Overview:

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. 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 scientists aren't sure whether low selenium levels are a cause or an effect of the disease.

In 2007, when researchers examined whether selenium had any effect on skin cancer, they found an unexpected result: People who took as little as 200 mcg of selenium per day for more than 7 years had a significantly higher risk of developing diabetes. More studies are needed to understand exactly what effect selenium might have. In the meantime, 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. 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, including 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

Studies show that low levels of selenium are linked to a higher risk of cancer death. Plus, research shows that selenium has a protective effect on various stages of cancer, including both the early and later stages of the disease. Scientists aren't sure how selenium works, but they have 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.

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 be effective. In one study of 1,312 people, those who took 200 micrograms of yeast per day had two-thirds lower risk of developing prostate cancer during the 4.5-year study compared to those who took a placebo. If you are at risk for prostate cancer, talk to your doctor before taking selenium.

Research hasn't determined 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 no 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 (RA)

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

Dietary Sources:

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.

Available Forms:

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.

How to Take It:

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

Pediatric

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

Adult

• Males 15 - 18 years: 55 mcg
• Males over 19 years: 55 mcg
• Females 15 - 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.

Precautions:

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. Over time, high doses of selenium 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 are at risk for, skin cancer should not take selenium without talking to their doctor. High selenium intake may increase the risk of diabetes and high cholesterol.

Possible Interactions:

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).