Selenium is an essential mineral found in trace amounts in the body. It works as an antioxidant, especially when combined with vitamin E, by scavenging damaging particles in the body known as free radicals. Free radicals can damage cell membranes and DNA, and may contribute to aging and a number of conditions including heart disease and cancer. Antioxidants such as selenium can neutralize free radicals and may reduce or even help prevent some of the damage they cause.

Selenium plays a role in thyroid function and is needed for the immune system to work properly. Scientists have observed that people with a number of conditions, ranging from rheumatoid arthritis to some types of cancer, often have low levels of selenium. However, in most cases they aren't sure whether low selenium levels are a cause or an effect of the disease.

A 2007 study that was examining whether selenium had any effect on skin cancer found an unexpected result: People who took as little as 200 mcg of selenium per day over a long period of time (more than seven years) had a significantly higher risk of developing diabetes. Although more studies are needed to understand exactly what effect selenium might have, you should not take more than the daily recommended allowance of selenium without your doctor's supervision.

If you are healthy and eat a well-balanced diet, you should get enough selenium. You may have low levels of selenium if you:

• Smoke cigarettes
• Drink alcohol
• Take birth control pills
• Have a malabsorption syndrome (such as Crohn's disease or ulcerative colitis)

Heart Disease

The evidence on taking selenium to treat heart disease is mixed at best. Scientists know that low levels of selenium can contribute to heart failure, and being deficient in selenium appears to make atherosclerosis (plaque build up in arteries which can lead to heart attack and stroke) worse. But studies show that taking selenium supplements doesn't seem to have any effect on the progression of heart disease, nor does it protect against heart attack. Selenium, in combination with other antioxidants vitamin E, vitamin E, and beta-carotene, may help lower LDL ("bad") cholesterol levels. But selenium can also interact with cholesterol lowering drugs (See "Possible Interactions"). If you have or are at risk for heart disease, talk to your doctor before taking selenium.

Cancer

The evidence is also mixed when it comes to selenium and cancer. Several animal and human studies suggested that selenium may protect against some kinds of cancer, and scientists observed that people who live in parts of the world where there is a good deal of selenium in the soil and foods have lower rates of cancer. In addition, people who have cancer often have low levels of selenium, but doctors don't know whether that is a cause or an effect of the disease.

The best evidence suggests that, if you have low levels of selenium, getting more selenium in your diet can reduce your risk of prostate cancer, and that taking a special kind of brewer's yeast (called selenized yeast) that's rich in selenium may also work. A large clinical trial is currently underway to determine whether selenium has any beneficial effect for men who already have normal levels of the mineral in their body. If you are at risk for prostate cancer, talk to your doctor before taking selenium.

Other scientific studies have not born out selenium's promise for most types of cancer. It doesn't appear to reduce the risk of lung or esophageal cancer, and evidence is mixed on whether it protects against colorectal cancer.

The situation is complicated when it comes to skin cancer. Although some early evidence led scientists to investigate whether selenium might protect against skin cancer, a large trial found that taking daily selenium supplements actually increased the risk of squamous cell carcinoma and nonmelanoma skin cancer.

Immune Function

Many studies suggest that the body needs selenium in order for the immune system to function properly. Selenium along with other minerals can help build up white blood cells, enhancing the body's ability to fight illness and infection. A few studies suggest that selenium might help prevent some infections (a bacterial skin infection that often occurs with lymphedema, and mycoplasma pneumonia). In addition, one study suggested that elderly people who took zinc and selenium supplements had a better immune response to the flu vaccine than those who received placebo.

Asthma

Evidence suggests that people with asthma tend to have low blood levels of selenium. In a study of 24 people with asthma, those who received selenium supplements for 14 weeks had fewer symptoms compared to those who received placebo. More studies are needed, however, to know if selenium has any real benefit.

HIV

The evidence concerning selenium and HIV/AIDS is contradictory. Studies have shown that levels of selenium decline consistently as HIV progresses. In one study, those taking a proprietary selenium supplement (Selenomax) slowed the increase in viral load and had higher CD4 cell counts. But another study found that taking selenomethionine (a type of selenium) had not effect. If you have HIV or AIDS, talk to your doctor before taking any supplement, as it may interact with medications you are taking.

Male Infertility

Selenium and other antioxidants play an essential role in the formation of certain proteins found in sperm. One study suggested that selenium supplements might improve male fertility in men who had low levels of selenium. However, high levels of selenium are associated with decreased sperm motility.

Rheumatoid Arthritis

Low levels of selenium in the blood may be associated with increased risk of rheumatoid arthritis. But it doesn't appear that selenium supplements have any beneficial effect once RA has already developed.

Brewer's yeast and wheat germ, liver, butter, fish (mackerel, tuna, halibut, flounder, herring, smelts) and shellfish (oysters, scallops and lobster), garlic, whole grains, sunflower seeds, and Brazil nuts are all good sources of selenium.

Selenium levels in food can vary. The level of selenium in the soil where food is grown determines how much selenium is in the food.

Selenium is destroyed when foods are refined or processed. Eating a variety of whole, unprocessed foods is the best way to get selenium in your diet.

Selenium may be taken as part of a vitamin-mineral supplement, a nutritional antioxidant formula, or as an individual supplement. Most supplements contain a form of selenium called selenomethionine.

The minimum daily recommended dietary allowances for selenium are listed below.

Pediatric

• Birth to 6 months: 15 mcg
• Infants 6 months to 1 year: 20 mcg
• Children 1 to 6 years: 20 mcg
• Children 7 to 10 years: 30 mcg
• Males 11 to 14 years: 40 mcg
• Females 11 to 14 years: 40 mcg

Adult

• Males 15 to 18 years: 55 mcg
• Males over 19 years: 55 mcg
• Females 15 to 18 years: 55 mcg
• Females over 19 years: 55 mcg
• Pregnant females: 60 mcg
• Lactating females: 70 mcg

Doses of 200 mcg per day have been used for some conditions, but evidence suggests that taking that amount over a long period of time could significantly increase your risk of developing diabetes. Talk to your doctor before taking more than the recommended daily allowance.

Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.

Talk to your doctor before taking more than the recommended daily allowance of selenium. Although doses for some conditions are 200 mcg, there is some evidence that this amount could increase your risk for diabetes. Do not exceed the tolerable upper limit of 400 mcg. High doses of selenium over time can be toxic. Side effects include fingernail loss, skin rash, fatigue, irritability, and weight loss.

People who have an underactive thyroid (hypothyroidism) or have had or at risk for skin cancer should not take selenium without talking to their doctor.

If you are being treated with any of the following medications, you should not use selenium supplements without first talking to your health care provider.

Drugs that affect selenium levels in the body -- These drugs may lower levels of selenium:

• Cisplatin, a chemotherapy drug
• Clozapine (Clozaril)
• Corticosteroids, such as prednisone
• Valproic acid (Depakote)

Anticoagulants and antiplatelet drugs (blood thinners) -- When combined with these drugs, selenium may increase the risk of bleeding:

• Clopidogrel (Plavix)
• Warfarin (Coumadin)
• Heparin
• Aspirin

Barbiturates -- In animal tests, selenium seems to make the sedative effects of these drugs last longer:

• Butabarbital (Butisol)
• Mephabarbital (Mebaral)
• Phenobarbital (Nembutal)
• Secobarbital (Seconal)

Chemotherapy -- Although selenium may help reduce side effects from drugs such as cisplatin, doxorubicin, and belomycin, it may also interfere with their cancer-fighting ability. If you are undergoing chemotherapy, talk to your oncologist before taking selenium or any other supplement.

Cholesterol-lowering medications -- Simvastatin (Zocor) and niacin have been shown to lower LDL ("bad") cholesterol and raise HDL ("good") cholesterol in people with heart disease. Taking antioxidants, including selenium, along with these drugs may reduce their effectiveness. Theoretically, selenium may also reduce the effectiveness of other statins, including atorvastatin (Lipitor), fluvastatin (Lescol), lovastatin (Mevacor), and prevastatin (Pravachol).

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