Drinking large amounts of coffee could increase the risk of a heart attack in some people but reduce it in others, researchers said today.

People with a genetic trait meaning their bodies are slow to break down caffeine were more than two-thirds more likely to suffer cardiac arrest if they drank four or more cups of coffee a day, a study found.

However, the study of 4,028 people, carried out in Costa Rica, also found that those with the opposite genetic trait - so-called "fast caffeine metabolisers" - appeared to reduce their chances of suffering a heart attack by drinking coffee.

The research, published in the Journal of the American Medical Association, also found age was another risk factor.

Slow caffeine metabolisers who were aged under 50 and drank coffee were up to four times more likely to have a heart attack than young slow metabolisers who drank little or no coffee.

The study looked at 2,014 men and women who had suffered a non-fatal heart attack between 1994 and 2004 and a control group of 2,014 healthy men and women.

Genetic tests were used to determine whether the participants were slow or fast caffeine metabolisers, and they were then given questionnaires to assess their coffee consumption.

"It appears that one cup a day is not associated with any harm, regardless of your genetic make-up," Ahmed el-Sohemy of the University of Toronto, one of the study's co-authors, told the Times.

"There may be some people in the population for whom several cups a day may not be harmful - but until such exceptions have been identified, moderation would appear to be best."