Selenium is an essential mineral found in small amounts in the body. It works as an antioxidant, especially when combined with vitamin E. Antioxidants like selenium help fight damaging particles in the body known as free radicals. Free radicals can damage cell membranes and DNA, and may contribute to aging and a number of conditions, including heart disease and cancer. Antioxidants can neutralize free radicals and may reduce or even help prevent some of the damage they cause.
Selenium plays a role in thyroid function and your immune system needs selenium to work properly. People with a number of conditions, ranging from rheumatoid arthritis to some types of cancer, often have low levels of selenium. However, in most cases scientists aren't sure whether low selenium levels are a cause or an effect of the disease.
When researchers examined whether selenium had any effect on skin cancer, they found something unexpected: People who took as little as 200 mcg of selenium per day for more than 7 years had a significantly higher risk of developing type 2 diabetes. More studies are needed to understand exactly what might be happening. In the meantime, you should not take more than the daily recommended allowance of selenium without your doctor's supervision.
If you are healthy and eat a well-balanced diet, you should get enough selenium. You may have low levels of selenium if you:
- Smoke cigarettes
- Drink alcohol
- Take birth control pills
- Have a condition that prevents your body from absorbing enough selenium such as Crohn's disease or ulcerative colitis
The evidence on taking selenium to treat heart disease is mixed. Scientists know that low levels of selenium can contribute to heart failure, and being deficient in selenium seems to make atherosclerosis worse. Atherosclerosis, or hardening of the arteries, happens when plaque builds up in arteries, which can lead to heart attack and stroke. But studies show that taking selenium supplements doesn't seem to have any effect on the progression of heart disease, nor does it protect against heart attack.
Selenium, combined with other antioxidants, including vitamin E and beta-carotene, may help lower LDL ("bad") cholesterol levels. But selenium can also interact with cholesterol-lowering drugs (See "Possible Interactions") and make them less effective. If you have or are at risk for heart disease, talk to your doctor before taking selenium.
Studies show that low levels of selenium are associated with a higher risk of cancer death. Scientists aren't sure how selenium works, but they have observed that people who live in parts of the world where there is a good deal of selenium in the soil and foods have lower rates of cancer. In addition, people who have cancer often have low levels of selenium.
But for most types of cancer, so far studies don’t show that selenium has much effect. One early study looking at whether selenium reduced risk of skin cancer found that although it didn’t seem to affect skin cancer risk, people who took selenium lowered their risk of death from cancer overall. However, later studies found that selenium doesn't seem to lower the risk of lung or esophageal cancer, and evidence is mixed on whether it protects against colorectal cancer. Two studies suggest that 200 mcg per day might help protect against colorectal cancer, but other studies don’t show any benefit.
The best evidence suggests that, if you have low levels of selenium, getting more selenium in your diet may lower your risk of prostate cancer. Taking a special kind of brewer's yeast, called selenized yeast, that's rich in selenium may also help. In one study of 1,312 people, those who took 200 mcg of selenium as selenized yeast per day had two-thirds lower risk of developing prostate cancer during the 4.5-year study compared to those who took a placebo. However, the men who got the benefit had low levels of selenium to start with.
In another study, taking 100 mcg of selenium along with 20 mg of zinc, 120 mg of vitamin C, 30 mg of vitamin E, and 6 mg of beta-carotene each day for about 8 years men who had normal PSA levels at the start of the study had a lower risk of prostate cancer. But men with higher PSA levels didn’t. PSA levels are one possible indicator of prostate cancer.
In another large study, men with normal PSA levels who took 200 mcg of selenium each day for about 5.5 years didn’t have a lower risk of prostate cancer than those who took placebo.
If you are at risk for prostate cancer, talk to your doctor before taking selenium.
The situation is complicated when it comes to skin cancer. Some early evidence led scientists to investigate whether selenium might protect against skin cancer. But a large trial found that taking daily selenium supplements actually increased the risk of squamous cell carcinoma and nonmelanoma skin cancer.
Many studies suggest that the body needs selenium in order for the immune system to work properly. Selenium, along with other minerals, can help build up white blood cells, which boosts the body's ability to fight illness and infection. A few studies suggest that selenium might help prevent some infections, such as a bacterial skin infection that often occurs with lymphedema, and mycoplasma pneumonia. In addition, one study suggested that when elderly people took zinc and selenium supplements, their immune systems responded better to the flu vaccine than those who took placebo.
Evidence suggests that people with asthma tend to have low blood levels of selenium. In a study of 24 people with asthma, those who took selenium supplements for 14 weeks had fewer symptoms compared to those who took placebo. But in a larger randomized, double-blind, placebo-controlled study, people who took a yeast supplement that contained selenium didn’t have any fewer symptoms than those who took placebo. More studies are needed.
The evidence concerning selenium and HIV/AIDS is contradictory. Studies have shown that levels of selenium go down consistently as HIV progresses. In one study, those taking a particular selenium supplement called Selenomax slowed the increase in viral load and had higher CD4 cell counts. But another study found that taking selenomethionine, a type of selenium, had no effect. If you have HIV or AIDS, talk to your doctor before taking any supplement, as it may interact with medications you are taking.
Selenium and other antioxidants play an essential role in how your body makes certain proteins found in sperm. One study suggested that selenium supplements might improve male fertility in men who had low levels of selenium. However, high levels of selenium are associated with decreased sperm motility.
Rheumatoid Arthritis (RA)
Low levels of selenium in the blood may be associated with increased risk of RA. But it doesn't seem that selenium supplements help once you have RA.
Brewer's yeast and wheat germ, liver, butter, fish (mackerel, tuna, halibut, flounder, herring, smelts) and shellfish (oysters, scallops, and lobster), garlic, whole grains, sunflower seeds, and Brazil nuts are all good sources of selenium.
Selenium levels in food depend on how much selenium was in the soil where the food was grown.
Selenium is destroyed when foods are refined or processed. Eating a variety of whole, unprocessed foods is the best way to get selenium in your diet.
Selenium may be taken as part of a vitamin-mineral supplement, a nutritional antioxidant formula, or as a separate supplement. Most supplements contain a form of selenium called selenomethionine.
How to Take It
The minimum daily recommended dietary allowances (RDA) for selenium are listed below.
- Children 1 - 3 years: 20 mcg
- Children 4 - 8 years: 30 mcg
- Children 9 - 13: 40 mcg
- Children 14 - 18: 55 mcg
- 19 and older: 55 mcg
- Pregnant women: 60 mcg
- Breastfeeding women: 70 mcg
As a supplement: Some studies have used 200 mcg per day for some conditions, but evidence suggests that taking that amount over a long time could increase your risk of developing diabetes. Talk to your doctor before taking more than the recommended daily allowance.
Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.
Talk to your doctor before taking more than the recommended daily allowance of selenium. Although some studies have used doses of 200 mcg, there is some evidence that this amount could increase your risk for diabetes. Do not exceed the tolerable upper limit of 400 mcg. Remember that you may also get some selenium in the foods you eat.
Over time, high doses of selenium can be toxic. Side effects include fingernail loss, skin rash, fatigue, irritability, and weight loss.
People who have an underactive thyroid (hypothyroidism) or have had, or are at risk for, skin cancer should not take selenium without talking to their doctor.
High selenium intake may increase the risk of diabetes and high cholesterol.
If you are being treated with any of the following medications, you should not use selenium supplements without first talking to your health care provider.
Drugs that affect selenium levels in the body -- These drugs may lower levels of selenium:
- Cisplatin, a chemotherapy drug
- Clozapine (Clozaril)
- Corticosteroids, such as prednisone
- Valproic acid (Depakote)
Anticoagulants and antiplatelet drugs (blood thinners) -- When taken with these drugs, selenium may increase the risk of bleeding:
- Clopidogrel (Plavix)
- Warfarin (Coumadin)
Barbiturates -- In animal tests, selenium seems to make the sedative effects of these drugs last longer:
- Butabarbital (Butisol)
- Mephabarbital (Mebaral)
- Phenobarbital (Nembutal)
- Secobarbital (Seconal)
Chemotherapy -- Although selenium may help reduce side effects from drugs such as cisplatin, doxorubicin, and belomycin, it may also interfere with their cancer-fighting ability. If you are undergoing chemotherapy, talk to your oncologist before taking selenium or any other supplement.
Cholesterol-lowering medications -- Simvastatin (Zocor) and niacin have been shown to lower LDL ("bad") cholesterol and raise HDL ("good") cholesterol in people with heart disease. Taking antioxidants, including selenium, along with these drugs may make them less effective. Theoretically, selenium may also reduce the effectiveness of other statins, including atorvastatin (Lipitor), fluvastatin (Lescol), lovastatin (Mevacor), and prevastatin (Pravachol).
Birth control pills -- Some researchers theorize that women taking birth control pills may have higher levels of selenium in their blood. If you take birth control pills, ask your doctor before taking extra selenium.
Gold salts -- may lower levels of selenium in the body and cause symptoms of selenium deficiency.
Beck MA, Nelson HK, Shi Q, Van Dael P, Schiffrin EJ, Blum S, Barclay D, Levander OA. Selenium deficiency increases the pathology of an influenza virus infection. FASEB J. 2001;15(8):1481-1483.
Bleys J, Navas-Acien A, Guallar E. Selenium and diabetes: more bad news for supplements.Ann Intern Med. 2007 Aug 21;147(4):271-2.
Bleys J, Navas-Acien A, Guallar E. Serum selenium levels and all-cause, cancer, and cardiovascular mortality among US adults. Arch Intern Med. 2008;168(4):404-10.
Bjelakovic G, Nikolova D, Simonetti RG, Gluud C. Antioxidant supplements for prevention of gastrointestinal cancers: a systematic review and meta-analysis. Lancet. 2004;364:1219-28.
Brawley OW, Panes H. Prostate cancer prevention trials in the USA. Eur J Cancer. 2000;36(10):1312-1315.
Cai J, Nelson KC, Wu M, Sternberg P Jr, Jones DP. Oxidative damage and protection of the RPE. Prog Retin Eye Res. 2000;19(2):205-221.
Cheung MC, Zhao XQ, Chait A, Albers JJ, Brown BG. Antioxidant supplements block the response of HDL to simvastatin-niacin therapy in patients with coronary heart disease and low HDL. Arterioscler Thromb Vasc Biol. 2001;21(8):1320-1326.
Duffield-Lillico AJ, Slate EH, Reid ME, Turnbull BW, Wilkins PA, Combs GF Jr, et al. Nutritional Prevention of Cancer Study Group. Selenium supplementation and secondary prevention of nonmelanoma skin cancer in a randomized trial. J Natl Cancer Inst. 2003 Oct 1;95(19):1477-81
Etminan M, FitzGerald JM, Gleave M, et al. Intake of selenium in the prevention of prostate cancer: a systematic review and meta-analysis.Cancer Causes Control. 2005 Nov;16(9):1125-31.
Fleshner NE, Kucuk O. Antioxidant dietary supplements: Rationale and current status as chemopreventive agents for prostate cancer. Urol. 2001;57(4 Suppl 1):90-94.
Geerling BJ, Badart-Smook A, Stockbrügger RW, Brummer R-JM. Comprehensive nutritional status in recently diagnosed patients with inflammatory bowel disease compared with population controls. Eur J Clin Nutr. 2000;54:514-521.
Ghadirian P, Maisonneuve P, Perret C, Kennedy G, Boyle P, Krewski D et. al. A case-control study of toenail selenium and cancer of the breast, colon, and prostate. Cancer Detect Prev. 2000;24(4):305-313.
Helzisouer KJ, Huang HY, Alberg AJ, Hoffman S, Burke A, Norkus EP, et al. Association between alpha-tocopherol, gamma-tocopherol, selenium, and subsequent prostate cancer. J Nat Cancer Inst. 2000;92(24):2018-2023.
Kneckt P. Serum selenium, serum alpha-tocopherol, and the risk of rheumatoid arthritis. Epidemiology. 2000;11(4):402-405.
Li H, Stampfer MJ, Giovannucci EL, et al. A prospective study of plasma selenium levels and prostate cancer risk. J Natl Cancer Inst. 2004;96:696-703.
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.
Mannisto S, Alfthan G, Virtanen M, Kataja V, Uusitupa M, Pietinen P. Toenail selenium and breast cancer -- a case-control study in Finland. Eur J Clin Nutr. 2000;54:98-103.
Meyer F, Galan P, Douville P, et al. Antioxidant vitamin and mineral supplementation and prostate cancer prevention in the SU.VI.MAX trial. Int J Cancer. 2005;116:182-6.
Navas-Acien A, Bleys J, Guallar E. Selenium intake and cardiovascular risk: what is new? Curr Opin Liidol. 2008;19(1):43-9.
Navarro-Alarcon M, Lopez-Martinez MC. Essentiality of selenium in the human body: relationship with different diseases. Sci Total Environ. 2000;249:347-371.
Peretz A, Siderova V, Neve J. Selenium supplementation in rheumatoid arthritis investigated in a double blind, placebo-controlled trial. Scand J Rheumatol. 2001;30(4):208-212.
Rayman MP. The importance of selenium to human health. Lancet. 2000;356:233-241.
Schrauzer GN. Anticarcinogenic effects of selenium. Cell Mol Life Sci. 2000;57(13-14):1864-1873.
Schrauzer GN. Selenomethionine: a review of its nutritional significance, metabolism, and toxicity. J Nutr. 2000;130(7):1653-1656.
Shaheen SO, Newson RB, Rayman MP, Wong AP, Tumilty MK, Phillips JM, Potts JF, Kelly FJ, White PT, Burney PG. Randomised, double blind, placebo-controlled trial of selenium supplementation in adult asthma. Thorax. 2007 Jun;62(6):483-90.
Sinclair S. Male infertility: nutritional and environmental considerations. Alt Med Rev. 2000;5(1):28-38.
Stranges S, Marshall JR, Natarajan R, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes. Ann Intern Med. 2007;147:217-23.
Stranges S, Marshall JR, Natarajan R, Donahue RP, Trevisan M, Combs GF, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007 Aug 21;147(4):217-23.
Witte KK, Clark AL, Cleland JG. Chronic heart failure and micronutrients. J Am Coll Cardiol. [Review]. 2001;37(7):1765-1774.
Wein A. Urology, 9th ed. Philadelphia, PA: Saunders Elsevier. 2007; Chapter 90.
Zeng H, Combs GF. Selenium as an anticancer nutrient: roles in cell proliferation and tumor cell invasion. J Nutr Biochem. 2008;19(1):1-7.
- Last Reviewed on 06/11/2011
- Steven D. Ehrlich, NMD, Solutions Acupuncture, a private practice specializing in complementary and alternative medicine, Phoenix, AZ. Review provided by VeriMed Healthcare Network.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2013 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
This page was last updated: May 7, 2013