Selenium is a trace mineral that is essential to good health but required only in small amounts. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.

There is evidence that selenium deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system. There is also evidence that selenium deficiency does not usually cause illness by itself. Rather, it can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses.
Three specific diseases have been associated with selenium deficiency:

What is Selenium?
Selenium is a mineral. It is taken into the body in water and foods. People use it for medicine.
Most of the selenium in the body comes from the diet. The amount of selenium in food depends on where it is grown or raised. Crab, liver, fish, poultry, and wheat are generally good selenium sources. The amount of selenium in soils varies a lot around the world, which means that the foods grown in these soils also have differing selenium levels. In the U.S., the Eastern Coastal Plain and the Pacific Northwest have the lowest selenium levels. People in these regions naturally take in about 60 to 90 mcg of selenium per day from their diet. Although this amount of selenium is adequate, it is below the average daily intake in the U.S., which is 125 mcg.

Selenium is used for diseases of the heart and blood vessels, including stroke and "hardening of the arteries" (atherosclerosis). It is also used for preventing various cancers including cancer of the prostate, stomach, lung, and skin.

Some people use selenium for under-active thyroid, osteoarthritis, rheumatoid arthritis (RA), an eye disease called macular degeneration, hay fever, infertility, cataracts, gray hair, abnormal pap smears, chronic fatigue syndrome (CFS), mood disorders, arsenic poisoning, and preventing miscarriage.
Selenium is also used for preventing serious complications and death from critical illnesses such as head injury and burns. It is also used for preventing bird flu, treating HIV/AIDS, and reducing side effects from cancer chemotherapy.

Where it is found
Plant foods are the major dietary sources of selenium in most countries throughout the world. The content of selenium in food depends on the selenium content of the soil where plants are grown or animals are raised. For example, researchers know that soils in the high plains of northern Nebraska and the Dakotas have very high levels of selenium. People living in those regions generally have the highest selenium intakes in the United States (U.S.). In the U.S., food distribution patterns across the country help prevent people living in low-selenium geographic areas from having low dietary selenium intakes. Soils in some parts of China and Russia have very low amounts of selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and eaten locally.

Selenium also can be found in some meats and seafood. Animals that eat grains or plants that were grown in selenium-rich soil have higher levels of selenium in their muscle. In the U.S., meats and bread are common sources of dietary selenium. Some nuts are also sources of selenium.
Selenium content of foods can vary. For example, Brazil nuts may contain as much as 544 micrograms of selenium per ounce. They also may contain far less selenium. It is wise to eat Brazil nuts only occasionally because of their unusually high intake of selenium. Selected food sources of selenium are provided in table below:

Selected Food Sources of Selenium

Benefits / uses
Selenium and cancer:
Several research reports indicate the inverse relationship between higher blood levels of selenium and mortality from cancer including lung, colorectal, prostate and skin cancer. Laboratory studies indicate the potentially beneficial role of selenium in the management of mammary cancer.

Selenium and cholesterol:
Based on epidemiological data, a lower antioxidant status has been linked to higher incidence of cardiovascular diseases due to increased levels of LDL oxidation. Selenium is one of the antioxidants that may help to inhibit LDL oxidation.
The National Academy of Sciences (NAS) released a report on April 11th reviewing the emerging science of antioxidants and their potential for disease protection. The Council for Responsible Nutrition Concludes that while there is not enough scientific evidence to permit recommended intakes based on disease prevention, the wide range of safe intakes for antoxidant vitamins is approved. It issued higher Recommended Dietary Allowances(RDAs) for vitamins C and E. The new RDAs for antioxidant nutrients are: vitamin C (75 milligrams (mg) for women and 90 mg for men), vitamin E (15 mg), and selenium (55 micrograms (mcg)).

The report stated that there is "reason to expect that the antioxidant vitamins (C and E) should decrease the risk of cardiovascular disease." The report concluded that vitamin E does inhibit LDL oxidation (a causative agent for cardiovascular disease) and could also affect the health of arteries in other ways. The report also noted that it is reasonable to expect that vitamin C would reduce oxidative damage to the eye, thus lessening the risk of cataracts and other conditions.

The NAS report also established tolerable upper intake levels (ULs) for vitamin C (2,000 mg), vitamin E (1,000 mg) and selenium (400 mcg). ULs for adults are set to protect the most sensitive individuals of the general population.

Selenium and healthy blood sugar levels
Selenium is reported to mimic the action of insulin. In laboratory studies performed in recent years, selenium has been shown to mediate a number of insulin-like actions such as stimulating glucose uptake and regulating metabolic processes including glycolysis, gluconeogenesis, fatty acid synthesis and the pentose phosphate pathway. Although the exact mechanism of the insulin mimicking action of selenium has as yet to be elucidated, it is reported that these actions are mediated through the activation of key proteins involved in the insulin-signal cascade. Selenium is also reported to play a role in reducing the oxidative stress associated with diabetes, thereby retarding the progression of the secondary complications of diabetes such as neuropathy, retinopathy and cataracts.

Selenium and arthiritis
Low selenium status has been associated with the incidence of arthritis. Preliminary studies indicate the beneficial role of selenium as a free radical scavenger that may delay the progression of arthritis.

Selenium and HIV
Malabsorption leading to deficiency state has been observed in cases of progression of HIV /AIDS. 24 children with HIV were observed for a period of five years. Those with low selenium levels had greater risk of mortality.

Low plasma selenium status has been associated with senility and cognitive decline in the elderly and with Alzheimer’s disease. Selenium supplementation was observed to reduce the severity of epileptic seizures in children. Selenium supplementation is also reported to improve confused and depressed mental states; mental fatigue and anxiety in adults.

When to take/ Types to take
Selenium supplements are best taken with meal. Since selenium works so closely with vitamin E, it makes sense to supplement that as well.
Selenium is available in several forms, both inorganic and organic. Inorganic salts like sodium selenite are thought to be less usable by the body, making the organic forms more desirable. The organic forms are selenomethionine and selenium rich yeast, or yeast bound selenium.

Doses
The daily recommended dietary allowances (RDAs) of selenium are:
Children 1-3 years, 20 mcg; children 4-8 years, 30 mcg; children 9-13 years, 40 mcg;

Possible Side effects / Precautions / Possible Interactions:
Selenium is likely safe for most people when taken by mouth in doses less than 400 mcg per day, short-term. Higher doses are possibly unsafe. They can cause significant side effects including nausea, vomiting, nail changes, loss of energy, and irritability. Poisoning from long-term use is similar to arsenic poisoning, with symptoms including hair loss, white horizontal streaking on fingernails, nail inflammation, fatigue, irritability, nausea, vomiting, garlic breath odor, and a metallic taste.

Selenium can also cause muscle tenderness, tremor, lightheadedness, facial flushing, blood clotting problems, liver and kidney problems, and other side effects.
There is concern that taking selenium for a long time might not be safe. Long-term consumption of selenium supplements appears to increase the chance of getting type 2 diabetes. It also seems to increase the risk of skin cancer recurrence. There is also some concern that having too much selenium in the body might increase the risk of overall death as well as death from cancer.

Special precautions & warnings:
Pregnancy and breast-feeding: Selenium use is possibly safe during pregnancy and breast-feeding when used short-term in amounts that are not larger than 400 mcg per day.

Fertility problems in men: Selenium might decrease the ability of sperm to move, which could reduce fertility. If you are trying to father a child, don’t take selenium supplements.

Prostate cancer: There is concern that taking large amounts of a multivitamin plus a separate selenium supplement might increase the chance of developing prostate cancer and dying from prostate cancer.

A history of skin cancer: Long-term use of selenium supplements might slightly increase the risk of skin cancer recurrence, but this is controversial. Until more is known about the possible increase in skin cancer risk, avoid long-term use of selenium supplements if you have ever had skin cancer.

Under-active thyroid (hypothyroidism): Taking selenium can worsen hypothyroidism especially in people with iodine deficiency. In this case, you should take iodine along with selenium. Check with your healthcare provider.

Surgery: Selenium might increase the risk of bleeding during and after surgery. Stop taking selenium at least 2 weeks before a scheduled surgery.

Do not take this combination:
Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)
Selenium might slow blood clotting. Taking selenium along with medications that also slow blood clotting might increase the chances of bruising and bleeding.

Some medications that slow blood clotting include aspirin, clopidogrel (Plavix), dalteparin (Fragmin), enoxaparin (Lovenox), heparin, ticlopidine (Ticlid), warfarin (Coumadin), and others.

Be cautious with this combination.
Medications used for lowering cholesterol (Statins)
Taking selenium, beta-carotene, vitamin C, and vitamin E together might decrease the effectiveness of some medications used for lowering cholesterol. It is not known if selenium alone decreases the effectiveness of medications used for lowering cholesterol.
Some medications used for lowering cholesterol include atorvastatin (Lipitor), fluvastatin (Lescol), lovastatin (Mevacor), and pravastatin (Pravachol).

Niacin
Taking selenium along with vitamin E, vitamin C, and beta-carotene might decrease some of the beneficial effects of niacin. Niacin can increase the good cholesterol. Taking selenium along with these other vitamins might decrease how well niacin works for increasing good cholesterol.

Sedative medications (Barbiturates)
The body breaks down medications to get rid of them. Selenium might slow how fast the body breaks down sedative medications (barbiturates). Taking selenium with these medications might increase the effects and side effects of these medications.

Warfarin (Coumadin)
Selenium might thin the blood. Selenium might also increase the effects of warfarin in the body. Taking selenium along with warfarin might increase the chances of bruising and bleeding.

Be watchful with this combination.
Birth control pills (Contraceptive drugs)
Some research shows that women who take birth control pills might have increased blood levels of selenium. But other research shows no change in selenium levels in women who take birth control pills. There isn't enough information to know if there is an important interaction between birth control pills and selenium.
Some birth control pills include ethinyl estradiol and levonorgestrel (Triphasil), ethinyl estradiol and norethindrone (Ortho-Novum 1/35, Ortho-Novum 7/7/7), and others.

Gold salts
Gold salts bind to selenium and decrease selenium in parts of the body. This might decrease the normal activity of selenium, possibly resulting in symptoms of selenium deficiency.
Gold salts include aurothioglucose (Solganal), gold sodium thiomalate (Aerolate), and auranofin (Ridaura).

Research studies / References