How much do you know about selenium? Not much, if you’re like most people. You may have noticed it in your multivitamin, but it usually takes a backseat to more popular and better-known nutrients.
Yet, while you only require tiny amounts of this trace mineral, its impact on your health is significant. If you don’t get enough selenium, your thyroid function falters, your immune system weakens, and your risk of cancer and other diseases increases.
What Exactly is Selenium?
Selenium is an integral component of selenoproteins, a family of 25 proteins that act primarily as enzymes, meaning they catalyze or speed up biochemical reactions in your body. Selenoproteins have two main functions:
- Thyroid hormone metabolism: Specific selenoprotein enzymes are required for the maturation of the thyroid gland and the conversion of T4, the most abundant thyroid hormone, into the more biologically active T3 hormone. T3 regulates your heart rate, brain development, muscle function, and more.
- Antioxidant defenses: Several selenoproteins, including five glutathione peroxidases and three thioredoxin reductases, are potent antioxidant enzymes that neutralize free radicals and protect against oxidative damage.
These two activities underscore the importance of selenium’s function in the body. It is required by the thyroid gland for normal development, growth, and metabolism. And as part of the body’s antioxidant defense system, selenium safeguards your mitochondria (the “energy factories” in your cells), DNA, and cell membranes.
Why Selenium Is Good for Your Thyroid
Because of selenoproteins’ essential role in thyroid hormone metabolism, selenium status obviously affects thyroid function:
- Goiter: Population studies have linked low selenium levels with an increased risk of goiter (enlarged thyroid gland) in women. Supplemental selenium has been used to prevent or treat this condition.
- Graves’ disease: Selenium deficiency can contribute to Graves’ disease, which causes hyperthyroidism (overproduction of thyroid hormones). In one study, 200 mcg of selenium daily reduced eye problems and improved quality of life in patients with this autoimmune disease.
- Hashimoto’s thyroiditis: Research has also found associations between selenium and hypothyroidism (low thyroid function), especially in individuals with Hashimoto’s thyroiditis, an autoimmune disease and the most common cause of hypothyroidism. Supplemental selenium, 200 mcg, has been shown to reduce antibodies that are elevated in Hashimoto’s.
Selenium Enhances Immune Function
The antioxidant activity of selenoproteins is vital throughout the body, including the immune system. Oxidative stress and associated inflammation significantly increase during viral and bacterial infections and other illnesses, and selenoproteins are called into action to help fight off invaders.
Because selenium is an indispensable element in these antioxidant enzymes, maintaining good selenium status is important for optimal immune function. Selenium also enhances immune function by promoting the formation and activity of natural killer cells, T cells, and antibodies.
Studies have shown that selenium levels are low in patients with chronic viral infections such as human immunodeficiency virus (HIV), and supplemental selenium has been shown to raise blood levels and improve outcomes.
Selenium & Cancer Prevention
This mineral also has chemopreventive properties. Population studies have found that cancer is more prevalent in geographic areas with low levels of selenium in the soils and, therefore, lower dietary intake. In China, for example, provinces with selenium-depleted soils have higher rates of cancer.
Supplemental selenium may help. In a clinical trial involving participants in these areas of China, 200 mcg of selenium daily significantly reduced prostate, lung, and colon cancer—and lowered the risk of death from cancer by 50%. Another study conducted in the US demonstrated that African-American women with the highest intake of supplemental selenium had a 30% reduction in risk of ovarian cancer.
Although some clinical trials have shown no protective effects of supplemental selenium, most of those studies included participants with normal selenium levels. Protection is greatest for those with inadequate selenium intake. If selenium status is already good, increasing it provides less dramatic protection.
Additional Selenium Benefits
Research suggests a number of other benefits of supplemental selenium:
- Cardiovascular disease: Selenium has been shown to increase levels of protective glutathione peroxidase and decrease C-reactive protein, a marker of inflammation and a risk factor for cardiovascular disease.
- Prostate problems: A number of men’s supplements contain zinc, saw palmetto, lycopene, beta-sitosterol, and selenium for prostate enlargement.
- Liver disease: Selenium is reportedly helpful because it boosts antioxidant defenses in the liver.
- Memory loss: Some studies suggest selenium may play a role in protecting against age-related cognitive decline.
- Male infertility: Because selenoproteins are required for fertility and sperm production, this problem could be related to selenium deficiency.
- Other conditions: Some researchers have proposed supplemental selenium for acne and other skin problems, hair growth, and weight loss, likely due to its effects on thyroid function.
Best Sources of Selenium
Brazil nuts are, hands down, the richest source of selenium. Foods that have selenium in reasonably high amounts include fish and seafood, meat, whole wheat, eggs, potatoes, and rice.
However, selenium content varies according to where the food was grown. Although the soils in some areas of Asia, Europe, and New Zealand are low in selenium, North America’s are relatively high, so grains, livestock, and vegetables from the US and Canada are generally good natural sources of selenium.
Nevertheless, your nutrient intake is only as good as your diet. That’s why I believe it is prudent to add selenium to your daily supplement regimen. This is particularly important if you have a thyroid disorder, poor immune function, a serious infection, cancer or another chronic disease, or any of the other conditions we have discussed. And because selenium status declines with age, I also recommend it for older people.
Yet, as important as selenium is, don’t go overboard. Selenium side effects and toxicity can occur when taken in excess. A daily dose of 100–200 mcg—the amount in some high-quality multivitamins—is safe, sufficient, and supportive of your overall health.