That morning cup of coffee may be enough to trigger a first-time heart attack in vulnerable people, a study suggests.

Researchers found that among middle-aged and older adults, light to moderate coffee drinkers had an elevated risk of heart attack in the hour after having a cup of coffee.

This was particularly the case when a coffee drinker got little regular exercise or had three or more risk factors for heart disease - such as diabetes, high blood pressure and smoking.

On the other hand, there was no risk of having a heart attack among people who were heavy coffee drinkers, downing four or more cups per day. The difference, according to the researchers, may be that these coffee devotees build up a tolerance to the cardiovascular effects of caffeine.

The findings suggest that light or moderate coffee drinkers who are already at risk of heart attack should consider giving up the beverage, according to lead study author Dr. Ana Baylin.

For them, that morning cup could become “the straw that broke the camel’s back,” Baylin, of Brown University in Providence, Rhode Island, said in a statement.

The findings, published in the journal Epidemiology, will likely add to the confusion that’s been brewing about the health effects of coffee.

For example, a recent large study of U.S. adults found no association between heavy consumption of coffee - at least the filtered kind - and heart disease. But another study suggested that the heart effects of coffee might depend on genetics; researchers found an elevated heart attack risk only among coffee drinkers with a particular variant of a gene that helps metabolize caffeine.

The current study included 503 adults from Costa Rica who suffered a first heart attack between 1994 and 1998. Baylin and her colleagues questioned them about their coffee intake and overall diet in the hours and days before the heart attack. They also collected information on patients’ medical history, background and lifestyle habits.

The researchers found that among those who typically drank less than one cup of coffee a day, the risk of heart attack in the hour after drinking a cup was four times higher than would be expected.

Similarly, moderate coffee drinkers - those who had two or three cups per day - showed a 60 percent higher-than-expected risk.

According to Baylin’s team, that jolt of caffeine may boost nervous system activity and blood pressure enough to rupture a pre-existing fatty deposit, or plaque, on the artery walls. When this happens, blood flow to the heart can be obstructed, causing a heart attack.

The researchers do acknowledge the limitations of their study, including the fact that patients were asked to recall their coffee intake in relation to the heart attack. In addition, it’s well known that heart attack risk naturally rises in the morning, a time when most coffee drinkers have at least one cup.

However, Baylin and her colleagues note, that would not explain why heavy coffee drinkers seem immune to the effects. Instead, they suspect that heavy consumers develop a complete tolerance to the short-term cardiovascular effects of caffeine, whereas light to moderate drinkers remain susceptible.

SOURCE: Epidemiology, September 2006.