Light coffee drinkers who normally have no more than one a day are twice as likely to suffer a blood clot on the brain if they increase that by an extra cup or two, it found.
The risk of a potentially fatal stroke was greatest in the hour following consumption. After two hours, the coffee’s effects had worn off and the risk of a stroke passed.
But the danger lies in occasional exposure to relatively higher caffeine levels, the findings in the journal Neurology said – making light coffee drinkers more susceptible.
This could be because coffee-lovers who get through several cups a day become ‘desensitised’ to the effects of caffeine, such as raised blood pressure, stiffening of the arteries and higher norepinephrine levels, a stress hormone that increases heart rate.
Experts at Harvard Medical School in Boston studied 400 stroke victims, comparing each one’s coffee intake in the hour before their stroke with their usual consumption over the previous year.
Around one in ten had drunk coffee less than an hour before falling ill.
They said: ‘Consumption was linked with a risk of stroke in the subsequent hour twice as high as during the periods where there was no coffee consumption.’
They found no such association with caffeinated tea or cola.The risk is mainly confined to those who exceed their normal intake of a cup or so a day.
Strokes kill around 200 people every day in the UK. A Stroke Association spokesman said: ‘Caffeine intake can contribute to increased blood pressure, the single biggest risk factor for stroke, but we have known for some time that antioxidants found in certain foods and drinks, including coffee, can help in the prevention of stroke.’