What is selenium?

Selenium is a trace element that functions within selenocysteine, an amino acid that makes up part of a category of proteins called selenoproteins. A few types of selenoproteins that have been studied include glutathione peroxidases, thioredoxin reductases, and thyroid hormone deiodinases.¹ Glutathione peroxidases exhibit antioxidant properties and have been found in tissues of the intestinal lining, lungs, thyroid gland, kidneys, and nose. Thioredoxin reductases are primarily involved in antioxidant functions within the body,² and thyroid hormone deiodinases are essential for the conversion of thyroxine (T4) to the active thyroid hormone triiodothyronine (T3).³

What happens if someone doesn’t consume enough selenium?

Due to its essential role in major metabolic pathways, including thyroid hormone metabolism, antioxidant defense systems, and immune function, maintaining proper selenium levels is important.⁴ While a deficiency in selenium will not lead to specific symptoms or clinical illness in most people, selenium deficiency has been identified as a public health concern in certain populations, and coordinated efforts have been made in the past to prevent selenium deficiency in these populations.⁵ Selenium has been studied in connection to several types of cancers, cardiovascular disease, and thyroid autoimmune disorders. 

Cancer. Studies on selenium intake and cancer risk suggests that adequate intake of selenium can protect humans against several types of cancers, including prostate, liver, and colorectal cancer.¹ However, reviewers of these studies point out some limitations that prevent us from saying with certainty that selenium supplementation prevents certain types of cancer.⁶ 

Heart disease. Cardiovascular disease (CVD) is related to oxidative stress. Remember those selenoproteins, such as glutathione peroxidases? These selenoproteins carry out antioxidant functions in the body and have been connected with lowered risk of CVD when measured at high circulating levels.⁷ 

Thyroid autoimmune disorders. Hashimoto’s thyroiditis, an autoimmune disorder in which antibodies are directed against the thyroid gland, leading to chronic inflammation, is the most common cause of hypothyroidism in the United States.⁸ Early research on selenium supplementation for the treatment of Hasimoto’s thyroiditis and Graves disease, another autoimmune disease that leads to hyperthyroidism, suggest that selenium supplementation can be an effective treatment by decreasing harmful thyroid antibodies. Ongoing trials set to be published within the coming months will provide further insight on the relationship between selenium and these diseases.⁹

What foods contain selenium?

Surveys conducted in the United States indicate a high intake of selenium in most Americans. Selenium can be found in abundance in Brazil nuts, yellowfin tuna, oysters, and clams.¹¹ An adult can easily obtain the recommended selenium intake by eating 2 to 3 brazil nuts per day. See Table 1 for more details on selenium-rich foods. 

Can you consume too much selenium?

High doses of selenium can be toxic. The upper limit for daily selenium intake is 400μg/day.¹¹ Symptoms of selenium toxicity include nail and hair brittleness, gastrointestinal disturbances, skin rashes, a garlicky breath odor, fatigue, irritability, and neurologic disorders.¹ Most people do not require selenium supplementation if they’re eating a balanced diet and regularly consuming nuts and seafood. See Table 2 to get a better idea of how much selenium you should be eating based on your age and sex. 

Sources

1. Higdon J, Drake VJ, Delage B, Tsuji PA. Oregon State University site. Selenium. 2015. Accessed 29 May 2019. https://lpi.oregonstate.edu/mic/minerals/selenium
2. Holmgren A. Thioredoxin and glutaredoxin systems. J Biol Chem. 1989 Aug 25;264(24):13963–13966.
3. Maia AL, Goemann IM, Souza Meyer EL, Wajner SM. Type 1 iodothyronine deiodinase in human physiology and disease. J Endocrinol. 2011;209(3):283–297.
4. Brown KM, Arthur JR. Selenium, selenoproteins and human health: a review. Public Health Nutr. 2001 Apr;4(2B):593–599.
5. Chen J. An original discovery: selenium deficiency and Keshan disease (an endemic heart disease). Asia Pac J Clin Nutr. 2012;21(3):320–326.
6. Vinceti M, Dennert G, Crespi CM, et al. Selenium for preventing cancer. Cochrane Database Syst Rev. 2014 Mar 30;(3):CD005195.
7. Flores-Mateo G, Carrillo-Santisteve P, Elosua R, et al. Antioxidant enzyme activity and coronary heart disease: meta-analyses of observational studies. Am J Epidemiol. 2009 Jul 15;170(2):135–147.
8. American Thyroid Association site. Hashimoto’s Thyroiditis (Lymphocytic Thyroiditis). Accessed 29 May 2019. https://www.thyroid.org/hashimotos-thyroiditis/
9. ClinicalTrials.gov. National Library of Medicine (US). Identifier NCT02013479. Selenium Supplementation in Autoimmune Thyroiditis (CATALYST). Available from: https://clinicaltrials.gov/ct2/show/NCT02013479
10. National Institutes of Health Office of Dietary Supplements sale. Selenium. 26 Sept 2019. Accessed 29 May 2019. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/.
11. Institute of Medicine (US) Panel on Dietary Antioxidants and Related Compounds. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington (DC): National Academies Press (US); 2000. Available at: https://www.nap.edu/read/9810/chapter/9#309.