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

Selenium plays a role in thyroid function. Your immune system also needs selenium to work properly. People with conditions ranging from rheumatoid arthritis (RA) to certain types of cancer often have low levels of selenium. However, in most cases scientists are not sure whether low selenium levels are a cause or an effect of the disease.

When researchers examined whether selenium had any effect on skin cancer, they found that people who took as little as 200 mcg of selenium per day for more than 7 years had a significantly higher risk of developing type 2 diabetes. More studies are needed to understand the meaning behind these findings. 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 probably get enough selenium. You may have low levels of selenium if you:

• Smoke cigarettes
• Drink alcohol
• Take birth control pills
• Have a condition that prevents your body from absorbing enough selenium, such as Crohn 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 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. Studies show that taking selenium supplements does not seem to have any effect on the progression of heart disease, nor does it protect against heart attack.

Selenium, combined 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, and make them less effective. 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 associated with a higher risk of cancer death. Scientists are not sure how selenium affects this risk. But they have observed that people who live in parts of the world where the soil is rich in selenium have lower rates of cancer, possibly because there is more selenium in food. In addition, people who have cancer often have low levels of selenium.

For most types of cancer, selenium does not appear to have much effect. One early study looking at whether selenium reduced the risk of skin cancer found that, although it did not seem to affect skin cancer risk, people who took selenium lowered their risk of death from cancer overall. However, later studies found that selenium does not seem to lower the risk of lung or esophageal cancer. Evidence is mixed on whether it protects against colorectal cancer. Two studies suggest that 200 mcg per day might help protect against colorectal cancer, but other studies do not show any benefit.

The best evidence suggests that if you have low levels of selenium, getting more selenium in your diet may lower your risk of prostate cancer. Taking a special kind of selenium-rich brewer's yeast, called selenized yeast, may also help. In one study of 1,312 people, those who took 200 mcg of selenium as selenized yeast per day (compared to those who took a placebo) had two-thirds lower risk of developing prostate cancer during the 4.5-year study. However, the men who got the benefit had low levels of selenium to start with.

If you are at risk for prostate cancer, talk to your doctor before taking selenium.

The situation is even more complicated when it comes to skin cancer. Some early evidence led scientists to investigate whether selenium might protect against skin cancer. But a large trial found that taking daily selenium supplements actually increased the risk of squamous cell carcinoma and non-melanoma skin cancer.

Immune function

Many studies suggest that the body needs selenium for the immune system to work properly. Selenium, along with other minerals, can help boost white blood cells, which improves the body's ability to fight illness and infection. A few studies suggest that selenium might help prevent some infections, such as a bacterial skin infection that often occurs with lymphedema, and mycoplasma pneumonia. In addition, one study suggested that when elderly people took zinc and selenium supplements, their immune systems responded better to the flu vaccine than those who took 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 took selenium supplements for 14 weeks had fewer symptoms compared to those who took placebo. But in a larger randomized, double-blind study, people who took a yeast supplement that contained selenium didn't have fewer symptoms than those who took placebo. More studies are needed.

HIV/AIDS

The evidence concerning selenium and HIV/AIDS is contradictory. Studies have shown that levels of selenium go down consistently as HIV progresses. In one study, those taking a particular selenium supplement called 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 how your body makes 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 may negatively impact sperm's ability to swim.

Rheumatoid arthritis (RA)

Low levels of selenium in the blood may be associated with increased risk of RA. But it does not seem that selenium supplements help once you have RA.

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

Selenium levels in food depend on how much selenium was in the soil where the food was grown.

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 a separate supplement. Most supplements contain a form of selenium called selenomethionine.

The minimum daily recommended dietary allowances (RDA) for selenium are as follows:

Pediatric

• Children, 1 to 3 years: 20 mcg
• Children, 4 to 8 years: 30 mcg
• Children, 9 to 13: 40 mcg
• Children, 14 to 18: 55 mcg

Adult

• 19 and older: 55 mcg
• Pregnant women: 60 mcg
• Breastfeeding women: 70 mcg

As a supplement

Some studies have used 200 mcg per day for some conditions. But evidence suggests that taking that amount over a long period of time could 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 some studies have used doses of 200 mcg, there is some evidence that this amount could increase your risk for diabetes. DO NOT exceed the upper tolerable limit of 400 mcg. Remember that you may also get some selenium in the foods you eat.

Over time, high doses of selenium can be toxic. Side effects include:

• Fingernail loss
• Skin rash
• Fatigue
• Irritability
• Weight loss

You should not take selenium without talking to your doctor if:

• You have an underactive thyroid (hypothyroidism)
• You have had skin cancer
• You are at risk for skin cancer

High selenium intake may increase the risk of diabetes and high cholesterol.

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

Drugs that may lower levels of selenium include:

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

Anticoagulants and antiplatelet drugs (blood thinners)

When taken 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 certain antioxidants together, including selenium, along with these drugs may make them less effective. In theory, selenium may also reduce the effectiveness of other statins, including atorvastatin (Lipitor), fluvastatin (Lescol), lovastatin (Mevacor), and prevastatin (Pravachol).

Birth control pills

Some researchers propose that women taking birth control pills may have higher levels of selenium in their blood. If you take birth control pills, ask your doctor before taking extra selenium.

Gold salts

These chemical compounds may lower levels of selenium in the body and cause symptoms of selenium deficiency.

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