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 health 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. Your immune system also needs selenium to work properly. People with conditions ranging from rheumatoid arthritis (RA) to certain types of cancer often have low levels of selenium. However, in most cases scientists are not 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 that 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 the meaning behind these findings. 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 probably 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 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. Studies show that taking selenium supplements does not 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, and make them less effective. If you have or are at risk for heart disease, talk to your doctor before taking selenium.Cancer
Studies show that low levels of selenium are associated with a higher risk of cancer death. Scientists are not sure how selenium affects this risk. But they have observed that people who live in parts of the world where the soil is rich in selenium have lower rates of cancer, possibly because there is more selenium in food. In addition, people who have cancer often have low levels of selenium.
For most types of cancer, selenium does not appear to have much effect. One early study looking at whether selenium reduced the risk of skin cancer found that, although it did not seem to affect skin cancer risk, people who took selenium lowered their risk of death from cancer overall. However, later studies found that selenium does not seem to lower the risk of lung or esophageal cancer. 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 do not 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 selenium-rich brewer's yeast, called selenized yeast, may also help. In one study of 1,312 people, those who took 200 mcg of selenium as selenized yeast per day (compared to those who took a placebo) had two-thirds lower risk of developing prostate cancer during the 4.5-year study. However, the men who got the benefit had low levels of selenium to start with.
If you are at risk for prostate cancer, talk to your doctor before taking selenium.
The situation is even more 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 non-melanoma skin cancer.Immune function
Many studies suggest that the body needs selenium for the immune system to work properly. Selenium, along with other minerals, can help boost white blood cells, which improves 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.Asthma
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 study, people who took a yeast supplement that contained selenium didn't have fewer symptoms than those who took placebo. More studies are needed.HIV/AIDS
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.Male infertility
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 may negatively impact sperm's ability to swim.Rheumatoid arthritis (RA)
Low levels of selenium in the blood may be associated with increased risk of RA. But it does not seem that selenium supplements help once you have RA.
Good sources of selenium include brewer's yeast and wheat germ, liver, butter, fish (mackerel, tuna, halibut, flounder, herring, and smelts), shellfish (oysters, scallops, and lobster), garlic, whole grains, sunflower seeds, and Brazil nuts.
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.
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 upper tolerable 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
- Weight loss
You should not take selenium without talking to your doctor if:
- You have an underactive thyroid (hypothyroidism)
- You have had skin cancer
- You are at risk for skin cancer
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
Drugs that may lower levels of selenium include:
- 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)
In animal tests, selenium seems to make the sedative effects of these drugs last longer:
- Butabarbital (Butisol)
- Mephabarbital (Mebaral)
- Phenobarbital (Nembutal)
- Secobarbital (Seconal)
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.
Simvastatin (Zocor) and niacin have been shown to lower LDL (bad) cholesterol and raise HDL (good) cholesterol in people with heart disease. Taking certain antioxidants together, including selenium, along with these drugs may make them less effective. In theory, selenium may also reduce the effectiveness of other statins, including atorvastatin (Lipitor), fluvastatin (Lescol), lovastatin (Mevacor), and prevastatin (Pravachol).
Birth control pills
Some researchers propose 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.
These chemical compounds may lower levels of selenium in the body and cause symptoms of selenium deficiency.
Algotar AM, Stratton MS, Ahmann FR, et al. Phase 3 clinical trial investigating the effect of selenium supplementation in men at high-risk for prostate cancer. Prostate. 2013;73(3):328-35.
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):1481-3.
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):1312-5.
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):205-21.
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):1320-6.
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.
Faghihi T, Radfar M, Barmal M, et al. A randomized, placebo-controlled trial of selenium supplementation in patients with type 2 diabetes: effects on glucose homeostasis, oxidative stress, and lipid profile. Am J Ther. 2014;21(6):491-5.
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;54:514-21.
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):305-13.
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):2018-23.
Hurst R, Hooper L, Norat T, et al. Selenium and prostate cancer: systematic review and meta-analysis. Am J Clin Nutr. 2012;96(1):111-22.
Kneckt P. Serum selenium, serum alpha-tocopherol, and the risk of rheumatoid arthritis. Epidemiology. 2000;11(4):402-5.
Kristal AR, Darke AK, Morris JS, et al. Baseline selenium status and effects of selenium and vitamin e supplementation on prostate cancer risk. J Natl Cancer Inst. 2014;106(3):djt456.
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.
Mistry HD, Broughton Pipkin F, Redman CW, Poston L. Selenium in reproductive health. Am J Obstet Gynecol. 2012;206(1):21-30.
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;249:347-71.
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):208-12.
Rayman MP. The importance of selenium to human health. Lancet. 2000;356:233-41.
Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-68.
Schrauzer GN. Anticarcinogenic effects of selenium. Cell Mol Life Sci. 2000;57(13-14):1864-73.
Schrauzer GN. Selenomethionine: a review of its nutritional significance, metabolism, and toxicity. J Nutr. 2000;130(7):1653-56.
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.
van Zuuren EJ, Albusta AY, Fedorowicz Z, Carter B, Pijl H. Selenium supplementation for Hashimoto's thyroiditis. Cochrane Database Syst Rev. 2013;6:CD010223.
Vinceti M, Dennert G, Crespi CM, et al. Selenium for preventing cancer. Cochrane Database Syst Rev. 2014;3:CD005195.
Witte KK, Clark AL, Cleland JG. Chronic heart failure and micronutrients. J Am Coll Cardiol. [Review]. 2001;37(7):1765-74.
Wein A. Campbell-Walsh Urology. 10th ed. Philadelphia, PA: Elsevier Saunders; 2011.
Wein A. Urology. 9th ed. Philadelphia, PA: Elsevier Saunders; 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.