Selenium. You’ve probably heard of it, maybe seen it on a multivitamin label, but do you really know what it does? This essential trace mineral is a small but mighty player in human health—one that influences everything from immune function to cognitive sharpness to reproductive health.
And yet, most people don’t think about selenium until they realize they might not be getting enough. Today, we’ll break down the science behind selenium—what it is, how it works in the body, and most importantly, how you can optimize your selenium intake to support long-term health.
What is Selenium and Why Does It Matter?
Selenium is a trace mineral, meaning we need it in small amounts, but its impact is anything but small. It serves as a critical component of 25 different selenoproteins, which help regulate key biological processes like:
- Thyroid function – Selenium is required for the production and conversion of thyroid hormones.
- Antioxidant defense – It protects cells from oxidative damage and inflammation.
- DNA synthesis and repair – Essential for maintaining cellular integrity.
- Reproductive health – Plays a role in male fertility and proper fetal development.
- Immune system function – Helps modulate immune responses and reduce susceptibility to infections.
Now, here’s where it gets interesting: the selenium content in food is largely determined by the soil in which it’s grown. This means selenium intake varies significantly depending on where you live and what foods you eat.
How Much Selenium Do You Actually Need?
The recommended dietary allowance (RDA) for selenium is:
- 55 mcg/day for most adults
- 60 mcg/day during pregnancy
- 70 mcg/day for lactation
This is enough to support normal bodily functions, but research suggests there may be added benefits to getting slightly more—within safe limits.
Top Food Sources of Selenium
Because selenium is bound to proteins, it’s most abundant in high-protein foods. Here are some of the richest sources:
- Brazil nuts – Just one nut can provide more than your daily requirement!
- Seafood (tuna, sardines, shrimp) – Excellent sources of bioavailable selenium.
- Meat and poultry – Pork chops, beef, and turkey are reliable sources.
- Eggs and dairy – Contribute smaller amounts but are still valuable.
- Whole grains – Brown rice, oatmeal, and whole wheat bread contain selenium, but levels depend on soil content.
If you follow a plant-based diet, selenium intake can be more variable. Soil depletion in certain regions—especially parts of China and Europe—can lead to lower selenium content in crops. If you’re vegan or vegetarian, you might want to track your intake more closely.
What Happens When You Don’t Get Enough?
Selenium deficiency is rare in North America but more common in parts of the world where soil selenium levels are low. However, even marginal deficiency can have consequences, including:
- Weakened immune function – Selenium plays a role in antiviral defense.
- Increased oxidative stress – Leading to higher inflammation and cellular damage.
- Thyroid dysfunction – Selenium is essential for producing and converting thyroid hormones.
- Reduced male fertility – Low selenium levels have been linked to poor sperm quality.
In extreme cases, severe selenium deficiency can lead to Keshan disease (a type of heart disease) or Kashin-Beck disease (a degenerative bone condition), both historically seen in selenium-poor regions of China.
Can You Get Too Much Selenium?
Yes. Selenium toxicity—also known as selenosis—can occur if you consume too much, particularly from over-supplementation. Symptoms include:
- Hair and nail brittleness or loss
- Gastrointestinal issues (nausea, diarrhea)
- Neurological problems (irritability, fatigue, nerve damage)
For adults, the tolerable upper intake level (UL) is 400 mcg/day. Brazil nuts, while a fantastic source of selenium, should be eaten in moderation—two or three per day is more than enough.
Selenium and Specific Health Conditions
Let’s take a look at some of the key health areas where selenium plays a significant role.
1. Selenium and Thyroid Health
Your thyroid contains more selenium per gram of tissue than any other organ in the body. That’s because selenium-dependent enzymes help regulate thyroid hormones. Research suggests that selenium supplementation may be beneficial in conditions like Hashimoto’s thyroiditis, an autoimmune disorder that causes hypothyroidism.
However, the effects are highly dependent on baseline selenium status—if you already get enough from food, extra supplementation may not provide added benefits.
2. Selenium and Cancer Prevention: Hype vs. Reality
Early research suggested that selenium could reduce cancer risk, particularly for prostate, lung, and colorectal cancers. However, large-scale trials like the SELECT trial found no significant protective effect from selenium supplements.
What does this mean? While maintaining optimal selenium levels is crucial for cellular health, supplementing beyond normal levels doesn’t appear to offer added protection against cancer.
3. Selenium and Brain Function
Cognitive decline and neurodegenerative diseases like Alzheimer’s have been linked to lower selenium levels. Selenoproteins help combat oxidative stress in the brain, which plays a role in cognitive aging.
Observational studies suggest that people with higher selenium levels tend to perform better on cognitive tests, but randomized trials haven’t shown strong evidence that supplementation prevents dementia. What’s clear is that selenium deficiency can accelerate cognitive decline—so maintaining sufficient levels is key.
4. Selenium and Male Fertility
Selenium is essential for sperm motility and integrity. Some studies suggest that supplementation can improve sperm health, particularly in men with low baseline selenium status. However, too much selenium can actually have the opposite effect, so balance is key.
Should You Supplement?
If you eat a varied diet with meat, seafood, eggs, and whole grains, you’re likely getting enough selenium. However, certain groups may benefit from supplementation, including:
- Vegetarians and vegans in selenium-poor regions
- People with digestive disorders affecting nutrient absorption (e.g., Crohn’s disease)
- Individuals with thyroid autoimmunity (under medical guidance)
- Men struggling with fertility issues (only if deficient)
If you choose to supplement, selenium-enriched yeast or selenomethionine are the most bioavailable forms. Dosages between 50–200 mcg/day are generally safe, but stay below the 400 mcg/day upper limit.
Key Takeaways
- Selenium is essential for thyroid function, immune health, brain function, and fertility.
- The best food sources include Brazil nuts, seafood, meats, and eggs.
- Deficiency is rare in North America but more common in regions with selenium-poor soil.
- While selenium is important, more is not always better—over-supplementation can be harmful.
- If you’re considering a selenium supplement, get your levels checked first.
Final Thought: Selenium is a perfect example of how small micronutrients can have a massive impact on health. Optimizing your selenium intake through diet first—and supplementation only when necessary—will support longevity, immune resilience, and cognitive function over the long term.