A daily dose of the mineral selenium may save your life, according to scientific research. Growing evidence says it boosts the body's immune system and helps in the fight against cancer. Even Health Editor Jeremy Laurance is convinced

We know one thing about the vitamin and mineral supplements sold by the sackload in every high street - they produce expensive urine. What the body can't absorb it excretes - and since most people eating a decent balanced diet are already getting a surfeit of most of the vitamins and minerals they need there is an awful lot of cash going down the toilet.

We know one thing about the vitamin and mineral supplements sold by the sackload in every high street - they produce expensive urine. What the body can't absorb it excretes - and since most people eating a decent balanced diet are already getting a surfeit of most of the vitamins and minerals they need there is an awful lot of cash going down the toilet.

Much as Jeremiah Colman is said to have made his fortune from the mustard that people left on the side of their plates, so the supplement makers are making theirs from what their customers do not need. In terms of nutritional status, there can surely never have been a better time to be a sewer rat.

This has been the standard response of sceptics, such as myself, to the claims of the food supplement industry. With few exceptions - those of us, for example, who through ill health or frailty, cannot eat a balanced diet - we do not need their pills and potions, and a pound of oranges is tastier, and better for you, than a tube of vitamin C.

However, I have had to revise my view in relation to one mineral - selenium - in the wake of an eight-page article published in The Lancet last month. It summarises evidence from 80 scientific studies published around the world and explains why we need it and, more importantly, why we are not getting enough of it.

Selenium is a remarkable mineral. It is essential to the proper functioning of the immune system, boosting our defences and resistance to infection. It improves male fertility and prevents miscarriage. Low levels are associated with depression and other mental disorders and high levels help prevent cancer. Deficient levels in the diet may even have played a role in the emergence of new strains of the flu virus in China and the crossing of the HIV virus from animals to man in Central Africa.

You can see why Margaret Rayman of the Centre for Nutrition and Food Safety at the University of Surrey, describes it in her review in The Lancet as "of fundamental importance to human health". One could, of course, say the same about many vitamins and supplements .

But the significance of the claim becomes apparent from the table below which shows that not a single country in Europe has a selenium intake high enough for the population to enjoy the maximum protective effect. In the UK, the average intake of 29-39 micrograms of selenium a day is half the recommended daily amount of 75 micrograms for men and 60 for women.

Selenium enters the food chain through plants which take it up from the soil but in many parts of the world, including Europe, iron and aluminium in the soil reduce uptake. Blood levels of the mineral are lowest in Spain, Greece and Eastern Europe and are generally lower in Europe than in the US because soil levels in Europe are lower. In the UK, intake of selenium has "declined considerably" in the last 25 years, according to Dr Rayman.

We obtain most of our selenium from meat, poultry and fish (the animals eat the plants that take up the mineral from the soil) which together account for over a third of our daily intake. Another fifth comes from bread and cereals made with wheat, which, although not a good source, is so commonly consumed it accounts for a significant proportion of the total.

The best sources of selenium, containing the highest level of the mineral, are brazil nuts, and kidneys. Other good sources are shellfish, especially crab, other fish and liver.

If a diet of brazil nuts and kidneys does not appeal, are supplements the answer? The jury is still out but the evidence is growing. In a study called the Nutritional Prevention of Cancer (NPC) trial in the US 1,300 people who had been treated for skin cancers other than melanoma (the most serious kind) were given selenium supplements or a placebo to see if the mineral reduced the recurrence rate.

It did not but the trial did show that deaths from all kinds of cancer were halved in the group given selenium and there were 63 per cent fewer cancers of the prostate, 58 per cent fewer cases of bowel cancer and 46 per cent fewer cases of lung cancer. The strongest effect was seen in those with the lowest blood level of selenium at the outset, below 106 micrograms per litre.

This trial is now being repeated in three European countries - the UK, Denmark and Sweden - to see if the findings of the US study can be replicated. Blood levels of selenium in 33,000 people, including 11,000 in the UK, are now being measured for what is called the PRECISE trial (Prevention of Cancer by Intervention with Selenium) of which the UK arm is being run by Dr Rayman. The effect of the mineral on mood and quality of life is also being studied.

The impact of supplements is expected to be greatest on those with the lowest levels of selenium in their blood. But even for those with relatively high levels, supplementation appears to have beneficial effects. One study in the US showed that people with natural levels between 120 and 134 micrograms per litre who took supplements of 200 micrograms a day showed evidence of "considerable immunoenhancing effects" - that is, a stronger immune system. Another study showed a near doubling in the activity of killer cells in the immune system.

Low levels of selenium appear to allow otherwise harmless viruses to become virulent and this could be a factor in the spread of infection. In experiments on mice, the mineral has been shown to be protective against heart damage caused by the coxsackie virus and if other viruses were implicated, such as polio, hepatitis, flu or HIV "there would be considerable public health implications," Dr Rayman says.

She adds: "The steady emergence of new strains of influenza in China [origin of most of the flu epidemics that subsequently travel round the world] with its selenium deficient belt or the first crossing over of HIV to human beings in the selenium-deficient population of Zaire, might also be explained."

If the results of the PRECISE trial are positive, and the cancer-preventive effect of selenium is confirmed, it could do more than trigger a rush for the supplement. With clear evidence that our national diet is deficient in the mineral more radical action may be necessary. In Finland, where selenium intakes were very low, the mineral has been added to fertilisers since 1984 to boost levels in the population.

The UK government is consulting on plans to require all flour to be fortified with folic acid to prevent the birth of spina bifida babies. Adding selenium to the food supply might one day cut cancer rates, boost immunity - and cheat the supplement makers of their market.

For those tempted to stock up on supplements, Dr Rayman adds a word of warning: selenium can be toxic in overdose. In some sensitive individuals the maximum safe intake may be as low as 600 micrograms a day, she says. "It would seem prudent to restrict adult daily intake to a maximum of 400 to 450 micrograms per day as recommended by several expert panels," she says.