Selenium is a trace element that is naturally present in many foods, added to others and available as a dietary supplement. It is nutritionally essential for humans.
Selenium is a mineral in the soil. It appears in water and some foods. People only need a very small amount, however it is important in metabolism. Selenium has attracted attention because of its antioxidant properties. Antioxidants protect cells from damage. Among healthy people in the U.S., selenium deficiencies are uncommon. But some conditions – such as HIV, Crohn’s disease and others, are associated with low selenium levels. People who are fed intravenously are also at risk for low selenium.
Selenium is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for reduction of antioxidant enzymes, such as glutathione peroxides and certain forms of thioredoxin reductase found in some animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium.)
Selenium also is important in the functioning of the thyroid gland and in every cell that uses thyroid hormone, by participating as a cofactor for the three of the four known types of thyroid hormone deiodinases which activate and then deactivate various thyroid hormones and their metabolites.
Four conditions in which Selenium may play a role:
Cancer – Because of its effects on DNA repair, apoptosis and the endocrine and immune systems, as well as other mechanisms, including its antioxidant properties, selenium may play a role in the prevention of cancer.
Cardiovascular disease - It may help in preventing platelets from aggregating.
Cognitive decline – Serum selenium concentrations decline with age and this decline may be associated with declines in brain function.
Thyroid disease – Selenium concentration is higher in the thyroid gland than in any other organ in the body and like iodine, selenium has an important function in thyroid hormone synthesis and metabolism.
More research is needed to better understand selenium and these relationships and to determine the role of selenium.
There are interactions between selenium and other nutrients, such as iodine and vitamin E.
Studies have implicated selenium deficiency in several serious or chronic diseases, such as cancer, diabetes, HIV/AIDS, and tuberculosis. In additions, selenium has been found to be a chemo preventive for some types of cancer in some rodents, however, in humans, such results have not been found.
Dietary selenium comes from nuts, cereals, meats, mushrooms, fish and eggs. Brazil nuts are the richest ordinary dietary source, though this is soil dependent. High concentrations of selenium are found in kidney, tuna, crab, and lobster.
Sources of Selenium – Seafoods and organ meats are the richest food sources of selenium. Other sources include muscle meat, cereals and other grains and dairy products. The major food sources in the American diet are breads, grains, meat, poultry, fish and eggs.
The human body’s content of selenium is believed to be in the 13-20 milligram range.
Recommended daily dietary allowances (RDA) for selenium are:
- Children 1-3 years - 20 mcg * 19 years and older – 55 mcg
- Children 4-8 years - 30 mcg * Pregnant women - 60 mcg
- Children 9-13 years – 40 mcg * Breastfeeding women – 70 mcg
- Children 14-18 years – 55 mcg
Selected Food Sources of Selenium:
. (cooked portions) (mcg)/serving
Brazil nuts, I ounce (6-8 nuts) 544
Tuna, 3 ounces 92
Halibut, 3 ounces 47
Sardines, 3 ounces 45
Ham, 3 ounces 42
Shrimp, 3 ounces 40
Macaroni, 1 cup 37
Beef steak, 3 ounces 33
Turkey, 3 ounces 31
Beef Liver, 3 ounces 28
Chicken, 3 ounces 22
Bread, whole wheat, 1 slice 13
Oatmeal, 1 cup 13
Spinach, 1 cup 11
Milk 1% fat, 1 cup 8
Cashew nuts, 1 ounce 3
Banana, 1 cup 2
Carrots, raw, 1 cup 0
Lettuce, raw, 1 cup 0
Most American consume adequate amounts of Selenium.
If you are healthy and eat a well balanced diet, you should get enough selenium.
Selenium deficiency is rare in healthy, well nourished individuals. It can occur in patients with severely comprised intestinal function, those undergoing total parenteral nutrition and in those of advanced age (over 90). Also persons dependent on food grown from selenium-deficient soil are at risk.
Selenium deficiency occurs only when a low selenium status is linked with an additional stress, such as high exposure to mercury or as a result of increased oxidant stress due to vitamin E deficiency.
Taken at normal doses, selenium does not usually have side effects.
Selenium deficiency produces biochemical changes that might predispose people who experience additional stress to develop certain illnesses.
Selenium deficiency is very rare in the United States or Canada. The following groups are among those most likely to have inadequate intakes of selenium:
People living in selenium-deficient regions – persons in countries whose diet consists primarily of vegetables grown in low selenium soils, such as areas in China, and in some areas in Europe among those mostly consuming vegan diets.
People receiving kidney dialysis – Selenium is removed from the blood during treatment and also from anorexia and dietary restrictions.
People living with HIV – Selenium is lower due to inadequate intake and excessive losses due to diarrhea and mal-absorption.
Low levels of selenium may occur if you:
- Smoke cigarettes
- Drink alcohol
- Take birth control pills
- Have a condition that prevents your body from absorbing selenium such as Crohn’s disease or ulcerative colitis.
Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Level of 400 micrograms per day can lead to selenosis.
Health risk from excessive intake of selenium – Early indicators of excessive intake are garlic odor in the breath and a metallic taste in the mouth. The most common clinical signs of high selenium intake, or selenosis, are hair and nail loss or brittleness. Other symptoms include lesions in the skin and nervous system, nausea, diarrhea, skin rashes, mottled teeth, fatigue, irritability and nervous system abnormalities. Acute selenium toxicity can cause severe gastrointestinal and neurological symptoms, acute respiratory syndrome, myocardial infarction, hair loss, muscle tenderness, tremors, sightedness, facial flushing, kidney failure, cardiac failure, cirrhosis of the liver, pulmonary edema, and in rare cases, death.
Although it is toxic in large doses, Increased dietary selenium intakes reduce the effects of mercury toxicity.
Selenium can interact with certain medications and some medications can have an adverse effect on selenium levels.
Selenium may interact with other medicines and supplements such as antacids, chemotherapy drugs, corticosteroids, niacin, cholesterol-lowering statin drugs and birth control pills. Selenium supplements are associated with a risk of skin cancer.
If one is being treated with any of the following medications, it is advised that one should not use selenium supplements without first talking to your health care provider.
- Drugs that may affect (lower) selenium levels in the body, such as Cisplatin ( a chemotherapy drug), Clozapine (Clozaril), Corticosteroids (such as prednisone), Valproic acid (Depakote).
- Anticoagulants and antiplatelet drugs (blood thinners), such as Clopidogrel (Plavix), Warfarin (Coumadin), Heparin, Aspirin.
- Barbiturates – (in animal tests, selenium seems to make the sedative effects of these drugs last longer) Butabarital, Mephabarbital, Phenobarbital, Secobarbital
- Chemotherapy – It is thought that Selenium may interfere with the cancer fighting ability of chemotherapy medicines.
- Cholesterol-lowering medication – Selenium may reduce the effectiveness of these medications such as (Zocor, Lipitor, Lescol, Mevacor and Pravachol).
- Birth control pills – Some researchers think that women taking birth control pills may have higher levels of selenium in their blood. If taking birth control pills, ask your doctor before taking additional selenium.
- Gold salts – may lower levels of selenium in the body and cause symptoms of selenium deficiency.
* NIH – National Institut of Health, ods.od.nih.gov/fact sheets/Selenium-HealthProfessional/
** Selenium – University of Maryland Medical Center – umn.edu/health/medical/altmed/supplement/selenium
**** Wikipedia – Selenium