What does it do? Selenium activates an antioxidant enzyme called glutathione peroxidase, which may help protect the body from cancer. Yeast-derived forms of selenium have induced "apoptosis" (programmed cell death) in cancer cells in test tubes and in animals.^{1 2 3} A double-blind trial that included over 1,300 people found those given 200 mcg of yeast-based selenium per day for 4.5 years had a 50% drop in the cancer death rate compared with the placebo group.⁴ Another study found that men consuming the most dietary selenium (assessed indirectly by measuring toenail selenium levels) developed 65% fewer cases of advanced prostate cancer than did men with the lowest levels of selenium intake.⁵

Selenium is also essential for healthy immune functioning. Selenium supplementation has reduced the incidence of viral hepatitis in selenium-deficient populations, presumably by enhancing immune function.⁶ Even in a non-deficient population of elderly people, selenium supplementation has been found to stimulate the activity of white blood cellsprimary components of the immune system.⁷ Selenium is also needed to activate thyroid hormones.

In a double-blind trial, selenium supplementation of infertile men improved the motility of sperm cells and increased the chance of conception.⁸

Where is it found? Brazil nuts are the best source of selenium. Yeast, whole grains, and seafood are also good sources. Animal studies have found that selenium from yeast is better absorbed than selenium in the form of selenite.⁹

Selenium may be of benefit relative to the following conditions:

Asthma
Atherosclerosis
Colon cancer (reduces risk)
Depression
Dermatitis herpetiformis
Heart attack
HIV support
Immune function (for elderly people)
Infections (to prevent hospital-acquired infections
in very low birth weight infants)
Infertility (male)
Lung cancer (reduces risk)
Pancreatic insufficiency
Phenylketonuria (if deficient)
Prostate cancer (reduces risk)
Rheumatoid arthritis

Cardiac arrhythmia
Cardiomyopathy (only for Keshan's cardiomyopathy)
Childhood diseases
Diabetic retinopathy (in combination with vitamin A, vitamin C and vitamin E)

Down's syndrome
Halitosis (if gum disease)
Hepatitis
Hypothyroidism (if deficient)
Liver cirrhosis
Macular degeneration
Osgood-Schlatter disease
Pap smear (abnormal)
Pre- and post-surgery health
Retinopathy (combined with vitamin A, vitamin C and vitamin E)

Who is likely to be deficient? While most people probably don't take in enough selenium, gross deficiencies are rare in Western countries. Soils in some areas are selenium-deficient and people who eat foods grown primarily on selenium-poor soils are at risk for deficiency. People with AIDS have been reported to be depleted in selenium.¹⁰ Similarly, limited research has reported an association between heart disease and depleted levels of selenium.¹¹ People who are deficient in selenium have an increased risk of developing certain types of rheumatoid arthritis.¹²

How much is usually taken? While the Recommended Dietary Allowance for most adults is 55 mcg per day, an adult intake of 100-200 mcg of selenium per day is recommended by many doctors.

Are there any side effects or interactions? Selenium is safe at the level people typically supplement (100-200 mcg); however, taking more than 900 mcg of selenium per day has been reported to cause adverse effects in some people.¹³ Selenium toxicity can result in loss of fingernails, skin rash, and changes in the nervous system. In the presence of iodine-deficiency-induced goiter, selenium supplementation has been reported to exacerbate low thyroid function.¹⁴ The National Academy of Sciences recommends that selenium intake not exceed 400 mcg per day, unless the higher intake is monitored by a healthcare professional.¹⁵

Selenium enhances the antioxidant effect of vitamin E,.

Links:

"Selenium is an antioxidant which is believed by proponents to protect you from free radical attack. Free radicals are believed to lead to cancer. ... Advocates argue that selenium helps reduce the incidence of breast, lung, colorectal and skin cancers." (Ontario Breast Cancer Information Exchange Project. Guide to unconventional cancer therapies. 1st ed. Toronto: Ontario Breast Cancer Information Exchange Project, 1994:144) at    http://www.bccancer.bc.ca/PPI/UnconventionalTherapies/Selenium.htm

References:
1. Lu J, Pei H, Ip C, et al. Effect on an aqueous extract of selenium-enriched garlic on in vitro markers and in vivo efficacy in cancer prevention. Carcinogenesis 1996;17:1903-7.
2. Wilson AC, Thompson HJ, Schedin PJ, et al. Effect of methylated forms of selenium on cell viability and the induction of DNA strand breakage. Biochem Pharmacol 1992;43:1137-41.
3. Redman C, Xu MJ, Peng YM, et al. Involvement of polyamines in selenomethionine induced apoptosis and mitotic alterations in human tumor cells. Carcinogenesis 1997;18:1195-202.
4. Clark LC, Combs GF, Turnbull BW, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. JAMA 1996;276:1957-63. Published erratum appears in JAMA 1997;277:1520.
5. Yoshizawa K, Willett WC, Morris SJ, et al. Study of prediagnostic selenium levels in toenails and the risk of advanced prostate cancer. J Natl Cancer Inst 1998;90:1219-24.
6. Yu S-Y, Li W-G, Zhu Y-J, et al. Chemoprevention trial of human hepatitis with selenium supplementation in China. Biol Trace Element Res 1989; 20:15 -20.
7. Peretz A, Nve J, Desmedt J, et al. Lymphocyte response is enhanced by supplementation of elderly subjects with selenium-enriched yeast. Am J Clin Nutr 1991;53:1323-8.
8. Scott R, Macpherson A, Yates RWS, et al. The effect of oral selenium supplementation on human sperm motility. Br J Urol 1998;82:76-80.
9. Yoshida M, Fukunaga K, Tsuchita H, Yasumoto K. An evaluation of the bioavailability of selenium in high-selenium yeast. J Nutr Sci Vitaminol 1999;45:119-28.
10. Dworkin BM. Selenium deficiency in HIV infection and the acquired immunodeficiency syndrome (AIDS). Chem Biol Iteract 1994;91:181-6.
11. Moore JA, Noiva R, Wells IC. Selenium concentrations in plasma of patients with arteriographically defined coronary atherosclerosis. Clin Chem 1984;30:1171-3.
12. Knekt P, Heliovaara M, Aho K, et al. Serum selenium, serum alpha-tocopherol, and the risk of rheumatoid arthritis. Epidemiology 2000;11:402-5.
13. Yang GQ, Zhou RH. Further observations on the human maximum safe dietary selenium intake in a seleniferous area of China . J Trace Elem Electrolytes Hlth Dis 1994;8:159-65.
14. Contempre B, Dumont JE, Ngo B, et al. Effects of selenium supplementation in hypothyroid subjects of an iodine and selenium deficient area: The possible danger of indiscriminate supplementation of iodine deficient subjects with selenium. J Clin Endocrinol Metabol
1991;73:213-5.
15. Panel on Dietary Antioxidants and Related Compounds, Food and Nutrition Board, Institute of Medicine , National Academy of Sciences. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. National Academy Press, Washington , D.C. , 2000