Body Effects of Coffee

Coffee lovers—and their doctors—have long wondered whether a jolt of caffeine could have negative effects on the heart.

New research finds that drinking caffeinated coffee did not significantly affect one kind of heart hiccup (小问题) that can feel like a skipped beat. But it did signal a slight increase in another type of irregular heartbeat in people who drank more than one cup per day. And it found that people tended to walk more and sleep less on the days they drank coffee.

Coffee is one of the most common beverages in the world. In America, two‑thirds of people drink coffee every day, more than bottled water, tea, or tap water, according to the National Coffee Association, a trade group for the US coffee industry. Coffee contains caffeine, a stimulant (兴奋剂), which is widely regarded as safe for healthy adults at about 400 milligrams per day, or roughly the equivalent of four to five cups brewed at home.

Coffee has been associated with multiple health benefits and even a lower risk of dying, based on large studies that observed participants’ behaviors. However, despite research that has shown moderate coffee consumption doesn’t raise the risk of heart rhythm (心律) problems, some medical societies still warn against consuming caffeine.

Researchers equipped 100 healthy volunteers with devices that continuously monitored their heart function, daily steps, sleep patterns and blood sugar levels. The volunteers, most of whom were below the age of 40, were sent daily text messages over two weeks instructing them to drink or to avoid drinking any caffeinated coffee on certain days.

This type of study directly measures the biological effects of drinking or not drinking caffeinated coffee in the same people. This is rare and provides a dense set of data points, said study co‑author Dr. Gregory Marcus.

Researchers found that drinking caffeinated coffee did not result in more daily episodes of extra heartbeats, known as premature atrial contractions (房性早搏). These extra beats that begin in the heart’s upper chambers (心腔) are common and typically don’t cause problems. However, they have been shown to predict a potentially dangerous heart condition called atrial fibrillation (房颤).

They also found slight evidence of another kind of irregular heartbeat that comes from the lower heart chambers, called premature ventricular contractions (室性早搏). Such beats are also common and not usually serious, but they have been associated with a higher risk of heart failure (心力衰竭). The researchers found more of these early beats in people on the days they drank coffee, but only in those who drank two or more cups per day.

The volunteers logged about 1,000 more steps per day on the days they drank coffee—and they slept about 36 minutes less, the study found. There was almost no difference in their blood sugar levels.

There is also one interesting result. People with genetic variants (基因变异) that make them break down caffeine faster experienced less of a sleep shortage while folks with variants that lead them to break down caffeine more slowly lost more sleep.