What Is It?
Health Benefits

Recommended Intake
If You Get Too Little
If You Get Too Much
General Dosage Information
Guidelines for Use

General Interaction

Evidence Based Rating Scale

What Is It?

The trace Mineral selenium makes its way into our bodies because it is contained in certain foods. Over time, it becomes part of nearly every cell, with particularly high concentrations in the kidneys, liver, pancreas, spleen, and testes.

The most concentrated food source for selenium is the Brazil nut; a single one contains 120 mcg, (which is about twice the RDA). Other good sources of selenium include seafood, poultry, meat, and grains (especially oats and brown rice). The concentration of selenium in all these food sources depends on a variable that's nearly impossible for the consumer to determine: the level of selenium in the soil in which the plant grew (and which the animal then ate).

Only over the past two decades scientists have begun to understand just what a vital role selenium plays in numerous biological functions. Perhaps its most crucial job is to prevent disease.

Health Benefits

Selenium has many tasks to perform in the body. It helps to boost the immune system and fight off infection, providing a general increase in the body's defense against dangerous bacteria, viruses, and cancer cells. On a basic metabolic level, every cell in the body needs a particular Hormone from the thyroid gland that selenium helps to convert to an active form.

Perhaps the most recognized use of selenium in supplement form is as an Antioxidant; it helps to mop up dangerous molecules known as Free radicals that can damage and alter healthy cells. It has also been recommended for staving off the effects of aging. In many cases there's relatively weak evidence for these uses, but not all.

Specifically, selenium may help to:

Prevent cancer. Test-tube studies indicate that in addition to fostering healthy cell growth and division, selenium discourages the formation of tumors. When researchers at Cornell University and at the University of Arizona pooled results from 5-year studies designed to assess the effects of selenium supplements (200 mcg daily), they came up with some startling findings: Compared with the rest of the population, participants had 63% fewer prostate cancers, 58% fewer colorectal tumors, and 46% fewer lung cancers. Overall, their death rate from cancer was 39% lower than the average. (1, 2) However, in the 2004 SU.VI.MAX study of more than 13,000 French men and women, taking a lower dose of selenium (100 mcg) along with 20 mg of zinc, 120 mg of vitamin C, 30 mg of vitamin E, and 6 mg of beta-carotene daily for more than seven years did not significantly lower the overall risk of cancer in either men or women. The combination did, however, seem to reduce overall risk of cancer and mortality rates in a subgroup of men with low baseline status of certain antioxidants, suggesting supplementation may be more effective in men with these low levels. (3)

The most evidence supporting selenium supplementation to prevent cancer is associated with prostate cancer. In men with deficiencies or low baseline levels, increasing dietary intake of selenium seems to reduce the risk of prostate cancer. (2, 4-6) However, in men with normal baseline levels, a large clinical trial, the Selenium and Vitamin E Cancer Prevention Trial (SELECT) reported in 2009 found that taking 200 mcg of selenium daily for about five and a half years did not significantly reduce prostate cancer risk compared to placebo. (7) Earlier studies had found that in people already diagnosed with cancer, compared to those whose selenium levels were highest, those with the lowest selenium levels developed more subsequent tumors, had a greater likelihood of cancer recurrence, were at higher risk of having the cancer spread to new organs, and generally survived for shorter periods of time. (8, 9)

Protect against heart attack and stroke. Selenium may decrease the "stickiness" of the blood, decreasing its tendency to clot and thus reducing the risk of heart attack and stroke. In addition, the mineral may encourage healthy heart function by increasing the proportion of HDL ("good") cholesterol to LDL ("bad") cholesterol. Increased selenium levels have been associated with a reduction in total cholesterol and LDL (bad) cholesterol in post-menopausal women. (30) People who have already had a heart attack or a stroke, or who smoke, appear to benefit the most from selenium. A five-year study of more than 29,500 adults in China – where residents had chronically low intakes of several nutrients and high rates of esophageal cancer and stroke – found that taking 50 mcg of selenium-enriched yeast in combination with 15 mg of beta-carotene and 60 mg of vitamin E daily may help to decrease the incidence of death from cancer and stroke, as well as total mortality. (10) However, administering selenium intravenously during stroke does not seem to affect mortality rates. (11) More research is needed.  

Guard against cataracts and macular degeneration. Selenium may help prevent the two most common causes of impaired vision and blindness in older Americans--cataracts and macular degeneration--by providing antioxidant actions that fight free radicals. (12) It's these free radicals, after all, that often damage the eye's lens (the site of cataracts) and the macula at the center of the eye's retina (the site of macular degeneration). Preliminary laboratory and animal studies have shown a deficiency in selenium may lead to the formation of cataracts. (13-15) A review of studies evaluating antioxidant properties of minerals such as selenium found these nutrients, particularly from food sources, have important roles in preventing cataracts and macular degeneration. (16) Conflicting evidence, however, was presented in another review, which indicated that low selenium intake resulting from inadequate dietary intake does not lead to selenium deficiency in the eye. The review concludes there is a lack of evidence for the theory that selenium has a preventive role against cataracts. (17) Complicating the issue further is that an overdose of selenium can actually cause cataracts to develop. Studies are underway to clarify the role of selenium in cataract prevention, treatment, or cause. (18) 

Promote healing of cold sores and shingles. When the herpes virus erupts from a dormant state in the body, painful cold sores and shingles may appear. Selenium, an immune-system booster, may help suppress this kind of eruption. Interestingly, findings published in Agriculture Research indicate that mice with low levels of selenium or Vitamin E were particularly prone to herpes virus outbreaks. (19) A later study found the degree of selenium deficiency was directly correlated to the severity in the number and size of viral lesions in mice. (20) Further studies are needed to confirm or refute efficacy in humans.  

Fight Inflammation associated with lupus. Selenium's antioxidant and anti-inflammatory actions may be enhanced when combined with vitamin E. For persons with lupus, an inflammatory autoimmune disease, this nutrient duo may foster healing of the skin and help to protect the heart, blood vessels, skin, joints, and other parts of the body prone to inflammation. Along the same lines, the selenium/vitamin E combination may benefit people with other types of inflammatory disorders too, such as psoriasis, eczema, and rheumatoid arthritis. (21) One early study found selenium supplementation led to significant improvement in the survival time of mice with lupus – 55 weeks in treated mice compared to 36 weeks in the control group. Mice receiving selenium supplementation also showed higher natural killer cell activity than controls. (22) Research has been limited since these early, preliminary studies. More is needed to evaluate the role of selenium supplementation in humans with lupus. 

Improve sperm quality in infertile men. When taken in conjunction with vitamin E, the antioxidant effects of selenium may help to inhibit free radical damage to the sperm membrane, thus boosting sperm quality in men with infertility. Selenium also is required for testosterone biosynthesis; thus normalizing selenium levels may help to boost fertility in some cases. In one study, taking 400 IU of vitamin E and 225 mcg of selenium daily significantly improved sperm quality in infertile men. (23) However, while taking vitamin E plus selenium seemed to improve sperm function, it did not improve fertilization rates. (24) Other trials have shown no significant effects of antioxidant supplementation in improving sperm quality, leading some researchers to believe antioxidant supplementation may only be beneficial when oxidative stress is present. (25-27) Larger trials are needed to confirm or refute efficacy and who may most benefit from these treatments. Evidence of selenium's potential effect in boosting testosterone levels may also be useful in treating andropause, the natural decline of hormone levels in men as they age. See the WholeHealthMD entry for andropause for more information. 

Treat acne. The anti-inflammatory and free-radical scavenging properties of selenium may be useful in protecting acne-prone skin. Taking selenium in conjunction with vitamin E may help to replenish glutathione peroxidase, a valuable antioxidant often deficient in people with acne. One study of 42 men and 47 women with moderate or severe acne found significantly lower levels of glutathione peroxidase and improvement of acne after treatment with selenium and vitamin E. (28) Confirmatory studies are needed.  

Alleviate symptoms of asthma. Researchers have found that asthmatics have low levels of selenium, and that supplementation may help those patients to breathe easier and improve their quality of life. (29) But a recent randomized, double-blind, placebo-controlled study found selenium supplementation alone did not seem to improve symptoms in adults with asthma. (31) 

Regulate hypothyroidism. Selenium is critical to the function of the enzyme needed to convert the thyroid hormone thyroxine (T4) to the more active triiodothyronine (T3) in the liver affecting the amount of the more potent thyroid hormone that is available to the cells. When too little thyroid hormone is available, the condition is called hypothyroidism, which results in a host of mental and physical symptoms. A deficiency in selenium may result in apparent hypothyroidism even when the thyroid gland is functioning normally. Assuring adequate intake is also especially important for hypothyroid patients reliant on T4 supplementation. (32) 

Selenium also may be useful in delaying signs of aging and in treating hair problems by helping to promote hair growth. For more information about treating these conditions, see the WholeHealthMD library entries for aging and hair problems. 

Recommended Intake

The RDA for selenium is 55 mcg for women and 70 mcg for men. Most adults readily obtain this amount through their regular diet, and it's rare that people living in an industrialized country become deficient in this important trace mineral.

If You Get Too Little

Symptoms of selenium deficiency include muscle weakness and fatigue. Poor selenium intake over time may even increase the risk for cancer, immune-system problems, heart disease, and various inflammatory conditions (especially skin-related ones).

If You Get Too Much

While many foods are rich in selenium--it's found in grain, meats, seafood, poultry, and especially brazil nuts--you're unlikely to ingest too much of this trace mineral from your diet.

Taking selenium in concentrated supplement form is a little different, however. Toxic reactions are a risk if you get an overdose. In one case, people who mistakenly took nearly 500 times the RDA for selenium--from bottles of 30 mg selenium pills that were erroneously labeled as 200 mcg pills--suffered hair loss, nausea, vomiting, and fingernail changes. Other signs of potential toxicity include depression, anxiety, and a garlicky odor emanating from the breath and perspiration.

General Dosage Information

  • For acne: Take 200 mcg a day.

  • For diminishing the effects of aging and protecting against cancer: Take 200 mcg a day.

  • For asthma: Take 200 mcg a day.

  • For guarding against cataracts and macular degeneration: Take 200 mcg a day. 

  • For infertility in men: Take 200 mcg a day. 

  • For hypothyroidism: Take 200 mcg a day. 

  • For cold sores and shingles: During flare-ups, take 600 mcg a day. Don't take more than this amount, and do not continue for more than a few days. 

Be sure to check out our Dosage Recommendations Chart for Selenium, which lists therapeutic dosages for specific ailments at a glance. 

Guidelines for Use

Selenium may be most effective when taken in combination with at least 400 IU vitamin E daily.

For general antioxidant benefits, consider a high-potency antioxidant blend containing selenium along with vitamins C and E, beta-carotene, grape seed and green tea extracts, alpha-lipoic acid, coenzyme Q10, and N-acetylcysteine (NAC). 

General Interaction

There are no known drug or nutrient interactions associated with selenium.


Because of the risk of toxic levels building up in your system, avoid taking high doses--900 mcg or more at one time, or 600 mcg daily for an extended period of time. Be sure to also take into account the amount of selenium you're getting from foods.


1. Clark LC, Combs GF Jr, Turnbull BW, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin: A randomized controlled trial. Nutritional Prevention of Cancer Study Group. JAMA. 1996 Dec 25;276(24):1957-63.
2. Clark LC, Dalkin B, Krongrad A, et al. Decreased incidence of prostate cancer with selenium supplementation: results of a double-blind cancer prevention trial. Br J Urol. 1998 May;81(5):730-4.
3. Hercberg S, Galan P, Preziosi P, et al. The SU.VI.MAX Study: a randomized, placebo-controlled trial of the health effects of antioxidant vitamins and minerals. Arch Intern Med. 2004 Nov 22;164(21):2335-42.
4. Yoshizawa K, Willett WC, Morris SJ, et al. Study of prediagnostic selenium level in toenails and the risk of advanced prostate cancer. J Natl Cancer Inst. 1998;90:1219-24.
5. Nomura AM, Lee J, Stemmermann GN, Combs GF. Serum selenium and subsequent risk of prostate cancer. Cancer Epidemiol Biomarkers Prev. 2000;9:883-7.
6. Brooks JD, Metter EJ, Chan DW, et al. Plasma selenium level before diagnosis and the risk of prostate cancer development. J Urol. 2001;166:2034-8.
7. Lippmann SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the selenium and vitamin E cancer prevention trial (SELECT). JAMA. 2009;301:39-51.
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9. Duffield-Lilico AJ, Reid ME, Turnbull BW, et al. Baseline characteristics and the effect of selenium supplementation on cancer incidence in a randomized clinical trial: a summary report of the Nutritional Prevention of Cancer Trial. Cancer Epidemiol Biomarkers Prev. 2002 Jul;11(7):630-9.
10. Mark SD, Wang W, Fraumeni JF Jr, et al. Do nutritional supplements lower the risk of stroke or hypertension? Epidemiology. 1998;9:9-15.
11. Avenell A, Noble DW, Barr J, Engelhardt T. Selenium supplementation for critically ill adults. Cochrane Database Syst Rev. 2004;(4):CD003703.
12. Stewart MS, Spallholz JE, Neldner KH, Pence, BC. Selenium compounds have disparate abilities to impose oxidative stress and induce apoptosis. Free Radic Biol Med. 1999 Jan;26(1-2):42-8.
13. Cai QY, Chen XS, Zhu LZ, et al. Biochemical and morphological changes in the lenses of selenium and/or vitamin E deficient rats. Biomed Environ Sci. 1994 Jun;7(2):109-15.
14. Karaküçük S, Ertugrul Mirza G, Faruk Ekinciler O, et al. Selenium concentrations in serum, lens and aqueous humour of patients with senile cataract. Acta Opthalmol Scand. 1995 Aug;73(4):329-32.
15. Naziroglu M, Karaoglu A, Aksoy AO. Selenium and high dose vitamin E administration protects cisplatin-induced oxidative damage to renal, liver and lens tissues in rats. Toxicology. 2004 Feb 15;195(2-3):221-30.
16. McDermott JH. Antioxidant nutrients: current dietary recommendations and research update. J Am Pharm Assoc (Wash). 2000 Nov-Dec;40(6):785-99.
17. Flohé L. Selenium, seleoproteins and vision. Dev Ophthalmol. 2005;38:89-102.
18. Dawczynski J, Winnefeld K, Königsdörffer E, et al. [Selenium and cataract – risk factor or useful dietary supplement?] Klin Monatsbl Auenheilkd. 2006 Aug;223(8):675-80.
19. Levander OA, Beck MA, McBride J, et al. Nutrient Deficiency Unleashes Jekyll-Hyde Virus. Agric Res. August 1994;14.
20. Gómez RM, Berría MI, Levander OA. Host selenium status selectively influences susceptibility to experimental viral myocarditis. Biol Trace Elem Res. 2001 Apr;80(1):23-31.
21. Juhlin L, Edqvist LE, Ekman LG, et al. Blood glutathione-peroxidase levels in skin diseases: effect of selenium and vitamin E treatment. Acta Derm Venereol. 1982;62(3):211-4.
22. O'Dell JR, McGivern JP, Kay HD, Klassen LW. Improved survival in murine lupus as the result of selenium supplementation. Clin Exp Immunol. 1988 Aug;73(2):322-7.
23. Gupta NP, Kumar R. Lycopene therapy in idiopathic male infertility – a preliminary report. Int Urol Nephrol. 2002;34:369-72.
24. Vezina D, Mauffette F, Roberts KD, et al. Selenium-vitamin E supplementation in infertile men. Effects on semen parameters and micronutrient levels and distribution. Biol Trace Elem Res. 1996;53:65-83.
25. Rolf C, Cooper TG, Yeung CH, Nieschlag E. Antioxidant treatment of patients with asthenozoospermia or moderate oligoasthenozoospermia with high-dose vitamin C and vitamin E: a randomized, placebo-controlled, double-blind study. Hum Reprod. 1999;14:1028-33.
26. Agarwal A, Nallella KP, Allamaneni SS, et al. Role of antioxidants in treatment of male infertility: an overview of the literature. Reprod Biomed Online. 2004;8:616-27.
27. Pizzorno JE, Murray MT. Textbook of Natural Medicine. Third Edition. "Male Infertility." 2006;1871.
28. Michaelsson G, Edqvist L. Erythrocyte glutathione peroxidase treatment. Acta Derm Venerol. 1984;64:9-14.
29. Kadrabova J, Maderic A, Kovacikova Z, et al. Selenium status is decreased in patients with intrinsic asthma. Biol Trace Elem Res. 1996;52:241-8.
30. Karita K, Yamanouchi Y, Takano T, Oku J, Kisaki T, Yano E. Associations of blood selenium and serum lipid levels in Japanese premenopausal and postmenopausal women. Menopause. 2008 Jan-Feb;15(1):119-24.
31. Shaheen SO, Newson RB, Rayman MP, et al. Randomised, double-blind, placebo-controlled trial of selenium supplementation in adult asthma. Thorax. 2007 Jun;62(6):483-90.
32. Gartner R, Gasnier BC, Dietrich JW, et al. Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. J clin Endocrinol Metab. 2002;87:1687-91.

Evidence Based Rating Scale

The Evidence Based Rating Scale is a tool that helps consumers translate the findings of medical research studies and what our clinical advisors have found to be efficacious in their personal practice into a visual and easy to interpret format. This tool is meant to simplify the information on supplements and therapies that demonstrate promise in the treatment of certain conditions.




Date Published: 04/19/2005
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