Drinking coffee may increase heart attack risk in people whose gene make them slow caffeine metabolizers, showed a study in Costa Rica to be published in Wednesday's Journal of American Medical Association.

Among slow-metabolizers, drinking two or more cups of coffee daily could increase the risk of a nonfatal heart attack by at least 36 percent, and by up to four times for people under 50, in comparison to those who drank little or no coffee in the corresponding age group.

The liver enzyme cytochrome P450 1A2, which is responsible for metabolizing caffeine, has a slow 1F version and a fast 1A version, said the study.

"We found in individuals who had the slow version of this gene, as little as two cups of coffee a day is associated with an increased risk of heart disease," said study co-author Ahmed El-Sohemy of the University of Toronto.

He conducted the study with researchers from Harvard's School of Public Health and the University of Costa Rica.

The study involved 2,014 men and women aged 58 on average who had had a nonfatal heart attack between 1994 and 2004, and another 2,014 healthy people in a control group. Researchers used genetic tests of blood samples to differentiate slow- and fast-metabolizers.

El-Sohemy said the study helped explain why previous studies to probe coffee-drinking effects on the cardiovascular system yielded mixed results.

There is currently no easy way to determine if a person is a fast or slow caffeine metabolizer. The genetic test used in the research is not commercially available.

Study authors recommended to limit coffee drinking to no more than four cups a day.