People who break down caffeine slowly could be increasing their risk of a heart attack if they drink large amounts of coffee, say Canadian researchers. The team, based at the University of Toronto, says that a genetic variation affecting caffeine breakdown in the body could help explain conflicting results on coffee intake and heart attacks. The scientists say their findings, published in the Journal of the American Medical Association, suggest that nobody should drink more than four cups of coffee per day.
Previous research on the links between coffee and health has produced mixed results, with a recent Dutch study suggesting that coffee drinkers have a lower risk of developing heart disease and type 2 diabetes. The Canadian team decided to see if genetic variations could explain different responses to coffee, and the caffeine it contains. They focused on the gene CYP1A2, one member of a 'family' of genes that make liver enzymes responsible for clearing potential toxins from the body. It comes in two different versions: the 'rapid' variety that quickly breaks down caffeine, and a 'slow' form that takes longer to clear the stimulant from the body.
The team studied over 4000 people living in Costa Rica between 1994 and 2004, half of whom had suffered a non-fatal heart attack. They asked the participants to fill out questionnaires about their coffee intake, and analysed DNA samples to see which version of the CYP1A2 gene they had. The scientists found that just over half those studied had inherited one copy of the 'slow' form of the gene, which means they break down caffeine more slowly than individuals with one or two copies of the 'fast' version.
The study showed that for 'slow' caffeine metabolisers, drinking two to three cups of coffee per day increased the risk of a heart attack by 36 per cent, while four or more cups raised the risk by 64 per cent. The scientists also found that for 'fast' metabolisers, drinking two or three cups of coffee actually decreased the chances of a heart attack, although consuming four or more cups per day did not have any further effect on their risk.
Study leader Ahmed El-Sohemy thinks that increased levels of caffeine in the blood could trigger a heart attack by blocking the hormone adenosine, which would cause blood vessels to narrow. However, he stresses that further studies are needed to confirm the link. He also says that how people react to coffee - how 'jittery' it makes them - is not a reliable way of finding out whether they are fast or slow metabolisers. The authors say that since there is no genetic test available, restricting coffee consumption to no more than four cups a day is sensible advice for everyone. Professor Peter Weissberg, of the British Heart Foundation, commented that 'for most people other lifestyle choices, such as smoking, diet and exercise, are far more likely to affect their heart health than the occasional cup of coffee'.