Introduction
Selenium yeast, produced by fermenting Saccharomyces cerevisiae in a selenium-rich media, is a recognized source of organic food-form selenium. In this process, virtually all of the selenium structurally substitutes for sulfur in the amino acid methionine thus forming selenomethionine via the same pathways and enzymes that are used to form sulfur-containing methionine. Owing to its similarity to S-containing methionine, selenomethionine is taken up nonspecifically and becomes part of yeast protein. It is this metabolic route that makes selenium yeast valuable in animal and human nutrition, since it offers the same organic form of selenium produced by food-chain autotrophs (i.e., most plants and certain blue-green algae)(Schrauzer G (2006). “Selenium yeast: composition, quality, analysis, and safety”. Pure Appl Chem 78: 105–109. doi:10.1351/pac200678010105.).

Selenium is a fundamental trace element which is closely related to human health. Selenium can prevent and inhibit tumor, anti-age and maintain the normal functions of cardiovascular system. Selenium-enriched yeast is the best source of organic selenium. The absorption and utilization of organic selenium is much higher than the inorganic selenium, and the toxicity of organic selenium is much lower.

Organic selenium in selenium-enriched yeast
French CNRS labs has analyzed the inorganic selenium, selenomethionine and selenocysteine in selenium-enriched yeast by using HPLC-ICP MS and found that there was no inorganic selenium in selenium-enriched yeast, and the organic selenium mainly existed in the form of selenomethionine and selenocysteine as shown in below spreadsheet.

Applications
Human health applications
Selenium is physiologically essential and may also offer a protective effect against several degenerative diseases.The organic form of selenium provided by selenium yeast has been shown to differ in bioavailability and metabolism compared with inorganic (e.g., selenate, selenite) forms of dietary selenium. Dietary supplementation using selenium yeast has been associated with increased ability to counteract oxidative stress. Furthermore, selenium yeast has been used in a wide range of studies aimed at examining the importance of selenium status in the incidence and progression of a variety of infectious and degenerative diseases(Lovell M, Xiong S, Lyubartseva G, Markesbery W (2009). “(Sel-Plex diet) decreases amyloid burden and RNA and DNA oxidative damage in APP/PS1 mice”. Free Radic. Biol. Med. 46: 1527–1533. doi:10.1016/j.freeradbiomed.2009.03.008.).

Cancer
Although selenium is promoted for its claimed anti-cancer effect, there is no good evidence of it being helpful in treating or preventing cancer, or with helping alleviate the side-effects of cancer treatment. Dietary supplementation with selenium yeast has been used to study the effects of selenium status and the risk of developing cancers or precancerous lesions(Combs GJ, Lu J. Selenium as a cancer preventive agent. In: Hatfield D, Berry M, eds. Selenium: Its molecular biology and role in human health, 2nd ed. New York: Springer, 2006: 249-264.).

Viral infection
Findings of increased viral virulence in selenium-deficient hosts support the need for further investigation into the interaction between host nutrition and viral evolutionary processes. Certain viral diseases have been shown to mutate more rapidly in selenium-deficient hosts producing more virulent viruses. This etiology has been demonstrated for both the Coxsackie B virus (associated with a Se-deficiency-related cardiomyopathy known as Keshan disease) and the influenza virus. High selenium yeast supplementation (200 μg/d) was evaluated in a 9-month double-blind, randomized, placebo-controlled trial in HIV-positive adult men and women. Daily supplementation was found to suppress progression of HIV-1 viral burden and provide indirect improvement to CD4 cell counts. (Selenium status diminishes with HIV disease progression; low selenium status has been shown to be a predictor of HIV-related mortality. Selenium deficiency has also been linked with increased infectiousness of people with HIV-1.)(Baeten J, Mostad S, Hughes M, et al. (2001). “Selenium deficiency is associated with shedding of HIV-1-infected cells in the female genital tract”. JAIDS 26: 360–364. doi:10.1097/00126334-200104010-00013.)

Animal feed and food-animal product applications
Selenium supplementation in yeast form has been shown to have beneficial effects in many species, especially on animal immune status, growth and reproduction The consequent improvements in productivity can be of economic benefit to livestock producers for many reasons, including greater overall efficiency of feedstuff use(Thatcher W, Santos J, Silvestre F, Kim I, Staples C (2010). “Perspective on physiological⁄endocrine and nutritional factors influencing fertility in post-partum dairy cows”. Reprod Dom Anim 45: 2–14. doi:10.1111/j.1439-0531.2010.01664.x.)

Selenium yeast supplementation of food-animal diets has an added nutritional benefit to human consumers of food-animal products. Dietary selenomethionine-containing plant or yeast protein can be also stored nonspecifically in animal protein, which can result in nutritionally useful selenium content in meat, milk, and eggs. Consequently, strategies to supplement animal feed with selenium yeast have led to the development of selenium-rich functional foods, including selenium-enriched eggs and meats for human consumption(Mahan D. Organic selenium fed to swine – its potential impact on human health issues. In: Midwest Swine Nutrition Conference. Indianapolis, IN, 2005.).

Safety
Since 2000, selenium yeast (S. cerevisiae CNCM I-3060) has been reviewed and received the following approvals for use in animal and human diets:
U.S. Food and Drug Administration (FDA) approval for use as a supplement to feed for chickens, turkeys, swine, goats, sheep, horses, dogs, bison, and beef and dairy.(Sel-Plex receives authorisation [online]. Available at:http://www.allaboutfeed.net/news/sel-plex-receives-authorisation-id98.html. Accessed January 21.)
European Food Safety Authority (EFSA) approval for use as a feed supplement for all animal species and categories.(Commission of the European Communities. Commission regulation (EC) No 1750/2006 of 27 November 2006 concerning the authorisation of selenomethionine as a feed additive [online]. Available at: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2006:330:0009:0011:EN:PDF. Accessed January 21.)
Organic Materials Review Institute approval for use as a feed supplement for all animal species(Organic Materials Review Institute. Sel-Plex [online]. Available at:http://www.omri.org/simple-opl-search/results/sel-plex. Accessed January 21.).
European Food Safety Authority (EFSA) approval for human health claims linking dietary intake of selenium yeast to “protection of DNA, proteins and lipids from oxidative damage, normal function of the immune system, normal thyroid function and normal spermatogenesis.”(European Food Safety Authority (2006). “Opinion of the scientific panel on additives and products or substances used in animal feed on the safety and efficacy of the product Sel-Plex®2000 as a feed additive according to regulation (EC) No 1831/2003”. EFSA Journal 348: 1–40.)