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Science / Molecule of the NMMonth: Too much of a good thing?: Selenium is known to prevent certain types of heart condition and can ward off cancer. But it may also cause foul breath, says John Emsley

Last month the US National Cancer Institute reported on a five-year study of 30,000 middle-aged Chinese living in the north central region of Linxian, where there is a high incidence of stomach cancer. The people had agreed to take part in a project in which they were given different combinations of dietary supplements such as vitamins A, B2, C, E, zinc or selenium. The group taking vitamin E and selenium showed a remarkable drop in cancer cases.

The Chinese have long had a special interest in selenium because large areas of that country have selenium-deficient soil, and this affects the health of the locals. Children in the Keshan region were particularly prone to a heart condition that killed half of those afflicted. It is known as Keshan disease and involves a swelling of the head; the cause is a lack of selenium in the body. A large-scale trial in south China in 1974 involved 20,000 children, half of whom were given selenium-containing tablets and half given a placebo. Of those on the placebo, 106 developed Keshan disease and 53 died, while of those on selenium, 17 succumbed to the illness and only one died.

Philip Whanger, an agricultural chemist at Oregon State University, studies selenium deficiency among Chinese farmers but finds that they are often unwilling to provide blood samples for analysis. Instead, he asks for hair and nail clippings from which he can assess their selenium level because the element concentrates in these. In Western countries hair and nails could not be used for monitoring because some anti-dandruff shampoos contain selenium, and nails may be cleaned with nail polish remover which removes some of the element.

As a dietary supplement, selenium is taken in the form of sodium selenite, Na2 SeO3 , which is a white crystalline material soluble in water, and the daily dose is 50 micrograms (millionths of a gram). Selenium was advocated as a dietary supplement 10 years ago by Alan Lewis, whose book Selenium: The Essential Trace Element You Might Not Be Getting Enough Of was published in 1982. Mr Lewis said that selenium could be used to treat rheumatism, arthritis, heart disease, cancer and that it would even delay old age.

Most of these claims appear fanciful and are based on anecdotal evidence, but two are well founded: selenium does prevent certain types of heart condition, and it can ward off cancer - at least if you live in parts of China. However, selenium deficiency is highly unlikely in Britain, says Roger Whitehead, director of the Medical Research Council's Dunn Nutrition Unit at Cambridge and chairman of the panel that in 1991 drew up selenium intake recommendations for the Department of Health.

'There is no doubt that selenium is essential, and it is especially important that pregnant and breast-feeding women, and children, get enough,' says Dr Whitehead. The average Briton takes in about 65 micrograms per day, which is more than adequate; symptoms of selenium deficiency only appear when the intake is below 20 micrograms. Some days our body may lose more selenium than it takes in, but because the average adult holds about 14,000 micrograms this does not pose a threat. We store most of our selenium in our skeleton, but the parts with the highest levels of selenium are hair, liver and testicles.

Britons get more than half their selenium from wheat products such as breakfast cereals and bread. Some foods are rich in selenium: seafoods, such as tuna, cod and salmon; offal, such as liver and kidney; nuts, such as Brazil, cashew and peanuts; wheat germ, bran and brewer's yeast. All contain more than 30 micrograms of selenium per 100g (4oz) although for wheat and meat products the level depends on the soil of the farm from which they came.

Some soil is deficient in selenium, but some has too much, and animals grazing on such pasture may go down with the so-called blind staggers. Marco Polo (1254-1324) wrote that the animals of Turkestan behaved this way. The plant responsible for the staggers was probably milk-vetch (Astragalus) which can concentrate up to 1.4 per cent of its weight as selenium. The cowboys of the Wild West also knew this plant could affect their herds and called it locoweed, from the Spanish for crazy.

In 1975 selenium was proved essential for humans by Yogesh Awasthi who discovered that it was part of a human enzyme, glutathione peroxidase. In 1991 Dietrich Behne at the Hahn-Meitner Institute in Berlin found selenium in another enzyme, deiodinase, which promotes hormone production in the thyroid gland. Essential though selenium is, we can have too much, and the recommended maximum daily intake is 450 micrograms. Above this we risk selenium poisoning, the most obvious symptom of which is extremely foul breath and body odour.

Selenium is a chemical element, atomic number 34, which exists in two forms, a silvery metal or a red powder. It was discovered in 1817 by Jons Jacob Berzelius in Stockholm, Sweden, and he called it after Selene, the Greek name for the moon. The metallic form has the curious property of generating an electric current when light falls on it, and it is used in photoelectric cells and photocopiers. The red form is not used on its own, but in the compound cadmium selenide it makes a striking and permanent red pigment for paints and plastics. Its toxicity, however, has curbed its use in recent years. Selenium is also added to glass to produce a deep ruby colour.

Most selenium comes from copper smelters and refiners. When these sources are exhausted, crops of milk-vetch could be grown on seleniferous soils and we could harvest the mineral at yields of 7kg per hectare (3kg per acre).

The author is science writer in residence in the department of chemistry, Imperial College, London.

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