Behind The Headlines: Coffee's effect on MI risk is genetic

More than three cups of coffee a day raises the risk of MI in some people.

WHAT IS THE STORY?

Drinking more than three cups of coffee increases your risk of a heart attack, according to newspaper reports.

Around a third of the British population might carry the *1F gene and, if they drink more than three cups of coffee a day, their risk of MI increases by over 60 per cent, compared to those without the gene. This is because the *1F gene slows the metabolism of caffeine.

But the media reported that people with another variant of the gene, *1A, benefit from drinking coffee.

People with this allele process caffeine faster and are therefore protected against MI by coffee drinking.

Unfortunately, only laboratory testing can determine which version of the gene a person has. Knowing whether or not coffee has a tendency to keep you awake does not indicate whether you carry the fast- or slow-processing gene.

WHAT IS THE RESEARCH?

The media reports are based on a case-control study that matched 2,014 patients who had a first acute non-fatal MI with 2,014 controls in Costa Rica over a 10-year period.

The *1F allele for slow caffeine metabolism was carried by 55 per cent of the cases, and 54 per cent of the controls.

The researchers analysed coffee-drinking habits and found that *1F carriers who drank more than four cups of coffee a day had a 64 per cent increased risk of MI, compared to carriers who drank less than one cup a day.

The impact of caffeine metabolism on MI risk was most marked in younger people. Conventional risk factors were more important in the older group.

When the researchers looked only at carriers of the *1F allele below the age of 50, drinking four cups of coffee or more increased the risk of MI fourfold, making it a risk comparable to heavy smoking.

But coffee was found to be protective against MI for roughly 45 per cent of people homozygous for the alternative *1A allele. These people metabolised caffeine quickly and four or more cups of coffee reduced their risk of MI by 17 per cent.

WHAT DO THE RESEARCHERS SAY?

Dr Ahmed El-Sohemy, a nutrigenomics researcher at the University of Toronto, told GP that his research could explain conflicting reports of caffeine's effect on the heart.

'Some studies said caffeine was harmful and others that it could be beneficial.

'We hypothesised that if caffeine was harmful, people unable to metabolise it would be affected,' he said.

Dr El-Sohemy added that he thought the effects were to do with caffeine levels in the blood.

Other studies suggest that the effects of caffeine are acute - they only last while it is still circulating in the body.

So if someone with a slow caffeine metabolism cut their caffeine consumption, they would return to a normal risk.

WHAT DO OTHER EXPERTS SAY?

Dr Toni Steer, of the MRC Human Nutrition Research Centre in Cambridge, said: 'The match between the food diaries and food frequency questionnaires means they can definitely say heavy coffee drinkers didn't raise their risk of MI by any other means.'

She added: 'The results for the under-50s were striking, but I suggested this might be a population-specific effect.'

However, Dr Steer added: 'This work doesn't support a drastic change to the Food Standards Agency's current guidelines,' he said.

Professor Peter Weissberg, medical director at the British Heart Foundation, said: 'This research suggests that heavy coffee drinkers who break down caffeine more slowly might have a slightly increased risk of MI.

'But 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,' he said.

JAMA 2006; 295: 1,135-41; www.food.gov.uk; Live links at GPonline.com

INFORMING PATIENTS

- MI risk can be reduced through lifestyle changes such as having a healthy diet and taking exercise.

- The Food Standards Agency recommends no more than five cups of coffee a day, or four if you are pregnant.

- The study's findings are unproven in the UK.

- To avoid dehydration ensure that caffeinated drinks are not your only source of fluid.

WHAT THE PAPERS SAID

"Enjoy that next cup of coffee: it may be your last" - The Times

"Three daily cuppas could stir up a heart attack" - The Scotsman

"Coffee health warning" - The Sun.

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