"What is Selenium?"
Selenium is a trace element essential for human health and wellbeing (1). It is integral to a number of proteins with important functional roles in the maintenance of health, including antioxidant enzymes which protect the body against free radical attack. Free radicals generated in the body can be increased by some diets and may cause significant degenerative diseases.
It is the 17th most common element on earth and is found in inorganic form in the soil. However, its concentration varies considerably across geographical regions. High concentrations occur in some parts of Russia, China, USA and Venezuela, while New Zealand, Australia, Finland, and other regions of China have areas which are significantly depleted of selenium (2).
I recommend readers to visit the Oregon State University Linus Pauling Institute Micronutrient Information Centre website at http://lpi.oregonstate.edu/infocenter/minerals/selenium for a complete synopsis on Selenium. Please note that the specified RDI's relate to the current US standards.
"How Long Have We Known About Selenium?"
Discovered in 1818, its importance in biology didn't become apparent until 1957 when it was demonstrated that selenium, along with vitamin E, helped to protect the liver of laboratory rats from necrosis. Selenium's potential as an anticancer agent became known in the 1970s, and knowledge of how this occurs is still developing.
"What other foods provide selenium?"
It is believed that grain foods, depending on their source, currently provide about 30% of our normal dietary intake of Selenium (21), with meat and seafood (eg octopus, squid, tuna) providing about the same. Dairy foods are a variable source (20).
Brazil nuts provide one of the richest natural sources of selenium known. However, the amount of selenium present in any food also depends on where and how it was produced. There is a shortage of information on the selenium content of common foods in the Australian diet (13).
"Why is Selenium important to my health?"
Selenium is needed to create a number of proteins which are important antioxidant enzymes. In the human body, Selenium is present within at least 20 identifiable proteins, some with significant biological functions in the maintenance of health. It is also found in the 21st amino acid selenocysteine.
Some of these proteins are Antioxidants, which help control free radicals which cause cellular damage which contributes to the development of chronic diseases.
Selenium has also been shown to be essential in the maintenance of a healthy immune system, the central nervous system, and in heart and skeletal muscle function. As well, selenium has been shown to be capable of preventing some cancers from developing (see below) (1-4).
"Do Australians need extra selenium?"
There has been no comprehensive research into selenium levels in the population across Australia, but studies in US and elsewhere suggest higher levels than present RDI recommendations might be required to achieve significant reduction in risk of several cancers (3).
Several small surveys of blood samples indicate Australians and New Zealanders are marginally deficient in selenium by North American standards (13, 14, 23 and 24). There needs to be more comprehensive study and surveying of the food and population.
This is probably due to the selenium deficiency in some Australian soils, the nature of those soils e.g. acidic and some modern farming practices.
Obviously, there can be an immense variation in diet from one person to another and the selenium levels in the food you eat varies naturally as well.
"What happens if we don't get enough selenium?"
Because selenium is vital to human life, a deficiency may have severe implications (1,5-7). It's accepted that 40 micrograms is the minimum daily intake, while less than 11 micrograms will put people at risk of genetic damage. Three specific diseases have been associated with selenium deficiency (1, 2):
- Keshan Disease, which results in an enlarged heart and poor heart function in selenium deficient children
- Kashin-Beck Disease, an endemic disease of the joints or bones
- Myxedematous Endemic Cretinism, which results in mental retardation
Further evidence indicates that selenium deficiency may contribute to development of a form of heart disease, and lead to hypothyroidism, or dysfunctioning thyroid gland. It can also result in a weakened immune system, which can result in a host of ailments of infectious (viral or bacterial) disease origin.
"Can we have too much selenium?"
Yes we can. Selenium is toxic at high intakes. With a number of nutrients, vitamins, trace elements and minerals if you ingest too much there are potential ill effects.
Several experts have recommended that an upper limit of 400-450 micrograms of Selenium per day is prudent, with the maximum safe dietary intake for sensitive individuals thought to be below 600 micrograms (2, 5, and 18) depending on the form of selenium ingested.
"Does selenium help prevent cancer?"
Yes! Cancer studies in the United States have shown that incidence of lung, prostate and colon cancer was halved in a group of people who took 200 micrograms of organic selenium everyday for five and a half years (3, 5).
Most importantly, this positive benefit was found only in those who had relatively low levels of selenium in their blood to commence with, and as it is estimated that a third to a half of the Australian population fits into this category, the health benefits of an diet higher in selenium could be significant.
Research in China has also shown a link between increased selenium intake and lower rates of cancer of the stomach, oesophagus and liver (7).
Clinical data suggests that selenium lowers the risk of cancer in two ways. As an anti-oxidant, it can help protect the body from damage caused by free radicals and selenium might also prevent or slow the growth of tumours by boosting the activity of immune cells and suppressing the development of the blood vessels (angiogenesis) which feed the tumour and lead to cancer spread.
Longer-term research projects in USA and Europe involving thousands of people are now underway to try to clarify the link between selenium and cancer prevention (1).
"Is your heart protected by selenium?"
Selenium is one of a group of antioxidants that may help limit the oxidation of low-density lipoproteins (LDL – so called "bad" cholesterol). It is this form that promotes plaque build-up (hardening) in the coronary arteries (19).
"Can selenium boost the immune system?"
Selenium appears to be able to reverse the age-related decline in immune response in the elderly. In a group of institutionalized elderly subjects who received daily supplements of 100 micrograms of Selenium for six months, their immune response was restored to the level of healthy young individuals (8).
Selenium has also been shown to improve immune function in healthy adults who had marginal selenium status. In the UK, 22 adult subjects with relatively low levels of selenium in their blood received 50 or 100 micrograms of selenium daily for 15 weeks, which resulted in improved cellular immune response (9).
Further research has reported that selenium is essential in the activation of all elements of the immune system, with the thymus gland, a key organ of the developing immune system, appearing to have enzymes exquisitely sensitive to selenium status (8-10).
"Organic and Inorganic Selenium"
A study conducted in 1995 suggests that the organic forms of selenium increased blood selenium concentration to a greater extent than inorganic forms (4, 8, and 21).
Selenium pills and other supplements may contain two inorganic forms of selenium - sodium selenite and sodium selenate which are not so well absorbed.
