The following correction was printed in the Guardian's Corrections and Clarifications column, Thursday June 12 2003

A report on homocysteine and the part it plays in heart disease, Alzheimer's and cancer quoted research published in the New English Journal of Medicine when we meant the highly respected New England Journal of Medicine.

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Only a couple of years ago, medical dictionaries had no entry for homocysteine. But today the amino acid is widely recognised as increasing the risk of heart disease and possibly several other illnesses including Alzheimer's and diabetes. And despite the recent debate over the dangers of some vitamin supplements, a book published later this month provides extensive evidence that supplementary B vitamins can dramatically reduce levels of homocysteine.

These vitamins are the only effective way of lowering the amount of homocysteine in your bloodstream. Researchers have known for years that high levels are linked with an increased risk of heart attack, but GPs in this country almost never test for it. This is partly because a quick and easy test has only recently become available, but also because there is no drug on the market that can lower it.

The links between cholesterol and heart disease are well known, and drugs to reduce it are a billion-dollar business; but there is far less widespread recognition of the dangers of homocysteine. As is revealed in the book The H Factor Diet (Piatkus), evidence is mounting that this amino acid is also linked with Alzheimer's, diabetes and cancer. "The link with heart disease is just the tip of the iceberg," claim the authors, Patrick Holford and Dr James Braly. "The discovery of homocysteine identified a fundamental cause and biological marker of ageing."

One of their key pieces of evidence is a study two years ago by researchers at Bergen University in Norway. The scientists measured the blood levels of homocysteine in 4,766 people aged 65 to 67, then recorded their death rates over a four-year period. They found that a five-point increase in homocysteine was associated with a 50% rise in the risk of death from cardiovascular causes, a 26% increase in the risk of cancer and a 104% boost in the risk of all non-cancer, non-cardiovascular causes of death. "These results should encourage studies of [homocysteine] in a wider perspective than one confined to cardiovascular disease," they concluded.

The original link between homocysteine and heart disease was made more than 30 years ago. In 1969 Kilmer McCully, a researcher at Harvard, was studying an obscure single-gene disorder called homocystinuria, sufferers of which were unable to remove homocysteine from the bloodstream. McCully published a paper suggesting that homocysteine could be linked to heart attacks, but the medical establishment, then committed to the notion that high fat was the cause, didn't want to know. He was denied further funding and lost his job. It was 20 years before he was vindicated, when a study of male doctors in the Journal of the American Medical Association showed that a homocysteine level of above 12 points was linked with a threefold increase in the risk of heart attack.

By the end of the 90s it was clear that there was a connection between homocysteine and heart disease and that people with lower levels of the substance had higher levels of B6, B12 and folic acid. But why should high homocysteine be so damaging, and why are B vitamins so effective at bringing it down? One way of getting a grip on what is going on is to think of cooking. Our metabolism is a vastly more complicated version of making a cake. To get from ingredients to product, you have to go through a halfway house - the dough. You might ask: why do we make homocysteine if it is so dangerous? The answer is that it is an inbetween stage, a dough. When you are healthy, homocysteine levels stay low because it is quickly turned into either a valuable antioxidant called glutathione or the chemical SAMe, which has, among other things, an antidepressant effect. This is why high homocysteine is such a good marker that something is wrong. If you are supposed to be making cakes and your kitchen is full of dough, it's worth checking that your oven is OK.

But where do the vitamins come in? They are the bicarbonate of soda or the dash of milk that make the process work properly. Turning homocysteine into something else requires enzymes, and these don't work properly without the right B vitamins.

Reports of the value of B vitamins have often concentrated on folate or B12, but a more varied combination is needed. Most of the research on homocysteine so far has been done on the links with heart disease, but The H Factor Diet shows that high levels increase the risk of a wide range of disorders. For instance, overweight kids with high insulin levels also have high levels of homocysteine, and patients with levels of over 13 are more likely to have signs of "silent" damage on a brain scan. Last year a study of more than 1,000 people in the New English Journal of Medicine concluded: "An increased plasma homocysteine level is a strong, independent risk factor for the development of dementia and Alzheimer's."

So how low should your homocysteine go? It is measured in micromoles per litre (Imol/l) and most experts agree that a score of over 11 or 12 is dangerous and that around nine is healthy. However, Holford and Braly make a case for saying that you should aim for around six. With the correct diet and supplements, this is normally achievable within a couple of months.

Specific features of a healthy diet include fish at least three times a week, lean wild - rather than intensively reared - meat no more than three times a week, and five servings (1lb) of fruit and vegetables a day. Limit wine, preferably red, to four glasses a week, and tea or coffee to two cups a day. "We generally don't get enough of the nutrients from our diet to get homocysteine down to a low level," say Braly and Holford. "The average intake of zinc, for instance, is only half the RDA, while most people get around 240mcg of folate when healthy people need 400mcg. If your homocysteine level is high, you can benefit from 2,000mcg of folate or more."

Men particularly benefit from vitamin supplements (they naturally have homocysteine levels about 20% higher than women), as do teenagers and the elderly. Strict vegetarians are at risk of raised homocysteine levels because you get B12 from meat, dairy products and seafood, and so may benefit more from supplements. B6 and folic acid are, however, abundant in vegetables.

· Details of a new test to assess your homocysteine levels can be obtained from British Cardiac Patients Association 020-8289 5591. This is an edited version of an article that appears in the May edition of the monthly newsletter Medicine Today.