By JONEL ALECCIA
AP Health Writer

Coffee lovers - and their doctors - have long wondered whether a jolt of java can affect the heart. New research published Wednesday finds that drinking caffeinated coffee did not significantly affect one kind of heart hiccup 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 tend to walk more and sleep less on the days they drank coffee.

Coffee is one of the most common beverages in the world. In the U.S., two-thirds of Americans drink coffee every day, more than bottled water, tea or tap water, according to the National Coffee Association, a trade group. Coffee contains caffeine, a stimulant, which is widely regarded as safe for healthy adults at about 400 milligrams per day, or roughly the equivalent of four or 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' behavior. Despite research that has shown moderate coffee consumption doesn't raise the risk of heart rhythm problems, some professional medical societies still caution against consuming caffeine.

The latest research:

THE EXPERIMENT

Researchers outfitted 100 healthy volunteers with gadgets that continuously monitored their heart function, daily steps, sleep patterns and blood sugar. The volunteers, who were mostly younger than 40, were sent daily text messages over two weeks instructing them to drink or avoid caffeinated coffee on certain days. The results were reported Wednesday in the New England Journal of Medicine.

This type of study, which directly measures the biological effects of drinking or not drinking caffeinated coffee in the same people, is rare and provides a dense array of data points, said study co-author Dr. Gregory Marcus, a cardiologist at the University of California, San Francisco, who specializes in treating heart arrhythmias.

THE FINDINGS

Researchers found that drinking caffeinated coffee did not result in more daily episodes of extra heartbeats, known as premature atrial contractions.