Selenium : Uses and Health Benefits

by MedlinePlus

Selenium is a trace mineral found in soil, water, and some foods. It is an essential element in several metabolic pathways, including the glutathione-peroxidase pathway. Selenium appears to promote antioxidant activity in the body via glutathione peroxidase (GPX), a selenium-dependent enzyme.

Selenium deficiency can occur in areas were soil content of selenium is low, and may affect thyroid function and cause conditions such as Keshan disease. Selenium deficiency is also commonly seen in patients on total parenteral nutrition (TPN) as their sole source of nutrition. Gastrointestinal disorders may decrease the absorption of selenium resulting in depletion or deficiency. Selenium may be destroyed when foods are refined or processed. For supplementation, selenium as selenomethionine has been reported to have nearly twice the bioavailability of selenium as selenite.

Specific dietary sources of selenium include brewer's yeast, wheat germ, butter, garlic, grains, sunflower seeds, Brazil nuts, walnuts, raisins, liver, kidney, shellfish (lobster, oyster, shrimp, scallops), fresh-water and salt-water fish (red snapper, salmon, swordfish, tuna, mackerel, halibut, flounder, herring, smelts). Selenium is also found in alfalfa, burdock root, catnip, fennel seed, ginseng, raspberry leaf, radish, horseradish, onion, chives, medicinal mushrooms (reishi, shiitake), and yarrow.

The role of selenium in cancer prevention has been the subject of recent study and debate. Initial evidence from the Nutritional Prevention of Cancer (NPC) trial suggests that selenium supplementation reduces the risk of prostate cancer among men with normal baseline PSA (prostate specific antigen) levels, and low selenium blood levels. However, in this study selenium did not reduce the risk of lung, colorectal, or basal cell carcinoma of the skin, and actually increased the risk of squamous cell skin carcinoma. The ongoing Selenium and Vitamin E Cancer Prevention Trial (SELECT) aims to definitively address the role of selenium in prostate cancer prevention.

Synonyms

Atomic number 34, Na2SeO3, selenium dioxide, selenized yeast, L-selenomethionine, Se, Sele-Pak, selenate, selenite, selenious acid, selenium sulfide, selenocysteine, selenomethionine (Semet), selepen, Se-methylselenocysteine (SeMCYS).

Evidence

These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider. Uses based on scientific evidence

Antioxidant

Selenium is a component of glutathione peroxidase, which possesses antioxidantactivity, and demonstrates antioxidant properties in humans. Long-term clinical benefits remain controversial.

Prostate cancer prevention

Initial evidence has suggested that selenium supplementation reduces the risk of developing prostate cancer in men with normal baseline PSA (prostate specific antigen) levels, and low selenium blood levels. This is the subject of large well-designed studies, including the Nutritional Prevention of Cancer Trial (NPC), and the ongoing Selenium and Vitamin E Cancer Prevention Trial (SELECT), as well as prior population and case-control studies. The NPC was conducted in 1312 Americans, and reported that 200mcg of daily selenium reduces the overall incidence of prostate cancer - although these protective effects only occurred in men with baseline PSA levels less than or equal to 4 ng/mL, and those with low baseline blood selenium levels (<123.2 ng/mL). The NPC trial was primarily designed to measure the development of nonmelanoma skin cancers, not other types of cancers, and therefore these prostate cancer results cannot be considered definitive. To settle this question, further study is underway: The SELECT trial is in progress, with a goal to include 32,400 men with serum PSA levels less than or equal to 4ng/mL. SELECT was started in 2001, with results expected in 2013.Laboratory studies have reported several potential mechanisms for selenium's beneficial effects in prostate cancer, including decrease in androgen receptors and PSA production, antioxidant effects, angiogenesis inhibition, or apoptosis. It is not known if selenium is helpful in men who already have been diagnosed with prostate cancer to prevent progression or recurrence of disease. It does appear that selenium may not be beneficial in those with elevated PSA levels, or with normal/high selenium levels. It remains unclear whether men at risk (or all men) should have their serum selenium values measured; results of the SELECT study may provide additional guidance. There is evidence that low selenium levels are associated with an increased risk of prostate cancer, and several mechanisms for the beneficial effects of selenium supplementation have been suggested. In the NPC trial, no benefits were seen in reducing the risk of colorectal or lung cancers. Although an overall reduction in cancer risk was observed, it is not clear which specific types of cancer besides prostate cancer prevention may benefit.

Keshan disease

Keshan disease is a cardiomyopathy (heart disease) restricted to areas of China in people having an extremely low selenium status. Prophylactic administration of sodium selenite has been shown to significantly decrease the incidence of this disorder. For supplementation, selenium as selenomethionine has been reported to have nearly twice the bioavailability of selenium as selenite. Selenium is used to treat and prevent selenium deficiency (for example in those with HIV or receiving enteral feedings).

Asthma

Preliminary research reports that selenium supplementation may help improve asthma symptoms. Further research is needed to confirm these results.

Intracranial pressure symptoms

Preliminary research shows a decrease of symptoms of elevated intracranial pressure (headaches, nausea, emesis, vertigo, unsteady gait, speech disorders, and Jacksonian seizures). More research is needed before a recommendation can be made.

Burns

Early study results suggest that supplementation with selenium and other trace elements (copper, zinc) may increase the rate of burn wound healing. Additional research is necessary before a clear recommendation can be made.

Cancer treatment

Several studies suggest that low levels of selenium (measured in the blood or in tissues such as toenail clippings), may be a risk factor for developing cancer, particularly prostate cancer. Population studies suggest that people with cancer are more likely to have low selenium levels than healthy matched individuals, but in most cases it is not clear if the low selenium levels are a cause or merely a consequence of disease. It remains unclear if selenium is beneficial in the treatment of any type of cancer.

Cardiomyopathy

Low selenium levels have been associated with the development of cardiomyopathy, and selenium supplementation is likely of benefit in such cases (for example in Keshan disease). However, most cases of cardiomyopathy are not due to low selenium levels, and therefore selenium may not be helpful.It has been suggested that low selenium levels may be a risk for coronary heart disease, although this remains unclear.

Cataracts

Preliminary research reports that selenium supplementation may affect the development of cataracts. Further research is needed before a clear conclusion can be drawn.

Chemotherapy side effects

Study results of selenium supplementation during chemotherapy are mixed. General concern has been raised that antioxidants may interfere with radiation therapy or some chemotherapy agents (such as alkylating agents, anthracyclines, or platinums), which themselves can depend on oxidative damage to tumor cells for anti-cancer activity. Therefore, patients undergoing cancer treatment should speak with their oncologist before taking selenium.

Cystic fibrosis

Preliminary research of selenium supplementation in CF patients yields indeterminate results. Further research is needed in this area before a conclusion can be drawn.

Dandruff

Studies report that selenium-containing shampoos may help improve dandruff, and selenium is included in some commercially available products.

Dialysis

The benefits of selenium supplementation in dialysis patients remain unclear. Some methods of dialysis may lower plasma selenium levels.

Fatigue

Evidence of benefit is inconclusive in this area.

Malabsorption

Low selenium status has been demonstrated in several malabsorptive syndromes and in some digestive and gastrointestinal allergic conditions. There is some evidence that children with food allergies have a higher risk of selenium deficiency. There is no clear benefit of selenium supplementation as a therapy for malabsorptive syndromes, although vitamin supplementation in general may be warranted.

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