Selenium is one of the "essential" nutrients for humans, meaning that our bodies cannot make it, and so we have to get it from our diet. Without it the heart, joints, eyes, immune system and reproductive system can suffer. Yet humans only need to eat a trace of selenium every day, about 55 micrograms or millionths of a gram.
Selenium was discovered as an element in 1817 by the Swedish chemist Jöns Jacob Berzelius, who determined the atomic weights of many elements and developed a system of chemical symbols. It was first thought to be a toxin, but scientists determined that selenium was an essential mineral in the 1950s. By the 1960s, doctors began researching selenium's possible tumor-fighting properties in animals, according to the American Cancer Society.
Scientists now know selenium is necessary in the body's production of selenoproteins, a family of proteins that contain selenium in the form of an amino acid. So far, 25 different selenoproteins in the body have been isolated, but only half of their functions have been identified, according to the Linus Pauling Institute at Oregon State University.
Human and animal research has found selenoproteins are involved in embryo development, thyroid hormone metabolism, antioxidant defense, sperm production, muscle function and the immune system's response to vaccinations.
Where selenium is found
"Plants pick up trace minerals that are in the dirt — including selenium — and convert it into a form that is consumable and useable to humans and other animals," said Dr. Stephen Neabore, a primary care doctor at the Barnard Medical Center in Washington, D.C. "Because it's found naturally in plants, generally if you're eating a varied diet you are going to get all the nutrients and minerals you need."
Soil around the world varies in its selenium concentration. The higher the concentration of selenium in soil, the higher the concentration of selenium in crops. Soil in Nebraska, South Dakota and North Dakota is especially rich in selenium, and people living in these areas typically have the highest dietary intake of selenium in the United States, according to the 2014 Harvard Health publication "The Truth About Vitamins and Minerals."
Soil in some areas of China and Russia is naturally low in selenium. Selenium deficiencies in the Keshan region in northeast China were severe enough to spur a form of heart disease called cardiomyopathy, now called Keshan's disease. Chinese government programs to supplement people's diets with selenium in the 1970s greatly reduced cases of Keshan disease, according to the National Institutes of Health's Office of Dietary Supplements. Low selenium levels in China, Tibet and Siberia may play a role in a type of osteoarthritis called Kashin-Beck disease.
Low selenium levels are more common in people with certain conditions across the world, which has raised questions and hopes that selenium supplements could bring health benefits.
Selenium as a supplement
Selenium is one of several nutrients known to have antioxidant properties, meaning selenium plays a part in chemical reactions that stop free radicals from damaging cells and DNA. Free radicals are unstable molecules from environmental toxins, or from byproducts of the human body's metabolism. Antioxidant supplements, including selenium, are often touted to help prevent to heart disease, cancer and vision loss.
Selenium supplements in particular are purported to help people with asthma, and reduce the risk of rheumatoid arthritis and cardiovascular disease. Selenium levels drop with age, so some have claimed selenium can slow the aging process, cognitive decline and dementia. Low selenium levels are also implicated in depression, male infertility, weak immune systems and thyroid problems.
Selenium is said to slow the progression of HIV and counteract the poisonous effect of heavy metals. Dandruff shampoos use selenium sulfide as a topical treatment; however, this form of selenium can cause cancer if ingested. Headaches, nausea and vertigo symptoms from intracranial pressure are said to be improved by selenium supplements. Selenium is also said to improve malabsorption in the digestive tract, particularly among children.
Do selenium supplements work?
"In general, we don't prescribe supplements unless someone is a strict vegan. Lifestyle changes often have a bigger effect than supplements do," Neabore said. That's especially true for supplements of selenium because humans need so little of it and because it can usually be easily acquired through food.
There are better ways to improve health before turning to supplements, Neabore said. "Avoid foods that are processed. A plant-based diet has been shown in many studies to be the most effective in terms of improving overall health. Address psychological issues like stress, sleep, and exercises before turning to supplements." People who smoke cigarettes or drink alcohol frequently may have lower selenium levels, but they should work on quitting smoking and drinking less alcohol, he said.
Selenium and cancer
Studies of selenium levels and large populations over time have found a correlation between people who eat a lot of selenium and a lower risk of some cancers, specifically bladder cancer, prostate cancer, lung cancer and some gastrointestinal cancers. More than 100 small animal experiments have shown selenium supplements reduce the number of new tumors, according to a January 2004 review published in the British Journal of Nutrition.
Regions with selenium-rich soil tend to have lower death rates from cancer than areas with low-selenium soil, particularly for cancers of the lung, esophagus, bladder, breast, colon, rectum, pancreas, ovary and cervix, according to the American Cancer Society. But these trends do not prove selenium is an underlying factor in cancer survival.
Controlled studies, in which groups of people were given either selenium supplements or a placebo, have found conflicting results about selenium's cancer fighting properties.
A study of more than 1,300 men and women with non-melanoma skin cancer found that the group of men assigned to take selenium supplements also had 52 percent fewer cases of prostate cancer, according to the 2003 paper published in the journal BJUI. But a larger study of more than 35,000 men called the Selenium and Vitamin E Cancer Prevention Trial (SELECT), found selenium supplements had no influence on prostate cancer risk. The Mayo Clinic does not recommend selenium supplements for cancer based on the current scientific evidence.
Selenium and heart disease
Preliminary studies show selenium may play a role in heart health. Selenium reduces inflammation and prevents platelets — a type of clotting cell in the blood — from aggregating, which is necessary for blood clots to form. Blood clots can lead to stroke, heart attacks, kidney failure, pulmonary embolism and other problems.
A selenium deficiency may make atherosclerosis (a hardening of the arteries) worse, according to the University of Maryland Medical Center. Animal studies have shown supplementing selenium after a long dietary deficiency reversed cardiovascular damage in mice. However, human studies on the subject have seen mixed results.
Some observational studies, where doctors track people but don't randomly assign them to taking a supplement or a placebo, found that the lower the selenium levels in the blood, the higher a person's risk of high blood pressure and coronary heart disease. But other observational studies didn't find a significant link between selenium levels and cardiovascular disease. Some observational studies even found the opposite trend; higher selenium levels were associated with worse cardiovascular health, according to the National Institutes of Health (NIH).
In one clinical trial, researchers assigned more than 450 older adults to take either a placebo or various strengths of selenium supplements for six months. They found the people taking selenium supplements had lower levels of bad cholesterol, and the group taking the highest amount of selenium (300 micrograms) also showed higher levels of HDL ("good") cholesterol, according to a May 2011 paper in the Annals of Internal Medicine.
However, a 2012 review by the Cochrane Collaboration concluded that taken together, the evidence from selenium studies — including nearly 20,000 people participating in 12 trials — did not support taking selenium supplements as a way to prevent major cardiovascular disease. However, the researchers did note that the vast majority of participants were men from the United States, where people already get a lot of selenium from food.
Selenium and rheumatoid arthritis
People with rheumatoid arthritis are more likely to have low selenium levels, but it is currently unclear whether low selenium levels are a result of the condition, or a contributing factor.
Once a person is diagnosed with rheumatoid arthritis, selenium supplements do not seem to help, according to the University of Maryland Medical Center.
Selenium and memory loss
Because selenium levels decline with age, there is a possibility that selenium influences age-related mental decline. Large observational studies have found either no link between selenium levels and memory test scores, or found that people with lower selenium levels are more likely to have cognitive decline over time, according to the NIH.
One eight-year study of more than 4,000 participants, ages 45 to 60, found that people taking antioxidant supplements had better verbal memory scores six years after the study ended, according to the September 2011 paper in the American Journal of Clinical Nutrition. However, because the antioxidant supplements included a mix of vitamin and minerals, the researchers could not separate what, if any, influence selenium had on memory scores. Selenium may have a role in cognitive decline because of its antioxidant properties, which can protect brain cells from damage over time. But overall, the limited number of studies on selenium and mental decline do not provide enough evidence to determine whether selenium can influence brain function, according to the NIH.
Selenium and thyroid problems
Evidence supports a link between selenium levels, iodine deficiencies and thyroid function, especially in women. Selenium is more concentrated in the thyroid than in other organ in the body, and it is important in the production and metabolism of thyroid hormone.
One analysis of 1,900 people in France found that among women a mild iodine deficiency, those with lower the selenium levels were more likely to develop goiters or thyroid damage compared with those with higher selenium levels, according to a 2003 article in the European Journal of Endocrinology. Subsequent studies have found similar links. However, controlled studies on thyroid health, where participants are given selenium supplements or placebos, have had mixed results. More research is needed to determine if selenium supplements can fight thyroid diseases.
Selenium and diabetes
There is limited research on selenium supplements and diabetes risk. A few large observational studies showed a correlation between higher selenium concentrations in toenails and a lower risk of diabetes.
However a placebo-controlled study of more than 1,200 people over seven years found selenium supplements didn't reduce the risk of type 2 diabetes, and may actually increase diabetes risk, according to the 2007 paper published in the Annals of Internal Medicine.
Selenium and HIV
Selenium levels tend to drop as HIV infections progress. Recent studies on selenium supplements in HIV-positive patients show some promise. One laboratory study of human blood cells found that adding a certain selenoprotein to HIV-infected cells slowed the replication of HIV by 10-fold, compared with HIV-infected human cells without the selenoprotein, according to the November 2008 paper in the Journal of Biological Chemistry.
A study on 878 HIV-positive people in Botswana, who had not taken antiretroviral medicines, showed a multivitamin in combination with selenium supplements slowed the progression of HIV symptoms and lowered the risk of death, according to the November 2013 paper published in the Journal of the American Medical Association. Interestingly, the selenium supplements alone or the multivitamin alone did not provide any better protection than placebo during the two-year study.
Another controlled study of HIV-positive people found that 200 micrograms of daily selenium supplements could suppress HIV viral burden (the amount of virus in the blood) and strengthen the immune system, according to a 2007 paper published in the Archives of Internal Medicine.
Selenium and other health conditions
People with asthma tend to have lower selenium levels than people without asthma. But the few epidemiological studies to touch on this topic haven't found that taking selenium supplements can reduce asthma symptoms, according to the University of Maryland Medical Center. And while selenium is needed to make healthy sperm, very high selenium levels are also linked to decreased sperm motility.
Selenium does help the production of white blood cells, which help fight infection. But there are limited studies as to whether selenium supplements beyond the recommended daily amount boost the immune system, according to the University of Maryland Medical Center.
Are selenium supplements safe?
Most people in the United States get more than the 55 micrograms of selenium recommended each day. However, some situations may lower a person's selenium levels. People who smoke cigarettes or drink alcohol often are more likely to have low selenium levels. Crohn's disease, ulcerative colitis, or other conditions that interfere with how the body absorbs selenium can lead to low selenium levels. Kidney dialysis patients may also become low in selenium. The chemotherapy drug cisplatin can lower selenium levels in the blood, but it is not known if cisplatin can lead to a significant deficiencies.
Supplements may deliver selenium in several forms, including sodium selenite and sodium selenate. But more forms of selenium are found naturally in food, according to the Linus Pauling Institute at Oregon State University.
"People can overdose on selenium supplements," Neabore said. In 2008, a liquid dietary supplement that was 200 times more concentrated than advertised led to selenium poisoning in more than 200 people in 10 states. The most common effects were diarrhea, fatigue, hair loss, joint pail, brittle nails and nausea. A third of the people affected continued to experience symptoms 90 days after taking the mislabeled supplements.
Taking too much selenium over time can lead to selenosis, which can cause hair loss, nail loss, nausea, irritability, fatigue and some nerve damage. Other symptoms of chronic selenium overdose are a metallic taste in the mouth, and a garlic scent on the breath. A selenium overdose can cause skin lesions and nervous system abnormalities. In severe cases, selenium toxicity can cause tremors, kidney failure, cardiac failure, respiratory distress or even death. Luckily, selenosis is rare in the United States.
The Institute of Medicine's Food and Nutrition Board caps the safe daily intake of selenium at 45 micrograms for infants, 60 to 90 micrograms in toddlers, 150 to 280 micrograms in prepubescent children and 400 micrograms in adults.
Selenium supplements can cause adverse reactions when combined with drugs, warned Ken Sternfeld, a New York-based pharmacist. It may increase the risk of bleeding if it's combined with blood thinners such as clopidogrel (Plavix), coumadin, heparin or aspirin, according to the University of Maryland Medical Center. Animal studies show selenium supplements may also extend the effects of sedatives. And antioxidant supplements that included selenium have been shown to interfere with cholesterol-lowering treatments.
If you are thinking of taking selenium supplements, talk to your doctor and pharmacist about possible side effects from drug combinations, Sternfeld said. "The only way the health care industry can help people is if they collaborate, and talk their health care providers — pharmacists and doctors — so they can collaborate. When you pick up medicines like blood thinners or sedatives, tell your pharmacist if you are taking selenium supplements," he added. "Interactions and adverse drug reactions are the backbone of what pharmacists should be focusing on."
Supplements, including selenium supplements, can be easily mislabeled because the FDA does not regulate them, Neabore said. According to the University of Utah, products have been pulled from the market because they contained 200 times more selenium than advertised and people became sick as a result.
"With medication, they have to prove that it's safe before you can market it. For a supplement, someone has to prove that it's not safe for it be taken off the market," Neabore said. "That can leave a lot of room for error and dishonesty."
Additional reporting by Jessie Szalay, Live Science Contributor.