May 15, 2008

-Selenium

Summary

Selenium is a trace mineral found in soil, water and some foods. It is required for the body to function normally, but only in small amounts. Selenium boosts the performance of enzymes, which help the body carry out many chemical reactions that are crucial for brain and body functions.

Selenium appears to provide many health benefits to the body. It may help prevent the development of illnesses such as heart disease and cancer, aids in regulating the function of the thyroid gland, and helps strengthen the immune system.

Several foods are naturally rich in selenium. The best sources of selenium include dairy products, meats (especially organ meats such as liver and kidney), seafood and certain nuts. In addition, selenium is present in grains, seeds, fruits and vegetables grown in selenium-rich soils.

Selenium deficiency is rare in the United States and Canada. However, when selenium deficiency does occur, it can lead to heart disease, hypothyroidism (low production of thyroid hormone) and a weakened immune system. In particular, selenium deficiency can lead to Keshan disease, in which the heart becomes enlarged and fibrous tissue replaces the muscle tissue in the middle layer of the heart’s walls.

Experts advise adults not to consume more than 400 micrograms of selenium in a single day. Long term consumption of excessive selenium can lead to a condition called selenosis and symptoms such as fatigue, garlic odor on the breath, and loss and brittleness of hair and nails.

Experts generally urge people to get selenium from dietary sources instead of taking selenium supplements. However, in some cases (such as patients receiving total parenteral nutrition (TPN) for extended periods of time) supplements may be required.

About selenium

Selenium is an essential mineral that is required only in small amounts in the body. This mineral is a key component of enzymes, which help the body carry out many chemical reactions that are crucial to brain and body functions.


Selenium is incorporated into proteins that make enzymes known as selenoproteins. These crucial antioxidant enzymes help prevent free radicals (byproducts of oxygen metabolism) from damaging cells. Free radicals contribute to the development of illnesses such as heart disease and cancer. Selenoproteins also help regulate the function of the thyroid gland and help strengthen the immune system.

Soil, water and some foods provide the most abundant sources of selenium. In addition, selenium is found in many foods that do not naturally contain high levels of selenium, but that obtain this mineral because they are grown in regions of the world with selenium-rich soil.

Many areas of the United States (especially the high plains of Nebraska and the Dakotas) and Canada have selenium-rich soil. People who do not live in these areas still are likely to have access to food rich in selenium that was grown in and shipped from selenium-rich areas.

People in areas of the world without access to foods grown in high-selenium soil are more likely to have low levels of selenium. Parts of China and Russia are examples of where this occurs.

The Food and Nutrition Board of the National Academy of Sciences has issued the following recommended dietary allowances (RDAs) for selenium. The figures are expressed in micrograms (mcg):

Gender and Age

Amount of Selenium

Males/females 0 to 6 months

15

Males/females 7 months to 3 years

20

Males/females 4 to 8 years

30

Males/females 9 to 13 years

40

Males/females 14 and older

55

Pregnant females

60

Lactating females

70



Health impact of selenium

Selenium appears to provide many health benefits to the body. It may help prevent the development of illnesses such as heart disease and cancer, aids in regulating the function of the thyroid gland and helps strengthen the immune system.

Some scientists believe that selenium may work along with vitamin E to provide antioxidant protection from heart disease. Others believe selenium prevents viruses from attacking the heart muscle. However, it is important to note that one recent study analyzing the results of a 13-year clinical trial concluded that selenium supplements do not reduce the risk of heart disease.

Studies also suggest that adequate levels of selenium can protect the body against certain forms of cancer, including those of the bladder, breast, colon, lung, prostate and rectum. Rates of nonmelanoma skin cancers also appear to be much lower in U.S. regions with high levels of selenium in the soil. The National Cancer Institute presently is conducting a long-term study to see if taking supplements of selenium, vitamin E or a combination of the two may help prevent prostate cancer.

One recent study also found early evidence that taking selenium substances may slow the progression of HIV, the virus that causes AIDS. People who are diagnosed as HIV-positive often experience reduced levels of selenium in their bodies. Reduced selenium levels also have been reported in people with rheumatoid arthritis, a chronic disorder that causes pain and swelling of the joints.

Selenium also aids the body in cell growth and may help stimulate antibody production when a person receives a vaccine. It may protect the body against the toxic effects of substances such as heavy metals and may help protein synthesis in the body. Finally, selenium may boost fertility, particularly in men.

Selenium deficiency is rare in the United States and Canada. In fact, average intakes of selenium in these countries actually exceed amounts recommended by experts. When selenium deficiency does occur, it can lead to heart disease, hypothyroidism and a weakened immune system. However, evidence suggests that these illnesses typically are not caused by selenium deficiency itself. Instead, such deficiency makes the body more vulnerable to other nutritional, biochemical or infectious stresses.

People with digestive disorders who require total parenteral nutrition (or TPN, in which nutrition is obtained intravenously) may be at increased risk of selenium deficiency. However, this is easily solved by adding selenium to the solution used during TPN.

Digestive disorders themselves also can decrease the absorption of selenium. Physicians treating patients with these conditions (e.g., Crohn’s disease or surgical removal of part of the stomach) typically monitor selenium levels (along with levels of other nutrients) to make sure they stay in balance.

People with low body stores of selenium are at increased risk for Keshan disease, in which the heart becomes enlarged and fibrous tissue replaces the muscle tissue in the middle layer of the heart’s walls. Keshan disease also causes rapid heartbeat. It is a rare disease that most commonly occurs in areas of China with low selenium levels in the soil. In severe cases, patients may experience heart failure, with young children and women of childbearing age at greatest risk. Other rare diseases associated with selenium deficiency are the bone disease Kashin-Beck disease and myxedematous endemic cretinism, which causes mental retardation.

Experts advise people not to consume more than 400 micrograms of selenium in a single day. Eating large amounts of selenium for a long period of time can lead to a condition called selenosis and symptoms including:

  • Fatigue
  • Garlic odor on the breath
  • Irritability
  • Loss and brittleness of hair and nails
  • Mottled teeth
  • Nausea/gastrointestinal upsets
  • Nervous system disorders
  • Skin rash and inflammation
Selenosis is a rare condition in the United States, and most of the cases that occur are due to accidental industrial exposure (such as in metal industries, painting and certain trades).


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