Consuming energy drinks high in caffeine and taurine can significantly increase a person’s heart contraction rate, according to a new study that raises concerns about the drinks’ effect on the heart.

German researchers studied 18 healthy people before and one hour after they consumed an energy drink. They used magnetic resonance imaging to measure participants’ heart function. The drinks contained 400 milligrams of taurine, a substance that is believed to enhance athletic performance, and 32 milligrams of caffeine, per 3.4 ounces of liquid.

After consuming the energy drink, participants had a 6 percent increase in their heart contraction rate, HealthDay reports. When the researchers studied a second group that drank energy drinks with caffeine but no taurine, they did not find a significant increase in heart contractions.

The findings were presented this week at the annual meeting of the Radiological Society of North America.

Researcher Jonas Dörner, MD, of the University of Bonn, Germany, noted in a news release that while long-term risks to the heart from consuming energy drinks remain unknown, he recommends that children, as well as people with known irregular heart rhythms, or arrhythmias, should avoid energy drinks, because of the possibility the drinks could trigger arrhythmias.

In March, researchers presented a study at an American Heart Association meeting that found energy drinks may increase blood pressure, and lead to changes in the heart’s natural rhythm. The drinks may make the heart more prone to electrical short circuits. The researchers noted it is not clear how much of the drinks’ effect is due to the caffeine in the drinks.