Selenium is a chemical element with the atomic number 34 and symbol Se. It has an atomic mass of 78.96 and is a non metal. In medicine it is referred to as a 'trace element'.
Selenium is found in nuts (especially Brazil nuts), meat, mushrooms, fish, eggs and cereals. In a 'normal diet', about 50 percent of selenium comes from cereals. The selenium content of plant foods varies with the selenium content of the soil.
Selenium acts as an antioxidant and helps protect the body against the damaging effects of free radicals. Selenium is essential for the activity of glutathione peroxidase, an enzyme that protects against reactive oxygen species and subsequent cell membrane damage.
Selenium as An Antioxidant
A free radical is formed when a weak bond, ie shared electrons, splits. Normally, bonds (ie when electrons are shared by bonding together) do NOT split in a way that leaves a molecule with an odd, unpaired electron.
Free radicals, molecules with an odd, unpaired electron, are very unstable and react quickly with other compounds, trying to capture the needed electron(s) to gain stability. Generally free radicals attack the nearest stable molecule, 'stealing' an electron from it. When the "attacked" molecule loses its electron, it becomes a free radical itself, which in turn attacks another molecule, resulting in a chain reaction. This can lead to a cascade, finally resulting in the disruption of a living cell.
Some free radicals arise normally as part of the metabolic processes continually going on in the human body. These free radicals can be regarded as chemical by-products of normal cellular metabolism.
Apart from normal cellular metabolism resulting in free radical formation, free radicals are also formed by environmental factors such as pollution (for example, car exhaust fumes), cigarette smoke (including second-hand smoke), pesticides and fungicides and chemicals in our foods such as colourings, flavourings, preservatives etc. Radiation, including ultraviolet radiation from the sun, is another cause of free-radical formation. Stress is yet another cause.
Free radicals cause damage to many components of a cell, including the cell wall, the mitochondria and DNA, the highly complex molecule of which our genes are composed. When enough DNA is damaged, cells begin to die.
Free-radical damage is referred to as oxidation.
The substances which protect against free-radical damage (oxidation) in the body are called ‘antioxidants’. Antioxidants neutralize free radicals by donating one of their own electrons, ending the electron 'stealing' reaction. The antioxidant nutrients themselves do not become free radicals by donating an electron, because they are stable in either form (with or without the donated electron). They act as scavengers, helping to prevent cell and tissue damage that could lead to cellular damage and disease.
Selenium has strong antioxidant properties. It is a critical component of several cellular enzymes that protect red blood cells and membranes. It has clinical applications in many disease conditions.
What Foods Provide Selenium?
Selenium is found in a wide variety of foods, the content of which varies with geographic sources of the food. Soil concentration can range from <0.01 microgram/gram to >1,000 microgram/gram, with plant foods reflecting this range, as do the animals that consume these plants, although the variability of selenium levels is not so marked in animal foods. Plant foods are the major dietary sources of selenium in most countries throughout the world.
In Australia and New Zealand, the main dietary sources are seafood, poultry and eggs and to a lesser extent, other meats. As mentioned, the contribution of cereal products depends on the source. Much plant selenium is in the form of selenomethionine, selenocysteine or selenocysteine metabolites. Meats and seafood also contain selenoproteins with selenium in the form of selenocysteine.
Selected food sources of selenium (from US Department of Agriculture Nutrient Database web pages):
|Micrograms of Selenium|
|Brazil nuts, dried, 30 gram||544|
|Tuna, light, canned in water, 90 gram||68|
|Cod, cooked, 90 gram||32|
|Turkey, roasted, 90 gram||27|
|Chicken breast, 90 gram||24|
|Beef, lean, 90 gram||23|
|Sunflower seed kernels, dry, 30 gram||23|
|Egg, 1 large||15|
|Oatmeal, 1 cup||12|
|Bread, whole-wheat, 1 slice||11|
|Rice, white, half cup||6|
|Walnuts, dried, 30 gram||5|
|Cheese, cheddar, 30 gram||4|
Recommended Dietary Intake for Selenium
|Men:||70 mcg / day|
|Women:||60 mcg / day|
|Children:||from 25 to 60 mcg depending on age|
The upper limit for adults is 400mcg / day. This is for foods and supplements. The absorption of selenium from food is about 55 - 70%. (from NHMRC Nutrient Reference Values web pages).
Human selenium deficiency is rare in Western Countries.
There is evidence that selenium deficiency may contribute to the development of a form of heart disease, hypothyroidism, and a weakened immune system. Deficiency of selenium can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses.
Current Issues and Controversies about Selenium
Observational studies indicate that death from cancer, including lung, colorectal, and prostate cancers, is lower among people with higher blood levels or intake of selenium. In addition, it has been shown that the incidence of non-melanoma skin cancer is significantly higher in areas of the United States with low soil selenium content.
Research suggests the selenium might affect cancer risk in two ways. As an antioxidant, selenium can help protect the body from the damaging effects of free radicals. Selenium may also prevent or slow tumour growth. Certain breakdown products of selenium are believed to prevent tumour growth by enhancing immune cell activity and suppressing development of blood vessels to the tumour.
· Heart Disease
Evidence suggests that the oxidative stress from free radicals may promote heart disease. For example, it is the oxidized form of low-density lipoprotein (LDL, often called the 'bad' cholesterol) that promotes plaque build-up in coronary arteries. Selenium is one of a group of antioxidants that may help limit the oxidation of LDL cholesterol and thereby help to prevent coronary heart disease.
Surveys indicate that individuals with rheumatoid arthritis have reduced selenium levels in their blood. In addition, some individuals with non-rheumatoid arthritis have a low selenium uptake.
The body's immune system naturally makes free radicals that can help destroy invading organisms and damaged tissue, but that can also harm healthy tissue. Selenium as an antioxidant may help relieve the symptoms of arthritis by controlling the levels of free radicals. Current findings are considered preliminary.
Selenium is a powerful antioxidant.
My January 2007 newsletter Free Radicals - Antioxidants gives a detailed review of this important topic. Antioxidant vitamins (A, C, and E) are essential for health. The non-vitamin food sources of antioxidants include: selenium and CoEnzyme Q-10. Zinc, copper and manganese, while not antioxidants in their own right, have a role in protecting against free-radical damage.
All of the above antioxidants are important for heath. Selenium is powerful as an antioxidant and ideally should be obtained from food sources. However, it is often recommended that an oral supplement be taken. The suggested dosage is 50-100 mcg per day.
*Copyright 2013: The Huntly Centre.
Disclaimer: All material in the huntlycentre.com.au website is provided for informational or educational purposes only. Consult a health professional regarding the applicability of any opinions or recommendations expressed herein, with respect to your symptoms or medical condition.
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