Sheril Marek, M.S.

---

Selenium is a trace element that is nutritionally essential for humans. Essential nutrients are required for normal body functioning that cannot be synthesized by the body. This shows the importance of consuming adequate selenium in one’s diet and supplements for enhanced exercise performance and decreased health risks. Selenium is a component of many selenoproteins that play a critical role in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection (5). Selenium is best known for the role it plays in the glutathione peroxidase (GSH-Px) enzyme system (1). Glutathione is a very important antioxidant that helps stabilize body immunity and neutralizes harmful effects of free radicals in the body. During exercise, the body produces even more free radicals than would be produced during daily living. The GSH-Px system is one of the major antioxidant defense systems within the body that acts to combat free radicals.

 

 

 

 

 

 

Deficiencies and Toxicity of Selenium

An inadequate intake of selenium brings many health risks. This is relatively rare in the average American diet because of the geographically diverse food supply. However, in selenium-deficient areas, such as China, residents are more susceptible to a form of viral cardiomyopathy called Keshan disease, Kashin-Beck’s disease, thyroid disease, cognitive decline, and muscle pain and weakness (2,3). There are also associated complications with selenium toxicity, which is more prevalent. The most common symptom with consumption of selenium above the upper limit is hair and nail brittleness. Acute selenium toxicity can cause severe gastrointestinal and neurological symptoms, myocardial infarction, muscle tenderness, respiratory distress, etc. (2,3).

 

 

 

 

 

Selenium and Exercise Performance

Current exercise research targets the effects of selenium on exercise-induced free radical formation. As part of the antioxidant defense system, selenium may play a role in prevention of fatigue or may aid in recovery from exhaustive exercise (2). Powers et al. showed that the glutathione peroxidase system increases as a chronic exercise training adaptation (4). The enzyme, glutathione peroxidase, is dependent upon selenium, which gives insight to possible inhibitory effects of selenium on muscle damage and assistance of antioxidant function.

 

Exercise training does not appear to increase the need for selenium intake. A limited number of studies have shown that selenium supplementation can decrease oxidative stress and improve antioxidant levels. The RDA for selenium is based on the maximization of GPx activity; therefore, any intake above this would show little to no benefits. Guidelines for selenium supplementation as an ergogenic aid have not been formed because it is relatively easy to consume adequate amounts in a well-balanced diet, and because of its toxic effects if too much is consumed. 

 

 

 

 

Dietary Sources and Recommended Daily Allowances (RDA) for Selenium

The recommended daily allowance for selenium varies by age, sex, and pregnancy status. However, the majority of individuals (males and females ages 19 and older) should consume 55 micrograms (mcg) per day. Selenium deficiencies are very rare in the United States because most men and women consume adequate amounts of selenium, 151 mcg and 108, respectively (3). However, there are health risks associated with selenium deficiencies and excessive intake. People aged 19 years and older should consume no more than 400 mcg per day. Chronic selenium toxicity is very common because of the high amounts of selenium in the food that is eaten on a daily basis. For example, one Brazil nut can contain anywhere from 68-91 mcg.

There are many sources that allow people to consume adequate amounts of selenium, such as food and supplements. Seafoods and organ meats are the richest food sources of selenium (5). Other sources include muscle meats, cereals and other grains, and dairy products. Even though most Americans consume more than enough in their diet, selenium is also available as a dietary supplement, mainly in the forms of selenomethionine and sodium selenite (6).

 

In conclusion, selenium deficiencies are not a major concern in the sports nutrition world. Most individuals from industrialized countries consume more than adequate amounts of selenium in a normal diet. Selenium supplementation should be avoided, even in athletes, due to the health risks and performance decrements associated with toxicity.

 

 

References:

1. Burk, R.F, and O.A. Levander. Selenium. In: Modern Nutrition in Health and Disease Ninth Edition, edited by M. Shils, J. Olson, M. Shike, and A. C. Ross. Baltimore: Williams & Wilkins, 1999, p. 265-276.
2. Fink H, Burgoon L, Mikesky A. Practical Applications in Sports Nutrition. 2^nd ed. Sudbury (MA): Jones and Bartlett Publishers; 2009. 213-214 p.
3. http://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/ 
4. Powers SK, Criswell D, Lawler J, J LL. Martin D, Herb RA, Dudley G. Influence of exercise and fiber type on antioxidant enzyme activity in rat skeletal muscle. Am J Physiol Regul Integr Comp Physiol February 1, 1994 266:(2) R375-R380. http://ajpregu.physiology.org/content/ajpregu/266/2/R375.full.pdf
5. Sunde RA. Selenium. In: Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler TR, eds. Modern Nutrition in Health and Disease. 11th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:225-37
6. Sunde RA. Selenium. In: Bowman B, Russell R, eds. Present Knowledge in Nutrition. 9th ed. Washington, DC: International Life Sciences Institute; 2006:480-97.