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Caffeinated coffee may come with a blend of short-term benefits and harms, according to a study that monitored its health effects in real time.

Researchers who closely monitored 100 volunteers for two weeks in a randomized controlled trial found participants logged more steps a day on days when they drank coffee than on days when they didn't. And more coffee was associated with fewer episodes of one type of abnormal heart rhythm.

But on days they drank coffee, participants had more incidents of another type of abnormal heartbeat. They also slept less.

"These results highlight the complex relationship between coffee and health," said study author Dr. Gregory Marcus, associate chief of cardiology for research and endowed professor of atrial fibrillation research at the University of California, San Francisco.

"More physical activity, which appears to be prompted by coffee consumption, has numerous health benefits, such as reduced risks of Type 2 diabetes and several cancers, and is associated with greater longevity," Marcus said in a news release. "On the other hand, reduced sleep is associated with a variety of adverse psychiatric, neurologic and cardiovascular outcomes."

The , which is considered preliminary until published in a peer-reviewed journal, was presented this past weekend at the ·¬ÇÑÊÓƵ's virtual Scientific Sessions.

"Coffee is the most commonly consumed beverage in the world, yet its health effects remain uncertain," Marcus said. Most long-term observational studies have suggested multiple potential benefits of drinking coffee, but he called this "the first randomized trial to investigate the real-time, physiologic consequences of coffee consumption."

Study volunteers wore continuous recording monitors to track heart rhythm and blood sugar levels. Wrist-worn devices tracked physical activity and sleep. The participants' average age was 38; 51% were women, and 48% were white.

Over two weeks, they were randomly assigned to either avoid or drink coffee for no more than two consecutive days each. Coffee and espresso consumption were tracked in real time via a "time stamp button" on the heart monitor. Participants completed daily questionnaires to detail how much coffee they drank, and trips to coffee shops were recorded with geotracking.

Compared to days when participants did not drink coffee, coffee consumption was associated with more than 1,000 additional steps per day, 36 fewer minutes of sleep per night, and a 54% increase in episodes of a type of abnormal heartbeat that originates in the lower heart chambers, called premature ventricular contractions. More frequent abnormal beats from the lower chambers increase the risk of heart failure, Marcus said.

Each additional cup of coffee was associated with nearly 600 more steps per day and 18 fewer minutes of sleep a night. But episodes of an abnormally rapid heart rhythm that arise from the upper heart chambers, called supraventricular tachycardia, were 12% less frequent for each additional cup.

Researchers had screened participants' DNA to look for genes that may affect caffeine metabolism. They found people with genes associated with faster caffeine metabolism exhibited more abnormal heartbeats originating in the lower chambers of the heart when more coffee was consumed. The slower a coffee drinker metabolized caffeine, the more sleep they lost.

The study found no differences in glucose levels based on coffee consumption.

Researchers looked at whether changes in exercise or sleep influenced coffee's effects on abnormal heart rhythms. No such association was seen.

Marcus said that because coffee was randomly assigned to the study participants – with repeated assessments of days when each participant drank coffee versus days when they did not – cause-and-effect can be inferred.

Find more news from Scientific Sessions.

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