What is selenium?
Selenium is an essential mineral for your immune system, thyroid, and heart. You might read that selenium supplements can help some diseases, but the studies on selenium are mixed.
Selenium is an essential trace mineral you need for proper body function and health. A trace mineral is an element you need only in small amounts.
Selenium is a vital building block of proteins called selenoproteins and some enzymes that speed up chemical reactions in your body. These enzymes and selenoproteins protect your cells from damage and help your thyroid make and use hormones. Your body combines selenium with an amino acid called methionine and stores the combination in your muscles.
You can naturally get this mineral from your food or supplements. It’s possible to get too much selenium, though, so supplements aren’t always a good idea.
Most people in the United States get enough selenium daily, so a deficiency is rare. Some people are more likely to have lower amounts of selenium, though. These include people who:
- Have HIV
- Get kidney dialysis
- Only eat foods grown in low-selenium soil
- Eat a vegetarian or vegan diet
- Have absorption problems from bowel diseases, like Crohn’s disease or ulcerative colitis
Health benefits of selenium
Selenium performs lots of important functions in the body. Supplements might have some benefits, but more research is necessary.
Your body naturally creates unstable molecules called free radicals or reactive oxygen species as part of its normal function. These molecules cause damage to your cells and lead to disease if they build up. Your body uses a mix of molecules called antioxidants to neutralize these free radicals and help prevent damage.
Selenium is an antioxidant and works best with others like vitamin C and vitamin E. Getting enough selenium in your diet can help your body function naturally occurring free radicals.
Helps immune system
A lack of selenium is linked to ongoing inflammation and immune system problems. Studies suggest you need selenium to help your immune cells work correctly and to make white blood cells, which fight off infection and sickness. One study seemed to indicate that older people who took selenium and zinc supplements had a better response to the flu vaccine than those who took a placebo pill.
Supports thyroid function
Your thyroid stores more selenium than any other organ or gland in your body. It uses selenium to make and use thyroid hormones. Studies indicate that low blood levels of selenium and iodine can lead to thyroid problems, including goiter, low hormone levels, and damage.
You need selenium for proper thyroid function, but experts don’t know yet if low selenium levels cause thyroid disease or if thyroid disease causes low levels. Taking selenium supplements might only be helpful if you have thyroid antibodies or autoimmune disease.
For example, in one study, older patients with underactive thyroid took 300 micrograms of selenium for six months. They had higher blood levels of selenium, but it did not affect the thyroid.
But other studies suggest positive outcomes for autoimmune disease. One study gave selenium, medication, or a placebo pill to patients with an eye-related autoimmune thyroid disease called Grave’s disease. Those who had selenium had a better quality of life and milder symptoms than those who didn’t.
Selenium was also helpful for pregnant women with thyroid peroxidase (TPO) antibodies, which can lead to low thyroid during pregnancy and thyroid inflammation afterward. A clinical trial suggested that pregnant women with TPO antibodies who took selenium during pregnancy had fewer cases of postpartum thyroid inflammation.
Selenium might also help another autoimmune thyroid disease called Hashimoto’s thyroiditis. In a few studies, patients who received selenium supplements and thyroid hormone pills had lower blood levels of antibodies. The studies didn’t prove whether their symptoms improved or they needed less medication, though.
Selenium might help autoimmune thyroid disease and prevent postpartum thyroid problems. More research is necessary to understand if or how supplements can help.
Might improve heart health
Selenoproteins help free radical cell damage and stop platelets from clumping together and clotting, which is vital for heart health. Some studies link low blood levels of selenium to a higher risk of heart disease, but others seem to demonstrate the opposite. The results on selenium supplements are mixed.
In a clinical trial, healthy older adults took selenium supplements or a placebo pill every day for six months. Those who took selenium had lower total cholesterol levels than those who had the placebo. Yet other studies indicated that taking selenium as part of a daily multivitamin didn’t lower the risk of heart disease or death from heart disease.
Selenium with vitamin E might also help lower cholesterol levels. Too much selenium can also interfere with your heart medication, though.
Might lower cancer risk
Early animal studies suggest that selenium supplements can lower the rate of tumors and that selenium has anti-cancer activities. Human studies have provided evidence that lower levels of selenium can lead to a higher risk of getting colon, prostate, rectum, bladder, lung, stomach, esophagus, and skin cancer.
But it’s not clear that selenium supplements help. Some results suggest that supplements can lower skin cancer risk, but others suggest taking selenium raises your risk of getting some types of skin cancer. Other results show selenium doesn’t reduce lung or esophageal cancer risk and show mixed results on colon cancer.
Selenium might be most helpful for prostate cancer. One study found that men who took brewer’s yeast selenium supplements for 4.5 years had a lower risk of prostate cancer. Those with the most effects already had low levels of selenium, though, so it might only be helpful if you have a selenium deficiency.
Might help male infertility
Your body uses selenium to make the protein in sperm, so getting enough of this mineral might help with sperm growth. Results indicate that selenium supplements might help infertility in men with low selenium levels or infertility with an unknown cause. The supplements might improve sperm growth and the ability of sperm to swim. But too much selenium can also cause problems with sperm, so more research is necessary.
Might help asthma symptoms
Asthma is a lifelong condition in which your airways swell and narrow, making it hard to breathe. People with asthma might have lower levels of selenium, but experts aren’t sure how that affects asthma.
Small studies suggest that asthma patients who took selenium supplements had fewer asthma symptoms than those who took a placebo. In contrast, more extensive studies suggest selenium supplements don’t help.
You can find selenium in different supplement forms ranging from a daily multivitamin or a single vitamin to a combined antioxidant or nutritional supplement. It’s usually available as selenomethionine or sodium selenate.
Dosage and safety
Selenium supplements come in different doses, some as high as 200 micrograms. The daily recommended amount (RDA) for selenium is 55 micrograms for adults, 60 micrograms during pregnancy, and 70 micrograms during breastfeeding.
The upper daily limit for selenium intake is 400 micrograms for adults and much lower for children. This means if you take more than this amount, you’re getting too much selenium. Over time, this can cause toxicity and make you sick. Too much selenium can cause symptoms like:
- Skin rashes
- Brittle hair or nails
- Discolored teeth
- Garlic breath
- Metallic taste in your mouth
- Nervous system problems
Very high doses can cause serious side effects like kidney failure, trouble breathing, tremors, heart attacks, and heart failure.
Selenium supplements can interact with your medications. It can interact with blood thinners, like warfarin, heparin, and aspirin, increasing your risk of bleeding. It might also interfere with chemotherapy drugs like cisplatin. This drug can lower your selenium levels, but taking selenium can also interfere with how well cisplatin works.
The studies on selenium use doses ranging from 100 to 300 micrograms per day. Taking high doses long-term might raise your cholesterol, contribute to diabetes, and lead to or worsen low iodine levels.
Food sources of selenium
Selenium naturally exists in many foods, including fortified foods. You can get your daily recommended allowance by eating a healthy, balanced diet. Good food sources of selenium include:
- Dairy products
- Grains, cereals, breads, and other products with added selenium
- Brewer's yeast
- Wheat germ
- Brazil nuts
Brazil nuts are an excellent source of selenium. You can get all the selenium you need by simply eating one Brazil nut a day.
Bottom line: Get it from food first
While selenium supplements can raise your selenium blood levels, it’s not clear yet that they treat disease. Unless you have a diagnosis or are at risk of low selenium levels, you probably don’t need to take selenium supplements. In most cases, getting the vitamins and minerals you need from a healthy diet is best and safest. If you’re considering taking selenium, talk to your doctor first.
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Health Solutions From Our Sponsors
Harvard T.H. Chan School of Public Health: "Selenium."
International Journal of General Medicine: "Selenium–vitamin E supplementation in infertile men: effects on semen parameters and pregnancy rate."
Mayo Clinic: "Asthma."
Mount Sinai: "Selenium."
National Institutes of Health Office of Dietary Supplements: "Selenium Fact Sheet for Consumers," "Selenium Fact Sheet for Health Professionals."
Oregon State University Linus Pauling Institute: "Selenium."
U.S. Department of Agriculture FoodData Central: "Brazil nuts."
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