The Day

Study: Coffee lovers gain 1,000 steps a day but get less sleep


A rigorous new study that examined the health effects of coffee consumptio­n found good news and bad news for coffee lovers.

The research showed that coffee has striking effects on physical activity levels, causing people to move more, taking, on average, 1,000 extra steps a day — a significan­t boost in activity that might help explain why coffee consumptio­n has long been linked to better health.

But the study, published in the New England Journal of Medicine, did find some downsides to a daily cuppa. It showed that people lost about 36 minutes of nightly sleep on days when they drank coffee — and the more coffee they drank, the less they slept.

The research also looked at coffee’s effect on heart palpitatio­ns, a relatively common experience for healthy coffee drinkers. The study found that in healthy men and women, coffee did not cause a common type of palpitatio­n known as premature atrial contractio­ns, even though some health authoritie­s have warned that this could be a side effect of drinking coffee.

But coffee consumptio­n can lead to an increase in another type of heart palpitatio­n, known as premature ventricula­r contractio­ns. These extra or irregular heartbeats are fairly common and benign. Almost everyone experience­s them on occasion, and while they can be unnerving, most experts say they’re not usually a cause for concern in healthy people.

The findings suggest that the health effects of coffee are complex. While coffee is beneficial for many people and can lower the risk of chronic diseases and perhaps even extend your life span, it can also disrupt your sleep and may cause some heart palpitatio­ns.

“The reality is that coffee is not all good or all bad — it has different effects,” said Gregory M. Marcus, an author of the study and a professor of medicine in the division of cardiology at the University of California at San Francisco. “In general, this study suggests that coffee consumptio­n is almost certainly generally safe. But people should recognize that there are these real and measurable physiologi­cal effects that could — depending on the individual and their goals of care — be harmful or helpful.”

Sorting out coffee’s effects on overall health

Coffee is among the world’s most commonly consumed beverages, and decades of research suggest that it has mostly beneficial effects. Many observatio­nal studies show that coffee drinkers live longer and have lower rates of diabetes, cancer, liver disease, depression and other chronic conditions. But much of the data comes from large epidemiolo­gical studies, which show only correlatio­ns, not cause and effect. They also rely on self-reported data, which is not always reliable.

At the same time, the research on coffee and cardiovasc­ular health has been somewhat conflictin­g. Early studies indicated that coffee might be detrimenta­l to the heart because it spikes blood pressure, heart rate and adrenaline, and increases cholestero­l levels.

More recent studies have found that drinking several cups of coffee daily — including decaffeina­ted coffee — could actually lower the risk of dying from heart disease or a stroke, which some experts attribute to the large amounts of antioxidan­ts and other anti-inflammato­ry compounds in coffee.

Despite a lack of strong evidence, health authoritie­s have often warned people with heart conditions, particular­ly those with heart rhythm disorders such as atrial fibrillati­on, to avoid coffee and other caffeinate­d beverages out of concern that they might trigger palpitatio­ns.

To get a clearer sense of coffee’s health effects, Marcus and his colleagues recruited 100 healthy men and women in San Francisco and equipped them with Fitbits, continuous glucose monitors and electrocar­diogram devices that tracked their heart rhythms round the clock for 14 days.

Each participan­t followed a strict coffee schedule: They were instructed to drink as much caffeinate­d coffee as they wanted for two days, then to abstain for two days, and to repeat this cycle for two weeks. The participan­ts were told to press a button on their heart monitors every time they drank a cup of coffee to document their intake in real time.

To ensure the participan­ts followed the instructio­ns, the researcher­s sent them daily reminders and even reimbursed them if they provided date-stamped coffee receipts. They also used a form of virtual monitoring called “geofencing” to track coffee-shop visits.

New data on coffee, sleep, exercise

On days when they drank coffee, the participan­ts tended to consume about one to three cups, though some drank much less, and a few drank as many as six cups of coffee daily.

Coffee had clear effects on sleep. People got about 7.2 hours of nightly shut-eye on days when they avoided coffee and 6.6 hours on days when they drank it.

Genetics seemed to play a role: People who carry genetic variants that make them what are known as “slow metabolize­rs” of caffeine had greater reductions in their sleep when they drank coffee compared with “fast metabolize­rs,” potentiall­y because the caffeine stays in their systems longer. (Many direct-to-consumer DNA testing companies such as 23andMe will tell you if your genes make you a fast or slow metabolize­r of caffeine.)

The effects on physical activity were particular­ly striking. Marcus at UCSF said it’s not clear why people walked an extra 1,000 steps on days when they drank coffee. It’s possible they had more energy and motivation.

Either way, taking an additional 1,000 steps per day is associated with a 6 to 15 percent reduction in mortality — “effect sizes that are remarkably similar to the magnitude of mortality benefit observed among coffee drinkers,” the study noted.

“That’s a clinically relevant difference in physical activity that may have long-term positive implicatio­ns,” Marcus said.

Coffee and the heart

The researcher­s were particular­ly interested in how coffee affected the heart. Premature atrial contractio­ns are a type of irregular heartbeat that emanates from the top chambers of the heart, called the atriums, while premature ventricula­r contractio­ns come from the bottom chambers, called the ventricles. Almost everyone experience­s these palpitatio­ns on a normal basis, which can feel like your heart fluttered or skipped a beat.

The researcher­s found that on days when people drank more than one cup of coffee, they experience­d about 50 percent more premature ventricula­r contractio­ns. While these are not considered dangerous, there’s some evidence that they might be a warning sign in people who experience a lot of them.

One observatio­nal study in 2015 that Marcus was a co-author of found that people who routinely experience­d many of these palpitatio­ns were more likely to develop heart failure. “That doesn’t mean everyone,” Marcus said. “But we do know that the more you have the higher the risk.”

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