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Selenium
What are some current issues and controversies about selenium?
Selenium and cancer
Observational studies indicate that death from cancer, including lung, colorectal, and prostate cancers, is lower among people with higher blood levels or intake of selenium [34-40]. In addition, the incidence of nonmelanoma skin cancer is significantly higher in areas of the United States with low soil selenium content [37]. The effect of selenium supplementation on the recurrence of different types of skin cancers was studied in seven dermatology clinics in the U.S. from 1983 through the early 1990s. Taking a daily supplement containing 200 μg of selenium did not affect recurrence of skin cancer, but significantly reduced the occurrence and death from total cancers. The incidence of prostate cancer, colorectal cancer, and lung cancer was notably lower in the group given selenium supplements [41].
Research suggests that selenium affects cancer risk in two ways. As an anti-oxidant, selenium can help protect the body from damaging effects of free radicals. Selenium may also prevent or slow tumor growth. Certain breakdown products of selenium are believed to prevent tumor growth by enhancing immune cell activity and suppressing development of blood vessels to the tumor [42].
However, not all studies have shown a relationship between selenium status and cancer. In 1982, over 60,000 participants of the Nurse's Health Study with no history of cancer submitted toenail clippings for selenium analysis. Toenails are thought to reflect selenium status over the previous year. After three and a half years of data collection, researchers compared toenail selenium levels of nurses with and without cancer. Those nurses with higher levels of selenium in their toenails did not have a reduced risk of cancer [43].
Two important long-term studies, the SU.VI.MAX study in France and the Selenium and Vitamin E Cancer Prevention Trial (SELECT) study in the U.S., are now underway to further investigate the selenium/cancer prevention link.
The SU.VI.MAX Study is a prevention trial looking at the effects of antioxidant vitamins and minerals on chronic diseases such as cancer and cardiovascular disease. Doses of the nutrients provided in the study are one to three times higher than recommended intakes, including a daily supplement of 100 μg selenium. The SU.VI.MAX study, which began in 1994, has followed more than 12,000 adult men and women. This study was designed to continue for eight years, and the research community is eagerly awaiting the results of this study [44].
The SELECT study, a long-term study sponsored by the NIH, is investigating whether supplemental selenium and/or vitamin E can decrease the risk of prostate cancer in healthy men. Past evidence as well as pre-clinical trials for the SELECT study suggests that these two nutrients may be effective in preventing prostate cancer. A daily supplement containing 200 μg of selenium will be given to individuals in the selenium-only study group, while men in the combined-nutrients group will receive a daily supplement containing 200 μg selenium and 400 mg vitamin E. The study, which will span from 2001 to 2013, will include 32,400 healthy adult men [45].
Selenium and heart disease
Some population surveys have suggested an association between lower antioxidant intake and a greater incidence of heart disease [46]. Evidence also suggests that oxidative stress from free radicals, which are natural by-products of oxygen metabolism, may promote heart disease [47-49]. For example, it is the oxidized form of low-density lipoproteins (LDL, often called "bad" cholesterol) that promotes plaque build-up in coronary arteries [48]. Selenium is one of a group of antioxidants that may help limit the oxidation of LDL cholesterol and thereby help to prevent coronary artery disease [47-49]. Currently there is insufficient evidence available to recommend selenium supplements for the prevention of coronary heart disease; however, the SU.VI.MAX study mentioned earlier is looking at the effects of antioxidant nutrients such as selenium on heart disease.
Selenium and arthritis
Surveys indicate that individuals with rheumatoid arthritis, a chronic disease that causes pain, stiffness, swelling, and loss of function in joints, have reduced selenium levels in their blood [50-51]. In addition, some individuals with arthritis have a low selenium intake [52].
The body's immune system naturally makes free radicals that can help destroy invading organisms and damaged tissue, but that can also harm healthy tissue [53]. Selenium, as an antioxidant, may help to relieve symptoms of arthritis by controlling levels of free radicals [54]. Current findings are considered preliminary, and further research is needed before selenium supplements can be recommended for individuals with arthritis.
Selenium and HIV
HIV/AIDS malabsorption can deplete levels of many nutrients, including selenium. Selenium deficiency is associated with decreased immune cell counts, increased disease progression, and high risk of death in the HIV/AIDS population [55,56]. HIV/AIDS gradually destroys the immune system, and oxidative stress may contribute to further damage of immune cells. Antioxidant nutrients such as selenium help protect cells from oxidative stress, thus potentially slowing progression of the disease [57]. Selenium also may be needed for the replication of the HIV virus, which could further deplete levels of selenium [58].
An examination of 125 HIV-positive men and women linked selenium deficiency with a higher rate of death from HIV [59]. In a small study of 24 children with HIV who were observed for five years, those with low selenium levels died at a younger age, which may indicate faster disease progression [60]. Results of research studies have led experts to suggest that selenium status may be a significant predictor of survival for those infected with HIV [61].
Researchers continue to investigate the relationship between selenium and HIV/AIDS, including the effect of selenium levels on disease progression and mortality. There is insufficient evidence to routinely recommend selenium supplements for individuals with HIV/AIDS, but physicians may prescribe such supplements as part of an overall treatment plan. It is also important for HIV-positive individuals to consume recommended amounts of selenium in their diet.
What is the health risk of too much selenium?
High blood levels of selenium (greater than 100 μg/dL) can result in a condition called selenosis [62]. Symptoms of selenosis include gastrointestinal upsets, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage [2].
Selenium toxicity is rare in the U.S. The few reported cases have been associated with industrial accidents and a manufacturing error that led to an excessively high dose of selenium in a supplement [63,64]. The Institute of Medicine of the National Academy of Sciences has set a tolerable upper intake level (UL) for selenium at 400 micrograms per day for adults to prevent the risk of developing selenosis [12]. Table 4 lists ULs for selenium, in micrograms per day, for infants, children, and adults.
| Age | Males and Females (μg/day) |
|---|---|
| 0 - 6 months | 45 |
| 7 - 12 months | 60 |
| 1-3 y | 90 |
| 4-8 y | 150 |
| 9-13 y | 280 |
| 14-18 y | 400 |
| 19 y + | 400 |
Selecting a healthful diet
The 2000 Dietary Guidelines for Americans states, "Different foods contain different nutrients and other healthful substances. No single food can supply all the nutrients in the amounts you need". For more information about building a healthful diet, refer to the Dietary Guidelines for Americans ttp://www.usda.gov/cnpp/DietGd.pdf and the Food Guide Pyramid http://www.nal.usda.gov/fnic/Fpyr/pyramid.html.What is the recommended dietary intake for selenium?
