By Dr. Mercola
Selenium is a trace element a Swedish chemist, Baron Jöns Jacob Berzelius, discovered almost 200 years ago. Today, modern scientists recognize it as "an essential mineral of pivotal importance for human health," with anti-inflammatory, antiviral and anti-cancer potential.1
This mineral is also a powerful antioxidant, which plays itself out in many ways in regard to your health. You need only a little, though, to help keep your immune system and other functions humming along in proper order.
As much as your body requires selenium, taking the proper amount is crucial, because too much (such as 400 micrograms [mcg] daily) is associated with an increased risk of diabetes.2
However, unless you're taking a supplement, it's not likely you'll overdose on selenium through the foods you eat. In fact, most people have trouble getting what they need, and as many as 1 billion people worldwide have a selenium deficiency.
Your chance of having a selenium deficiency is higher if you smoke cigarettes, take birth control pills, drink alcohol or have a condition that keeps you from absorbing the nutrients you need through the foods you eat.
Free Radicals: The 'Bad Guys' You Don't Want Lurking in Your Body
As previously mentioned, one of the most important aspects of selenium is that it functions as a free-radical-zapping antioxidant. What does that mean, exactly?
When you take the word apart, "anti" is something you're against and the word or phrase that follows it is the "bad guy." In this case, what you're against is oxidation because it can cause oxidative stress, which in turn can lead to tissue and organ damage. According to News-Medical:
"Oxidative stress is essentially an imbalance between the production of free radicals and the ability of the body to counteract or detoxify their harmful effects through neutralization by antioxidants"3
While "free radicals" may be another murky term, in short, free radicals and other assorted reactive oxygen species (ROS) are caused by either normal, internal metabolic processes or via outside influences such as nicotine and X-rays, or exposure to harmful chemicals like those used to kill mosquitoes, germs in your bathroom or weeds around your patio. One study explains:
"Free radicals, reactive oxygen species (ROS) and reactive nitrogen species are generated by our body by various endogenous systems, exposure to different physiochemical conditions or pathological states. A balance between free radicals and antioxidants is necessary for proper physiological function.
If free radicals overwhelm the body's ability to regulate them, a condition known as oxidative stress ensues. Free radicals thus adversely alter lipids, proteins and DNA and trigger a number of human diseases. Hence application of external source of antioxidants can assist in coping (with) oxidative stress."4
It may be helpful to remember that free radicals can cause cell damage, and antioxidants fight free radicals.
Thyroid Function and the Role of Selenium
Your thyroid contains more selenium per gram of tissue than any other organ. One study explains:
"In 1957, studies investigating the requirements of nutrients in rodent diets revealed selenium (along with vitamin E) to be essential for prevention of liver necrosis. This led to the realization that selenium deficiency was responsible for a number of disorders observed previously …
(Selenium is) a contributing factor to Keshan disease in humans. Although toxicity at higher levels is still a serious problem, the importance of selenium as an essential micronutrient is now recognized."5
Another study states that the value of selenium supplementation for people with autoimmune thyroid problems is becoming more understood and deficiency even appears to have an impact on the development of thyroid problems, possibly due to selenium's ability to regulate the production of ROS and their metabolites.
In patients with Hashimoto's disease, selenium supplementation "decreases anti-thyroid antibody levels and improves the ultrasound structure of the thyroid gland."6 Further, studies for pregnant women regarding selenium say that supplementation significantly lowers the risk of postpartum thyroiditis.7
Selenium Strengths: Proper Amounts Cut Your Risk of Serious Disease
According to one meta-analysis:
"Selenium may play a beneficial role in multi-factorial illnesses with genetic and environmental linkages … Tissues particularly sensitive to changes in selenium supply include red blood cells, kidney and muscle.
The meta-analysis identified that for animal species selenium-enriched foods were more effective than selenomethionine at increasing (glutathione peroxidase) activity."8
One of the most important functions of selenium is its ability to help your body fight disease. It raises your white blood cell count so you're more able to resist infections.
An example is a study showing that selenium may help prevent a skin infection prevalent in people with lymphedema (swelling of the tissues in your arms and/or legs, usually as a result of chemotherapy or injury), and mycoplasma pneumonia, aka "walking" pneumonia.9
In 2012, researchers reported that in areas of the world where selenium levels are naturally low, supplementing with selenium may be cancer protective.10 Study author and professor John Hesketh of Newcastle University, U.K., explained:
"The difficulty with selenium is that it's a very narrow window between levels that are sub-optimal and those that would be considered toxic.
What our study shows is a possible link between higher levels of selenium and a decreased risk of colorectal cancer and suggests that increasing selenium intake may reduce the risk of this disease."11
While it should be noted that some researchers say taking selenium supplements doesn't appear to influence heart disease one way or the other or protect against heart attack, the University of Maryland Medical Center reported:
"Scientists know that low levels of selenium can contribute to heart failure, and being deficient in selenium seems to make atherosclerosis worse. Atherosclerosis, or hardening of the arteries, happens when plaque builds up in arteries, which can lead to heart attack and stroke."12
Another study found that patients who took selenium supplements on a regular basis are "far less likely" to have another heart attack.13
Asthma sufferers tend to have higher incidences of low selenium levels in their blood. Scientists found that diets containing high amounts of antioxidants are associated with lowered asthma prevalence in epidemiologic studies, as a report on accumulated data revealed:
"Accumulated data indicate that asthma is associated with reduced circulatory selenium (Se) … In the Se-supplemented group there were significant increases in serum Se
… Further, there was a significant clinical improvement in the Se-supplemented group, as compared with the placebo group."14
Among 24 subjects with asthma, those who took supplements for 14 weeks had fewer symptoms than those taking a placebo, one study found. However, scientists agree that more studies are needed.15
Proteins found in sperm and involved in their formation are impacted by selenium and other antioxidants.
An interesting dichotomy, however, is that while studies show male infertility may be improved by the selenium in a man's system, levels that are too high can inhibit the sperm's ability to swim, according to the University of Maryland Medical Center.16 Another study concluded:
"Selenium-enriched probiotics or inorganic selenium supplementation gave better results than probiotics supplementation and may be used to improve animal and human male fertility compromised by hyperlipidemia or obesity."17
Most of the African continent is selenium deficient. Simultaneously, AIDS is the most common cause of death. News-Medical, examining diseases impacted by selenium, reported:
"Taken as a whole, the geographical evidence, therefore, strongly suggests that selenium is protective against HIV infection.
Such a relationship is not limited to this virus. A frequently fatal illness of the heart, known as Keshan disease, is widespread in the population of the low selenium belt that crosses China from northeast to southwest. Keshan disease occurs only in individuals who are both selenium deficient and infected by the coxsackievirus"18
While the highest death rates from AIDS affect several of the southwestern-most portions of the continent, such as Botswana, Uganda and Kenya, "the prevalence rate for HIV infection still hovers at an unusually low 0.5 percent among women attending antenatal clinics" in Dakar, the capital city of Senegal.
The difference, scientists say, is that Senegal is located on the far western coast of Africa, where the soil is enriched with trace elements of selenium, contrasting the eastern portion, where the soil is devoid of the selenium that might help make a difference in this regard.
A similar situation is taking place in Finland where, to combat heart disease, legislation was passed in 1984 ordering sodium selenite to be added to all fertilizers throughout the country. Perhaps as a result, the country's HIV rates are half that of other Scandinavian countries.
Selenium From Food: Seafood, Mushrooms and Meat
The best selenium sources from food include salmon (although only wild-caught Alaskan salmon is recommended due to widespread pollution in other fish), free-range organic turkey, lamb and grass-fed organic beef. You can also find high amounts of selenium in Brazil nuts, sunflower seeds, onions and garlic and certain mushrooms.19 SFGate says:
"Mushrooms are one of the top vegetable sources for selenium. One cup of cooked shiitakes or white button mushrooms provides 19 micrograms of selenium, or 35 percent of the RDA. A more typical serving of ¼ cup provides less than 10 percent of the daily value.
A cup of cooked lima or pinto beans averages 9 to 11 micrograms of the mineral, or about 15 to 20 percent of the RDA. Frozen cooked spinach, which is packed more tightly per cup than fresh cooked, provides 10 micrograms of selenium, or 18 percent of the RDA."
It's not just how much selenium is in your food, though, that determines how much you're getting. It's also about how much selenium is in the soil your food is grown in. Related factors include how much selenium was in the grass eaten by the cattle producing your grass-fed beef.
(Grass-fed beef, by the way, contains a healthy ratio between omega-6 and omega-3 fats. Naturally, you also want it to be free of hormones and antibiotics.)