Genetics may play a role in how much of a java jolt your heart can take, suggests a new study.

The findings, reported in Tuesday's Journal of the American Medical Association, are preliminary, but may help explain conflicting earlier research that found both beneficial and negative effects of coffee on the ticker.

Researchers at the University of Toronto and their colleagues in the United States and Costa Rica said people possess a gene that metabolizes caffeine, and how well the heart takes caffeine depends on the form of that gene.

Four cups of coffee may be heart-health limit: researchers
Four cups of coffee may be heart-health limit: researchers

Canadian researcher Ahmed El-Sohemy, one of the authors of the study, said genetics are increasingly being studied to determine how the body reacts to various foods and drinks.

"We are approaching the era of personalized dietary advice," where genetic tests may help determine what's good and bad for you, El-Sohemy, a professor of nutritional sciences at U of T, said in a release.

Caffeine is found in coffee, tea, chocolate and other products. When it lingers in the body, it may cause blood vessels to constrict and eventually trigger a heart attack, El-Sohemy said.

El-Sohemy and other scientists looked at the genetic profile of more 2,014 men and women in Costa Rica. The average age was 58, and all had had a non-fatal heart attack between 1994 and 2004.

The subjects were compared to a control group of the same number of people who were matched for age, sex and place of residence.

Blood samples were tested to determine what form of the gene subjects carried, and they filled in questionnaires about their caffeinated coffee intake.

Slow and fast metabolizers

About 55 per cent of participants were considered slow coffee metabolizers, carrying the CYP1A2*1F form of the caffeine gene, the team reported.

In the rest of the participants, the stimulant broke down quickly and seemed to reduce their risk of heart attack.

In the slow-metabolizing group, study participants who drank two or more cups of coffee daily were about 36 per cent more likely to have a non-fatal heart attack, compared to those who drank little or no coffee.

And slow metabolizers under age 50 were up to four times more likely to suffer a heart attack than slow metabolizers of the same age who didn't drink much of the beverage.

Among fast metabolizers, those drinking two to three cups of coffee a day showed a 22 per cent reduction in risk of heart attack compared to other fast metabolizers who drank one cup a day or less.

So how much caffeine is too much of a bad or good thing?

The study's authors note there is no commercial test to pinpoint what form of the caffeine metabolism gene a person possesses, but they recommend drinking no more than four cups a day.