Bad news for young coffee lovers: Gulping down lots of your favourite pick-me-up might raise your risk of heart attack if you've already got high blood pressure, a new study suggests.
Italian investigators looked at 12 years of heart health data for a group of about 1,200 adults ages 18 to 45. This type of study design means the research could only point to an association, not cause and effect.
But experts said the findings might be worth bearing in mind.
"Although some limitations exist with this type of study, this association – especially in heavier coffee drinkers – cannot be totally discounted," said one U.S. expert, cardiologist Dr David Friedman.
Read: Genes may determine how much coffee you drink
"Patients with known heart disease who are at elevated risk should limit their intake of daily caffeine products," said Friedman, who is chief of heart failure services at North Shore-LIJ's Franklin Hospital in Valley Stream, New York.
The new research was presented in London at the annual meeting of the European Society of Cardiology. Experts note that findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.
The new study was led by Dr Lucio Mos, a cardiologist at Hospital of San Daniele del Friuli in Udine, Italy. His team tracked about 1,200 young adults with untreated stage 1 high blood pressure. That means they had systolic blood pressure (the top number of a reading) between 140 and 159 mm Hg and/or diastolic blood pressure (the bottom number) between 90 and 99 mm Hg.
Over 12 years, there were 60 heart or circulation-related events – incidents such as heart attacks or strokes – among the participants. About 80 percent were heart attacks, while the others included strokes, peripheral artery disease (poor circulation in the legs) and kidney failure.
Compared to those who didn't drink coffee, the risk of these events was four times higher among the heavy coffee drinkers and three times higher in moderate coffee drinkers.
Moderate coffee drinking was defined as one to three cups of the caffeinated beverage per day, while "heavy" consumption was defined as four or more cups daily.
Amount of coffee important
Drinking coffee also increased the risk of "prediabetes" – a precursor to type 2 diabetes – in some of the participants. This risk is linked to factors such as genetics and being overweight and obese, according to the study.
In a meeting news release, Mos acknowledged that "there is controversy surrounding the long-term cardiovascular and metabolic effects of coffee consumption in patients with hypertension."
He believes the new findings point to a "linear" relationship between coffee drinking and heart risks in younger adults with high blood pressure, meaning that the risks rise along with the amount of coffee consumed.
As to why that might be so, Mos said that it could be linked to coffee's effects on blood pressure and/or blood sugar.
Read: Is coffee sabotaging your diet?
For his part, Friedman said that differences in each person's processing of caffeine might play a role, too. "There was an association [in the study] with those patients having higher risk of development of heart problems who were also seen to be so-called 'slow caffeine metabolisers,'" he said.
Dr Kevin Marzo is chief of cardiology at Winthrop-University Hospital in Mineola, New York. He said that "the health effects of coffee have been controversial, with some studies suggesting benefit, and some, like the most recent in young hypertensive adults, showing potential harm by raising blood pressure."
He believes that, based on the findings, "physicians may want to encourage a healthy lifestyle with regular exercise, good nutrition and keeping the coffee consumption to less than three cups per day" in younger adults with high blood pressure.
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