Selenium yeast

Selenium yeast is Saccharomyces cerevisiae (baker's yeast) grown in a selenium-rich media.^[1] It contains selenium in the form of organic and inorganic compounds. It is used both as a feed additive for livestock and as a dietary supplement for humans. It is approved in the US, EU, and the UK.^[2] Because selenium yeast can be patented, its producers can demand premium prices.^[3]

The other source of selenium is inorganic selenium in the form of pure chemicals. Forms used in animal feed include sodium selenate and sodium selenite. These too are effective in supplying selenium to the livestock.^[3]

The main claimed benefit of selenium yeast is that it contains organic selenium, mainly in the form of selenomethionine and selenocystine-containing proteins. Because these organic chemicals are also found in common natural sources of selenium such as wheat, it is claimed that they are more easily absorbed by animals including humans.^[1][4] Unfortunately, there is considerable variability in products described as "selenium yeast", specifically in the selenium compounds found within. Many products on the market are simply mixtures of largely inorganic selenium and some yeast, which defeats the point of using selenium yeast.^[5]

Molecular biology

Yeast cell walls are able to bind to inorganic selenium-containing ions via chemisorption. The ions do not pass through the cell membrane, so special transporter proteins are required to let them in. Inside of the yeast cell, the selenium is reduced to hydrogen selenide.^[6] Yeast has no natural pathway that makes use of selenium; it has no specialized machinery for making selenoproteins, unlike mammals.^[7]

However, yeast does produce organic selenium compounds. This is because its promiscuous enzymes do not effectively distinguish between selenium and sulfur, allowing them to make selenium analogs of the usual sulfide compounds. Selenide first becomes homoselenocysteine. On one branch of the pathway, homoselenocysteine is converted to selenomethionine, Se-adenosyl selenomethionine, Se-adenosyl-homoselenocysteine and back to homoselenocysteine (the seleno version of the S-adenosyl methionine cycle). On the other branch, homoselenocysteine is converted to selenocystathionine, then selenocystine, Se-methylselenocysteine, and γ-glutamyl-Se-methylselenocysteine.^[6] Selenomethionine and selenocystine can be randomly incorporated into proteins in lieu of the regular amino acid.^[8] The net effect is that the yeast converts inorganic selenium into organic selenium stored in its body.^[6]

The higher the concentration of the noncanonical substrate, the more promiscuous reactions. In other words, yeast grown in higher selenium concentrations accumulate selenium faster and accumulate more.^[6] But there is a limit to how much selenium the yeast can handle: excess random misincorporation of selenocystine leads to non-functional proteins aggregating in the cell.^[8] The selenium accumulator plant Astragalus bisulcatus has a more selective version of the cystine-tRNA ligase that reduces the chances of misincorporation of selenocystine when put in yeast. This genetic modification is, however, not expected to be used in selenium yeast as a food ingredient, but for use in the production of designer proteins in biotechnology.^[9]

Animal feed additive

Main article: Selenium in biology

Large amounts of selenium are toxic; however, it is physiologically necessary for animals in extremely small amounts. Many other uncharacterized selenium-containing organic chemicals are also produced by a method similar to that of selenomethionine; some have recently been characterized but remain relatively unknown, such as S-seleno-methyl-glutathione and glutathione-S-selenoglutathione.^[1][5] Due to this, the European Union has questioned the safety and potential toxicity of this food supplement for humans, and it may not be used as an additive after 2002.

G.N Schrauzer, who has written two papers about selenomethionine, claims it should be an essential amino acid, and that the product is completely safe.^[1] The European Food Safety Authority does allow the use of selenomethionine as a feed additive for animals.^[10] Because organic forms of selenium appear to be excreted from the body slower than inorganic forms, products enriched with organic selenium might detrimentally bioaccumulate in the body. Because selenium-enriched foods contain much more selenium than natural foods, selenium toxicity is a potential problem, and such foods must be treated with caution.^[5] The EU allows up to 300 micrograms of selenium per day, but one long-term study of selenium supplementation showed no evidence of toxicity at a dose as high as 800 micrograms per day.^[11]

An organic selenium-containing chemical found in selenium yeast has been shown to differ in bioavailability and metabolism compared with common inorganic forms of dietary selenium.^[12] Dietary supplementation using selenium yeast is ineffective in the production of antioxidants in bovine milk compared to inorganic selenium (sodium selenate).^[13] One study examined if increased selenium in the diet of mutant mice (via a selenium yeast product) caused a higher production of selenium-containing enzymes which have an antioxidant effect. The effect was modest.^[14]

Selenium supplementation in yeast form has been shown to increase pig selenium-containing antioxidant enzymes,^[15] broiler growth and meat quality,^[16][17] the shelf life of turkey and rooster semen,^[18][19][20] and possibly cattle fertility.^[21]

Selenium supplementation in animal feeds may be profitable for agribusinesses. It may be possible to market selenium-fortified foods to consumers as functional foods, such as selenium-enriched eggs, meat,^{[22][23][24][25]} or milk.^[13]

Specific products

Sel-Plex

A patented cultivar of yeast (Saccharomyces cerevisiae 'CNCM I-3060') marketed as Sel-Plex® has been approved for use in animal fodder:

• U.S. Food and Drug Administration approval for use as a supplement to feed for chickens, turkeys, swine, goats, sheep, horses, dogs, bison, and beef and dairy cows.^[26]
• Organic Materials Review Institute approval for use as a feed supplement for all animal species.^[27]
• As of 2006, the European Food Safety Authority's Scientific Panel on Additives and Products or Substances used in Animal Feed allows the use of Sel-Plex® in animal fodder for poultry, swine, and bovines, as the selenium is not significantly bio-accumulated by the human consumer.^[3]

Precautions

Only a small amount should be used when blending animal feeds. An excess of selenium, specifically at 10× the European authorized maximum selenium intake of Sel-Plex, causes a drop in animal productivity.^[3]

Appropriate measures to minimize inhalation exposure to the product should be taken.^[3]

Analytical chemistry

Total selenium in selenium yeast can be reliably determined using open acid digestion to extract selenium from the yeast matrix followed by flame atomic absorption spectrometry.^[28] Determination of the selenium species selenomethionine can be achieved via proteolytic digestion of selenium yeast followed by high-performance liquid chromatography with inductively coupled plasma mass spectrometry.^[29][30][31]

See also

• Nutrition portal

Nutritional muscular dystrophy