The findings of a new medical study might be enough to make you spit out your morning coffee.
And that could be a good thing -- at least for half of you.
A study of 4,000 coffee drinkers has found that two or more cups each day can increase the risk of heart disease -- but only for those with a genetic mutation that slows the breakdown of caffeine in the body.
In diverse urban areas, the mutation is found in 54% of the population.
People without the mutation can drink as much coffee as they like with no added risk of a heart attack, the scientists said.
Unfortunately, there's no commercial test for the mutation, which now puts coffee die-hards in a bit of a quandary.
"I don't know what to think," said Donna Carmichael, 50, as she waited for a train at Union Station.
"How do you know if you've got the gene?" she asked, adding that she usually drinks about three cups of coffee a day. "If it's genetic, though, who's going to go out and get tested?"
The findings, published today in the Journal of the American Medical Assn., could explain why previous investigations into caffeine's effect on the heart have produced conflicting results. Some reports have tied coffee to an increase in heart disease, though others have found no effect.
Ahmed El-Sohemy, a University of Toronto scientist involved in the study, said earlier reports failed to account for the genetic differences among people.
"One size does not fit all," he said.
People who metabolized caffeine slowly and drank two to three cups of coffee each day had a 32% higher risk of heart attack, according to the study. Those consuming four cups or more had a 64% greater risk.
A single cup of coffee had no effect on heart attack risk, researchers found.
How this will affect America's addiction to caffeine remains to be seen.
Coffee drinkers in the U.S. -- about half of all adults -- have an average of three cups daily and spend more than $17 billion a year keeping themselves fully caffeinated.
"That's crazy," said Sophia Guevara on being told about the new research while waiting for a train at Union Station.
"I'll definitely cut back on coffee now," said the Santa Monica resident, who usually drinks about two cups a day. "I'm 35. I have kids. So it's something to do for my health and for them too."
Her son Danny, 13, started talking about how he likes caramel macchiatos and his mother cut in.
"I don't think you're ever going to start drinking coffee," she said.
Others aren't ready to kick their habit.
Stephen Seeser, 57, and his wife, Ruth, 53, said they weren't particularly surprised by the study as they waited for their train to New Mexico.
"Almost everything you have is bad for you or will kill you one way or another," said Stephen Seeser, who said he drank four to six cups of coffee a day.
"If it's not coffee, it's the kids," his wife said.
Starbucks Corp., the Seattle company that has built a global $6-billion empire on coffee, had no comment on the study.
The findings are "interesting, plausible and worthy of further study," said Dr. David Robertson, a professor of medicine and pharmacology at Vanderbilt University.
But he said the results needed to be confirmed through additional research and that consumers shouldn't worry about changing their coffee-drinking habits.
"I think we should keep in mind that coffee has been studied more than any other drug," he said. "This is not something that should scare people to death about coffee."
Caffeine's possible effect on heart disease has been studied for years because the chemical is a stimulant that causes blood vessels to constrict, interfering with their normal function.
Whatever the risk attributable to caffeine, the overall risk of heart attack is small, ranging from 2% to 5% a year among elderly Americans, depending on age, race and gender.
The new research, which involved scientists from the United States and Costa Rica, compared 2,014 men and women in Costa Rica who had recovered from a first heart attack with an equal number of healthy adults. Genetic tests determined whether participants had the gene for breaking caffeine down slowly.
The study was conducted in Costa Rica because health records for a large number of patients were available and one of the researchers was there on another study.
The gene, CY1A2, was first identified in the 1990s. Everyone has two copies of the gene. People with a mutation in one or both copies will break down caffeine slowly.
The "slow metabolizers" broke down caffeine four times more slowly than people without the mutation, El-Sohemy said. The longer caffeine remains the blood stream, the greater potential for harm, he said.
Women with the defective gene had a higher risk than men, and younger adults -- those younger than 59 -- faced worse odds than older coffee drinkers with the gene mutation.
El-Sohemy said that coffee was less of a factor in the elderly because they faced multiple cardiac risks, including weight gain, higher cholesterol and other problems that would obscure the effects of caffeine.
The study didn't look at the effects of any other caffeinated beverages or of decaffeinated coffee.
"Decaf differs from caffeinated coffee by more than just the caffeine content -- that's partly why it tastes different -- so we can't conclude what, if any, effects decaf might have in those with different genotypes," El-Sohemy said.
Not that decaf is much of an option for "real" coffee drinkers.
"I've given up alcohol, cigarettes and sugar," said Ruth Seeser, who said she drank about six cups a day. "I gotta have something.
"If I didn't have coffee, I'd definitely have a heart attack," she said, clutching the foam cup of coffee steaming in her hand.
Times staff writer Alan Zarembo contributed to this report.