A new study has some heartening news for coffee lovers: That morning cup is unlikely to make your heart skip a beat.
The study, published March 23 in the New England Journal of Medicine, found that drinking coffee does not seem to predispose healthy people to premature atrial contractions.
PACs are a normal occurrence for a healthy heart, but some people sense them as a "skipped" or extra heartbeat — which can be unsettling. And if they do arise frequently, that might foretell a more serious heart arrhythmia down the road.
Experts said the new findings are in line with the body of research on coffee and heart health: Despite caffeine's bad reputation, many studies have found that coffee drinkers have lower risks of various diseases — including heart disease — than non-drinkers.
But while those past studies have offered reassurance, they also had limitations, said first study author Dr. Gregory Marcus, a cardiologist and professor at the University of California, San Francisco.
There could be many differences between coffee drinkers and abstainers that affect their disease risks, Marcus said. And even though studies try to control for those differences, he added, it's impossible to account for everything.
So Marcus and his colleagues conducted a clinical trial to actually test whether coffee can have particular acute effects on the heart. Their primary question was whether a person's usual cup (or two or three) can boost the number of PACs the heart experiences.
PACs happen during moments when the heart's upper chambers (the atria) contract a bit too soon, when there may not be much blood in them. That, in turn, may be followed by a pause and a stronger-than-usual contraction.
While that feeling may be unnerving, Marcus said, PACs are a normal part of human heart function.
"We all have these," he said. "If you put a heart monitor on someone walking down the street, you'd find PACs."
That said, recent research suggests that older adults with a relatively high frequency of PACs are at increased risk of developing atrial fibrillation — a heart arrhythmia where the atria contract erratically instead of maintaining a steady beat.
A-fib is not immediately life-threatening, but it can cause distressing symptoms like heart palpitations and dizziness. And over time, it can raise the risk of heart failure or stroke.
So it's important, Marcus said, to find out whether a daily habit like coffee drinking affects PAC frequency.
Based on his team's findings, the answer may be no.
The researchers recruited 100 healthy adults who were moderate coffee drinkers — no more than one cup a day in most cases.
Over two weeks, participants were randomly assigned to have caffeinated coffee on certain days, and to avoid caffeine altogether on other days. The whole time, they wore an electrocardiogram patch that recorded the heart's electrical activity.
Overall, the study found, people averaged a similar number of PACs on coffee days as they did on no-caffeine days: 58 and 53, respectively.
There was a small difference when it came to premature ventricular contractions, which involve the heart's lower chambers. On coffee days, people averaged 154 premature ventricular contractions, versus 102 on caffeine-free days.
Premature ventricular contractions, when frequent, may raise the risk of chronic heart failure, based on past research.
However, a cardiologist who was not involved in the trial said it's important to see things in context.
"Most people have around 100,000 heartbeats per day," said Dr. David Kao, an associate professor of medicine and cardiology at the University of Colorado School of Medicine.
Doing the math, he said, on days when trial participants drank coffee, about 0.15% of their heartbeats involved a premature ventricular contraction — versus 0.1% on no-caffeine days.
Kao said the study is unique in that it's a clinical trial testing some acute effects of coffee. And the bottom line, he said, is that it supports what large population studies suggest: Coffee is, at least, not harmful to heart health.
The trial did look at a couple of additional outcomes, and found good and not-so-good news.
On coffee days, people were a little more physically active (as recorded by Fitbit devices). On the flip side, they got about a half-hour less sleep at night.
To Marcus, the findings mean that coffee lovers need to tailor their choices. If you have trouble sleeping, he said, try limiting yourself to that morning cup. If you need help to get moving, try having your coffee an hour before you plan to be active.
For people who do feel bothered by heartbeat changes when they drink coffee, Marcus suggested refraining from the habit and seeing what happens.
"But overall," he said, "I think these findings are reassuring."
Marcus did caution against downing highly caffeinated energy drinks as exercise motivation, however.
"Sometimes people think that if a little is good, a lot must be better," he noted.
But, in fact, Marcus said, energy drinks have been known to trigger arrhythmias in otherwise healthy young people.
According to the U.S. Food and Drug Administration, healthy adults can safely have up to 400 milligrams of caffeine a day — the amount in four to five cups of coffee.