking a multivitamin that already contains selenium.
Interactions With Medications
If you’re undergoing cancer treatment, selenium supplementation is a more nuanced question. Some research suggests it may reduce certain treatment side effects, including fatigue, nausea, hair loss, and appetite problems. Lab studies have also shown selenium can protect heart tissue from damage caused by certain chemotherapy drugs. However, the evidence is still limited, and the effects of selenium during active treatment can vary depending on the specific drugs involved and your baseline selenium levels. Supplementing during cancer care is something to coordinate with your treatment team rather than doing independently.