9.4 Selenium

Selenium

Selenium is a mineral, rather than a vitamin as we’ve been discussing, that can be divided into 2 categories: organic and inorganic. The organic forms contain carbon, while the inorganic forms do not. The primary inorganic forms of selenium are selenite (SeO₃) and selenate (SeO₄).

Selenite and selenate are not commonly found alone in nature; they are usually complexed with sodium to form sodium selenite (Na₂SeO₃) and sodium selenate (Na₂SeO₄)1.

Selenomethionine is the most common organic form of selenium.

While selenium has both animal and plant sources, the selenium content of plants is dependent on the soil where they are grown.

As shown in Figure 9.41, soil selenium levels vary greatly throughout the United States, meaning that the selenium content of plant foods also greatly vary.

Figure 9.41 United States soil selenium levels2

The above map is interactive, so to see the soil selenium levels in a certain county or state, click on it in the link below. What are soil selenium levels where you live?

Required Web LinkUSGS Soil Selenium Levels

Inorganic forms of selenium are commonly used in supplements. Selenomethionine is the most common organic form of selenium in supplements and food. It is found in cereal grains such as wheat, corn, and rice as well as soy. Yeast are typically used to produce selenomethionine for supplements.

It should be noted that selenomethionine accumulates at much higher levels in the body than other forms of selenium. This is because it can be nonspecifically incorporated into body proteins in place of the amino acid methionine.

For more information on selenium, see the Required Web Link below.

Required Web LinkSelenium Fact Sheet

Subsections:

• 9.41 Selenoproteins
• 9.42 Selenium Absorption, Excretion, Toxicity & Its Questionable Deficiency

References & Links

• Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
• http://tin.er.usgs.gov/geochem/doc/averages/se/usa.html

Link

USGS Soil Selenium Levels – http://tin.er.usgs.gov/geochem/doc/averages/se/usa.html Selenium Fact Sheet – https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/

Selenoproteins

Selenium’s antioxidant function is not due to the mineral itself, but a result of selenoproteins.

Selanoprotein refers to any protein containing the amino acid selenocysteine.

25 human selenoproteins have been identified. For the vast majority of the other selenoproteins, their function isn’t known. However, there are 2 primary functions of selenoproteins that are known1-3:

• They act as an antioxidant eyzyme
• They play a role in thyroid hormone (T3) metabolism

Now see how selenium completes the antioxidant network we’ve seen a few times before. Thioredoxin reductase (a selenoprotein) can regenerate ascorbate from dehydroascorbate in the theorized antioxidant network (shown in Figure 9.411).

Figure 9.411 The theorized antioxidant network4

References & Links

• Gladyshev V, Kryukov G, Fomenko D, Hatfield D. (2004) Identification of trace element- containing proteins in genomic databases. Annu Rev Nutr 24: 579-596.
• Beckett G, Arthur J. (2005) Selenium and endocrine systems. J Endocrinol 184(3): 455-465.
• Stipanuk MH. (2006) Biochemical, Physiological, & Molecular Aspects of Human Nutrition. St. Louis, MO: Saunders Elsevier.
• Packer L, Weber SU, Rimbach G. (2001) Molecular aspects of alpha-tocotrienol antioxidant action and cell signalling. J Nutr 131(2): 369S-373S.

Selenium Absorption, Excretion, Toxicity & Its Questionable Deficiency

Selenium is highly absorbed by the body. Thus, selenium levels are not regulated by absorption, but rather by urinary excretion. Organic selenium forms may be absorbed slightly better than inorganic forms, as one study found that 98% of a dose of selenomethionine (organic form) was absorbed, compared to 84% of selenite (inorganic form)1.

Selenium is primarily excreted in the urine, but at high levels it can be eliminated in exhaled air, producing garlic odor breath.

Selenium toxicity can be a problem, especially for animals living in or around bodies of water in areas with high soil selenium levels. This is because runoff from the soil causes selenium to

collect in the water at high levels. Then the selenium starts working its way up the food chain and causing problems, as shown in the following link.

Required Web LinkSelenium Toxicity

In humans, the initial symptoms are nausea, fatigue, and diarrhea. If continued, the person may develop hair and nail brittleness, rash or skin lesions, and nervous system abnormalities.

One disease known to be caused by a selenium deficiency is Keshan disease. Keshan disease is a congestive cardiomyopathy (heart disease) caused by a combination of dietary deficiency of selenium and the presence of a mutated strain of coxsackievirus. Researchers determined that a selenium deficient diet caused the virus undergo a mutation turning it into a more virulent (infectious) form3. Similar results have been seen in recent experiments using vitamin E5.

Researchers are also examining the effects of vitamin deficiencies on other viruses, such as influenza (flu) and HIV. Finding similar phenomena in these or other viruses could shed light on new methods of treating or preventing some diseases.

References & Links

• Stipanuk MH. (2006) Biochemical, Physiological, & Molecular Aspects of Human Nutrition. St. Louis, MO: Saunders Elsevier.
• http://en.wikipedia.org/wiki/File:China_100.78713E_35.63718N.jpg
• Beck M, Handy J, Levander O. (2004) Host nutritional status: The neglected virulence factor.

Trends Microbiol 12(9): 417-423.

• https://en.wikipedia.org/wiki/Keshan_disease
• Beck, M.A. (2007) Selenium and Vitamin E Status: Impact on Viral Pathogenicity. J. Nutr. 137(5): 1338-1340

Links

Selenium Toxicity – http://www.sci.sdsu.edu/salton/SeTooMuchTooLittle.html