Selenium is a 'trace element' which means it is required in very small quantities by the body and in larger quantities can be toxic. Don't let this fool you into thinking it is not an important nutrient though - this could not be further from the truth. Selenium is a component of many important enzymes, often in the form of selenoproteins (proteins containing selenium).
Two of these are:
Thyroid Hormone Deiodinases
Thyroid hormones are composed of a molecule of the amino acid L-tyrosine bound to various numbers of iodine atoms. For example thyroxine contains four iodine atoms and is thus referred to as 'T4'. The thyroid produces primarily T4 but this is considered to be a storage form. The more active thyroid hormone is known as triiodothyronine, or 'T3' because it has only three iodine atoms. This is where selenium comes in. The enzyme 5'-deiodinase, of which selenium is a major component, is responsible for removing an atom of iodine from T4 to T3. Without sufficient quantities of this selenium-dependent enzyme a deficiency of T3 may result and the symptoms of hypothyroidism develop (e.g. fatigue, depression, weight gain, constipation, cold intolerance, loss of libido).
Another selenium-containing enzyme of great importance to health is glutathione peroxidase. This enzyme is one of the body's most potent antioxidant defences. It protects cells from damage by free radicals, specifically lipid peroxides and hydrogen peroxide; reducing them to alcohols and water respectively which can then be detoxified further and removed from the body or in the case of water used as needed. Deficiency of glutathione peroxidase is associated with accelerated aging and degenerative diseases, as well as environmental illnesses. In the case of chronic fatigue syndrome (ME/CFS) for example there is chronic infection and immune activation. It is no surprise that glutathione peroxidase and selenium become depleted as immune cells actually generate hydrogen peroxide themselves as a means to kill invading pathogens in what is known as the 'respiratory burst'.
Selenium and selenium compounds are also able to bind to toxic metals such as mercury, lead and cadmium and facilitate their removal from the body, while also protecting the body from their toxic oxidative effects as part of glutathione peroxidase.
Studies have also shown that selenium may be protective against cancer and even beneficial as a adjunct to regular cancer therapy. Selenium may reduce both incidence of, and mortality from, various forms of cancer, based on current evidence particularly liver, prostate, colo-rectal and lung cancers. It is thought selenium exerts its anti-cancer effects by reducing DNA damage, protecting cells from free radical damage (through glutathione peroxidase) and perhaps by other mechanisms yet to be discovered.
All of selenium's functions and properties make it a nutrient of great importance to environmental illness sufferers who invariably have issues with poor thyroid function and glutathione peroxidase depletion (due to acute or chronic exposure to heavy metals and other toxins).
Selenium is available in various supplemental forms with those containing selenium bound to amino acids (e.g. selenomethionine, methylselenocysteine) considered the most bioavailable and therefore effective.
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