Just when medicine had re-established cholesterol as the major cause of heart attack, new research has thrown up a new culprit that nobody had considered before, or possibly had even heard of homocysteine.

Homocysteine is an amino acid that is found in higher levels among patents who have suffered strokes or other heart diseases. It was first identified in research back in 1969 (Am J Pathol, 1969; 56: 111-28) when extraordinarily high levels of homocysteine were discovered in postmortem findings of patients suffering from sclerosis, or hardening of the arteries. Any link was dismissed because the levels were due to rare metabolic rates among the study's subjects.

But later research, in particular a study carried out by P M Ueland et al in 1992 (Marcel Dekker, 1992: 183-236), discovered that homocysteine could have an effect at far lower levels. Consistent findings among 2,000 case studies suggest that levels of homocysteine only slightly above normal ranges could have a bearing.

Two more recent studies have produced similar results, suggesting that high levels of homocysteine came first, rather than being raised by the condition. One of these, carried out in Oslo, Norway in 1993, found that men whose homocysteine levels were in the highest 5 per cent and three times as likely to suffer a heart attack. This discovery was endorsed by the influential Physicians' Health Study (JAMA, 1992; 268: 877-81).

Animal experiments with baboons injected with the amino acid for three months also showed a link to heart disease.

Homocysteine levels tend to rise when there is a deficiency of folic acid. This, coupled with B6 and B12, seems to control its metabolic rate of growth. However, the level of folic acid needed to control homocysteine is about twice the level presently recommended in the US. (Ironically, the amount needed was the daily intake level that used to be recommended before publicity about the dangers of too high levels).

Folate supplements of between 1 and 2 mg a day are sufficient to reduce high homocysteine levels.

Dr Meir Stampfer of Harvard School of Public Health, who headed research into the subject in 1992, is now calling for a major and immediate study into the issue. Writing in the New England Journal of Medicine (2 February 1995), he says a major overhaul of heart treatment may have to be made.