Does drinking coffee reduce the risk of atrial fibrillation? Maybe just a little.

Atrial fibrillation is the most common type of heart arrhythmia in the U.S. and Europe, affecting millions of people every day. A-fib is a condition where your heart beats irregularly and less efficiently than normal. Some people experience a-fib without even being aware of it, but it is a serious condition that leads to an estimated 750,000 hospitalizations per year in the U.S. alone.

The causes of a-fib are not completely understood, but one widespread view is that too much caffeine might trigger it. For example, the American Heart Association’s website says that “Avoiding atrial fibrillation and subsequently lowering your stroke risk can be as simple as foregoing your morning cup of coffee.” Although the site goes on to describe more substantial treatments such as beta blockers and calcium channel blockers, the idea of simply cutting out coffee seems very appealing.

As appealing as it sounds, this advice is wrong, at least for men.

In a study published in 2019 in the Journal of the American Heart Association, Vijaykumar Bodar and colleagues at Harvard Medical School looked at data from nearly 19,000 men who participated in the long-term Physicians’ Health Study. They looked at the risk of atrial fibrillation in men drinking anywhere from no coffee at all to 4 or more cups per day.

They found, somewhat surprisingly, that men who drank 1-3 cups of coffee per day had a 15% lower risk of a-fib compared to men who never or almost never drank coffee. They also found a small hint of a “dosage” effect, with the greatest reduction in risk at about 1.5 cups per day, and less benefit as consumption rose to 4 or more cups.

In a commentary published along with the study, Ryan Aleong and Amneet Sandhu point out that coffee contains a number of ingredients that might explain its cardiovascular benefits. They also point out, though, that the benefits are modest at best.

Unfortunately for women, though, coffee doesn’t seem to have the same benefits for them. Back in 2010, the Women’s Health Study reported that higher caffeine consumption did not increase the risk of a-fib in women, but it didn’t decrease it either. In a subgroup of women drinking the highest amount of coffee, they reported a slight increase in the risk of a-fib, but those women also smoked more often.

(Actually, if you look closely at the numbers in Table 2 of the Women’s Health Study report, women in the group who consumed an average amount of caffeine had a 20% lower risk of atrial fibrillation, consistent with the more recent study in men.)

For those of us who like coffee, this seems to be good news. At worst, coffee isn’t bad for heart health, and at best it might slightly reduce the risk of atrial fibrillation.

Some caveats: although these are large studies with thousands of men and women followed for many years, they rely on self-reporting of coffee consumption, which isn’t perfect. That’s probably the best we can do, though, since it’s impractical to measure caffeine consumption precisely over a long period of time.

The best advice, then, appears to be to keep drinking one or two cups of coffee per day, if you enjoy it. For those who have atrial fibrillation, cutting out that morning coffee, as the American Heart Association suggests, is very unlikely to help.

Can the Apple Watch monitor heart health?

Can the Apple Watch accurately detect atrial fibrillation?

When I first heard that scientists were conducting a study to answer this question, I was deeply skeptical. A simple wrist device detecting heart arrhythmias? This seemed too simplistic to be possible.

But a new study, just published in the New England Journal of Medicine, lays out some pretty compelling evidence that the Apple watch can do just that.

Atrial fibrillation (or atrial flutter) is a type of irregular heartbeat that is the most common cardiac arrhythmia in the U.S., affecting some 6 million people a year. The NEJM article estimates that the lifetime risk for "a-fib" might be as high as 1 in 3. Atrial fibrillation isn't necessarily a problem on its own, but it can greatly increase the risk of strokes. Many people have episodes of a-fib without even being aware of them, which is why a simple, convenient way of detecting them could be medically valuable.

The Apple Watch study was truly enormous, with 419,297 subjects monitored for about 4 months. Only a company like Apple, with a popular product like its watch, can even hope to recruit this many people to join a study. The idea was pretty simple: use the Apple Watch to detect an irregular pulse, which might be sign of atrial flutter or arrhythmia.

During the course of the study, 2,161 subjects had at least one report of an irregular pulse, about 0.5% of the total. Each of these subjects was then sent an electrocardiogram (ECG) patch, which they were supposed to wear for several days to determine if they really were having arrhythmias.

Of course, not everyone wore the ECG patch as they were supposed to, but in the end, 450 subjects wore the ECG patch (for an average of about 6 days), filled in all the required questionnaires, and returned the patch for analysis. Among these subjects, 34% of them genuinely did have atrial fibrillation as confirmed by the ECG patch. Some of them had only brief episodes, but a subgroup had nearly continuous symptoms.

The study also tried to determine the false positive rate of the Apple Watch warnings–that is, how often did it report an irregular pulse when the subject was not experiencing a-fib or atrial flutter? The researchers evaluated all of the reports from watches that were being worn by people who also had an ECG patch. In this analysis, 71% of the irregular pulse reports from the watch corresponded to atrial fibrillation simultaneously measured by the ECG patch. The other 29% weren't normal either: three-fourths of those reports were due to "frequent premature atrial contractions."

So overall, the Watch was surprisingly accurate, with an impressively low false positive rate.

How does it work? Well, the back of the watch contains several sensors that detect light (photodiodes), along with green and infrared LEDs that emit light. As Apple's website explains,

"By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute."

This works because your skin is partially transparent: as everyone knows, you can see some of your blood vessels underneath your skin. In addition to measuring blood flow, the watch can also measure electrical signals using electrodes on the back of the watch and on a small dial on the side of the watch, called the Digital Crown. Here's how Apple explains this:

"When you place your finger on the Digital Crown, it creates a closed circuit between your heart and both arms, capturing the electrical impulses across your chest."

In other words, it behaves like an ECG monitor on your wrist. The NEJM study didn't use this feature of the Apple Watch, which suggests that the watch could be even more effective as a heart monitor in the future.

The Apple Watch is far from perfect, though. For one thing, we don't know how many episodes of atrial fibrillation it missed. Only 0.5% of subjects had a report of irregular pulse, but we don't know how many of the remaining 99.5% had an episode that the watch didn't detect. The authors of the study pointed out that they weren't trying to measure the watch's sensitivity, and they emphasized that

"the absence of an irregular pulse notification does not exclude possible arrhythmias."

Another caveat is that the study was funded by Apple, although it was led by scientists at Stanford and included scientists from multiple other highly regarded universities and medical schools. The funding was clearly disclosed in the NEJM article.

On the other hand, using the Apple Watch is far easier than other currently available procedures for monitoring your heart. Patients who have episodes of arrhythmia are typically told to wear a heart-rate monitor for days or weeks at a time. This involves taping electrodes to half a dozen places on your body, all of which are connected by wires to a device (basically a cell phone) that records the readings and sends them to a monitoring company. These monitors are very expensive to operate, far more than the Apple Watch.

So despite its imperfections, the Apple Watch might be the vanguard of a new wave of lightweight, less intrusive devices for monitoring our health. The technology is only going to get better.