Selenium

What Is Selenium?

Selenium is a chemical element, represented by the symbol, Se. Selenium is a non metal and is related to other chemical elements, sulfur and tellurium. Selenium was discovered in 1817 by Swedish chemist, Jons Berelius. Selenium is derived from the Greek word, 'selene' which translates to 'moon'.

What role does selenium play in the body?

Selenium is an antioxidant and helps protect the body from free radicals [1]. Oxygen free radicals are highly reactive atoms or compounds produced during metabolism and can damage cells.

An essential micro nutrient, traces of selenium are needed for cellular function in humans and animals.

In the body, selenium is found within certain enzymes including glutathione peroxidase which helps protect the body from oxidative damage.

Selenium is an important factor in the functioning of the thyroid gland and is a cofactor in 3 known thyroid enzymes. These selenium dependent enzymes are known as selenoproteins.

There is evidence which suggests a link between selenium intake and a reduced risk of cancer [2].

Benefits Of Selenium

• Helps promote a healthy liver
• Protects against toxic metals in the body including mercury
• Protects against heart disease
• Neutralizes alcohol, smoke and fats
• Acts as an antioxidant against free radicals
• May help prevent cancers

Where does it occur?

Selenium was initially discovered as a byproduct of sulfuric acid production. However, selenium occurs in the environment in various forms.

Depending on location, selenium can be found in soil in a soluble form called selegate. Many but not all plants will use selenium as a micro nutrient if present in the soil and there are a number of plants which need selenium.

Plants which thrive when selenium is present in the soil are known as indicator plants. Astragalus species, false goldenweed prince's plume, and woody aster are examples of selenium indicator plants.

Levels of selenium within the soil varies across the world. For example, Europe has lower levels of selenium compared to the US.

There are also natural inorganic forms of selenium which include selenide, selenate and selenite.

Dietary sources of selenium

Selenium can be found in some foods.

Dietary sources of selenium include nuts, cereals, eggs and fish (tuna, crab and lobster have high levels). Brazil nuts contain the highest source of selenium. Kidney also contains high levels of selenium.

(source: National Institute Of Health - Office Of Dietary Supplements )

Selenium Poisoning – Selenium Toxicity

As a trace element, selenium is a micro nutrient required by the body for cellular function. However when consumed in excess, this element can become toxic to the body. Selenium poisoning is known as 'selenosis'.

Symptoms of selenium poisoning include:

• abdominal pain
• convulsions
• decreased cognitive function
• dermatitis
• garlic breath odor
• hair nail and tooth loss
• hyper salivation
• muscle spasms
• nausea
• vomiting
• weakness

In extreme cases selenosis can be fatal.

In humans selenium poisoning is more likely to occur where people have had excessive environmental exposure to selenium. Selenosis can often occur in livestock which feed on plants containing high levels of selenium taken up from selenium rich soil below.

Upper tolerated levels - avoiding poisoning

A number of studies have investigated the effects of different levels of selenium in the body. This has resulted in conflicting data regarding the levels of selenium which cause selenosis. The general agreement is that in adults an upper level daily limit of 400mg is safe although up to 800mg of selenium per day may be tolerated.

Table: upper tolerated selenium levels

(source: Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes: Vitamin C, Vitamin E, Selenium, and Carotenoids. National Academy Press, Washington, DC, 2000 )

Given that brazil nuts have very high levels of selenium (1 ounce/28 grams can contain has over 500mg) they provide an economical alternative to selenium supplements but should be eaten in small quantities.

References

1. Garewal HS. Antioxidants and disease prevention. New York: CRC Press, 1997:106.
2. Fleet JC. Dietary selenium repletion may reduce cancer incidence in people at high risk who live in areas with low soil selenium. Nutr Rev. 1997 Jul;55(7):277-9.