March 7, 2006 -- Drinking a lot of coffee may raise the risk of heart attack in people with a particular gene variation.
That finding comes from researchers including Ahmed El-Sohemy, PhD, of the University of Toronto's nutritional sciences department.
However, drinking one cup of coffee per day shouldn't raise heart risks, El-Sohemy tells WebMD. However, his study -- published in The Journal of the American Medical Association -- doesn't make recommendations about coffee consumption.
"At this point, I don't think most people would make recommendations on the basis of a single study," Tufts University nutrition professor Alice Lichtenstein, DSc, tells WebMD.
"I think it's an interesting study," says Lichtenstein, who studies lifestyle factors that affect heart disease. "It's certainly very well done, but it raises a lot of questions." She didn't work on El-Sohemy's study.
How Much Coffee?
Participants, who all lived in Costa Rica, completed surveys about their diets, including coffee and other caffeine sources (chocolate, tea, and cola beverages). The researchers focused on people who drank fully caffeinated coffee.
Participants rated how much coffee they drank, ranging up to more than six daily cups. They also noted how they made their coffee; almost all drank paper-filtered coffee.
The researchers also screened participants for a gene that governs the metabolism, or breakdown, of caffeine. About half of both groups of participants had the "slow" variant of that gene, meaning that caffeine lingers longer in their blood.
Heart Attack Findings
Coffee intake was linked to higher risk of heart attack in people with slow caffeine metabolism and who were younger than 59, the study shows.
The study doesn't prove that caffeine caused any heart attacks. Also, coffee intake was actually linked to a lower risk of heart attack in people with fast caffeine metabolism.
"We find that the increased risk is in only those who cannot get rid of it very quickly," says El-Sohemy. He and his colleagues found "absolutely no increased risk in fast caffeine metabolizers. "In fact, there was even a protective effect [for those people], at moderate intakes," El-Sohemy says.
"I think one of the take-home messages is that not everybody responds in the same manner," he observes. "We apply a one-size-fits-all model to advice that we give, but what we're finding is that some people can respond in the opposite direction than others."
"In terms of the specifics of coffee and heart disease, it appears that drinking one cup a day is not associated with any harmful effects, regardless of what a person's genetic makeup is," El-Sohemy says. "And drinking four or more certainly doesn't have any beneficial effects for anybody" -- at least, in this particular study.
Are You a Slow Metabolizer?
Past studies on coffee, caffeine, and heart disease have been "all over the map," El-Sohemy says.
"We reason that if caffeine is in fact bad for you, then maybe [in] people who cannot get rid of it as efficiently we would expect to see more harmful effects in those individuals, and that's exactly what we saw."
The gene test isn't commercially available, El-Sohemy notes. He cautions that people can't tell if they're fast or slow caffeine metabolizers without the test.
"Some people say to me when I present this, 'Oh, I must be a slow metabolizer, because if I have a cup of coffee in the afternoon, I'm up all night,'" says El-Sohemy. However, "although an individual can feel the effects of caffeine on the nervous system, the stimulating effect or the anxiety effect, they cannot feel it just circulating in their blood."
How common is the slow-gene variant? "We don't know the exact prevalence in all the different populations around the world, but there's evidence to suggest that it is quite common, even in a typical North American population," says El-Sohemy.
Pros and Cons of Coffee
El-Sohemy, who drinks a daily cup of coffee, isn't slamming coffee and caffeine.
"One has to balance the risks of heart disease and the risks of other health outcomes, because coffee and caffeine have been associated with a number of different health outcomes, both good and bad," he says.
For instance, he says caffeine has been linked to a lower risk of Parkinson's disease. Caffeine's effects on blood pressure has also been "controversial,' he adds. "There's some indication that it causes a transient increase, but in habitual drinkers, it might actually be associated with a lower blood pressure."
"But even with all these studies now, we have to ask the question, 'Well, what was the genetic makeup of those subjects in those previous studies relating caffeine and heart disease and blood pressure, and any health outcome for that matter?'" El-Sohemy asks.
What About Tea?
Coffee contains lots of other compounds besides caffeine. So does tea, another caffeine source. That complexity can complicate research. Those compounds weren't part of this study.
Why not just compare drinkers of caffeinated and decaf coffee? "That's very difficult, because the decaffeination process gets rid of more than just caffeine," El-Sohemy says.
Also, "people who purposely drink decaf are very different people, so there are other confounders. They have, maybe, a healthier lifestyle and other issues that make it very difficult to understand," he says.
Role of Genetics
Genetics may be one reason why studies on heart disease and nutrition sometimes yield mixed results, says El-Sohemy.
"It may not have been, necessarily, because studies were flawed, as we've often argued. It could very well be that it's just the genetics of the people that were studied that are different."
"I think we need to move toward incorporating genetic markers into these studies to better understand what exactly is going on, who are the exceptions, and what are the rules that we should be using to be guidelines," says El-Sohemy.
Lichtenstein says more research is needed to see why the pattern was strongest in younger participants, and what contribution, if any, came from caffeine sources besides coffee.
"They did state that the coffee was the major contributor of the caffeine, but you don't know if it's 51% or 99% of the caffeine," Lichtenstein points out. She adds that although the researchers made adjustments for other factors, it's hard to do that perfectly.
"I think people need to look at the data quite seriously, and then they need to see what's going on in some other populations that have similar data, and also to look a little more closely at all sources of caffeine," Lichtenstein says.