Coffee drinkers get more steps but also less sleep, study finds
Coffee is one of the most consumed beverages worldwide, but the pendulum has swung back and forth about its benefits and drawbacks.
New findings from a small study published Thursday in The New England Journal of Medicine, suggest both upsides and downsides: Drinking at least a cup of coffee per day might make you move more but sleep less — and it might put you at higher risk for one type of heart palpitation.
"The big picture finding is that there isn't just one single health-related consequence of consuming coffee, but that the reality is more complicated than that," said the study's lead author, Dr. Gregory Marcus, a cardiologist and professor of medicine at the University of California, San Francisco.
"The great majority of research on the topic has been observational, meaning we just look and see at what happens to people who do and don't drink coffee, which is profoundly limited by the possibility that ... there may be some other characteristic that is driving whether someone happens to drink coffee," Marcus said. "The only way to mitigate those potential effects was to conduct a randomized interventional trial."
To get a better idea of coffee's immediate health effects, the authors recruited 100 healthy adults who were age 39 on average and from the San Francisco area. They equipped the participants with Fitbits to track their steps and sleep, continuous blood glucose monitors and electrocardiogram devices that tracked their heart rhythms. Each participant was randomly assigned to drink as much coffee as they wanted for two days, then abstain for two days, repeating that cycle over a two-week period.
On coffee-drinking days, participants got an average of 1,058 more steps than they did on abstention days, the authors found. But on those days, sleep took a hit, with participants getting 36 fewer minutes of shut-eye. The more coffee they drank, the more physical activity and the less sleep they got.
Coffee seemed to affect the heart, too. Researchers found no evidence of a significant relationship between coffee consumption and premature atrial contractions, which are "very common, early heartbeats that we all experience arising from the top chambers of the heart," Marcus said. They can feel like a flutter or skipped beat in your chest.
"People with more premature atrial contractions are at higher risk of developing a very clinically significant heart rhythm disturbance called atrial fibrillation," he added.
But drinking more than one cup per day resulted in about a 50% higher incidence of premature ventricular contractions, or PVCs, compared with days of no coffee intake.
These heartbeats arise from the lower chambers of the heart, and they can also feel like a skipped beat or heart palpitations.
"So this provides some compelling evidence that experimenting with (going) off coffee might be worthwhile in those individuals who experience bothersome palpitations related to PVCs," Marcus said.
"There's also evidence that in some people, more PVCs can lead to a weakening of the heart or heart failure," Marcus added. "So it may be that if someone is especially concerned about risks of heart failure — such as they have a family history of it or there's some other indication that their physician tells them (makes) them at risk — they may want to steer clear of coffee."
Peter Kistler, who is head of electrophysiology at the Alfred Hospital in Melbourne, Australia, described the study as strong but cautioned that "this is a short-term study in healthy volunteers."
"This provides no information on the long-term benefits or adverse effects of long-term coffee drinking," said Kistler, who was not involved in the study. "This does not provide information on the impact of coffee in people with other health conditions, and generally (the study participants) consumed modest amounts of coffee."
Health effects of coffee
When people drink coffee, they might have more motivation to exercise or have enhanced performance once they start moving, Marcus said.
But people "shouldn't extrapolate that to taking energy drinks or high-dose caffeine as a way to enhance workouts," since high doses can lead to disturbances, he said.
That coffee drinking led to less sleep perhaps isn't surprising, but a potential genetic aspect to that finding might be. Researchers collected DNA samples from participants, and those who had greater reductions in sleep when they consumed coffee had genetic variants associated with slower caffeine metabolism. People with genetic variants associated with faster caffeine metabolism, on the other hand, had more premature ventricular contractions.
These findings suggest that an individualized approach to coffee consumption might be the most appropriate method for determining effects on health, according to the study.
Kistler had another view on the finding of diminished sleep.
"Coffee is the most common 'drug' for cognitive enhancement," he said. "People who drink coffee are less tired. This is not necessarily a negative."
Regarding the links between coffee intake and premature ventricular contractions, caffeine can contain active metabolites such as aminophylline, which is used in asthma medications and, in high doses, is known to induce arrhythmias, Marcus said.
Additionally, "coffee tends to increase activity in the sympathetic nervous system, or the adrenaline side of the nervous system, that can promote PVCs," he added.
What this means for you
The effects the study found are worth considering based on your personal health goals, Marcus said.
"Individuals can be reassured that there's certainly no imminently dangerous effects of drinking coffee," Marcus said.
Wondering whether you're a fast or slow metabolizer of caffeine? There aren't really any clinical tests in the consumer market, Marcus said, but you might be able to find out by using a DNA test that measures this.
You can also pay close attention to what your experiences might be telling you about your tolerance.
"If they begin to feel anxious (and) tremulous on one cup of coffee, then they are a slow metabolizer," Kistler said via email. "But if they have a higher tolerance then they are metabolizing coffee more rapidly."
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