The results of a small study prompted the researchers to advise people who have high blood pressure or heart disease to avoid energy drinks because they could impact their blood pressure or change the effectiveness of their medications.

The drinks generally have high levels of caffeine and taurine, an amino acid found in protein-rich foods like meat and fish that can affect heart function and blood pressure, the researchers said.

"We saw increases in both blood pressure and heart rate in healthy volunteers who were just sitting in a chair watching movies. They weren't exercising. They were in a resting state," James Kalus of Henry Ford Hospital in Detroit, who led the study, said in an interview.

The increases did not rise to dangerous levels in the group of 15 healthy volunteers, whose average age was 26, the researchers said.

But the increases potentially could be significant in people with cardiovascular disease or those taking drugs to lower heart rate or blood pressure, they told a meeting of the American Heart Association in Orlando, Florida.

"While the amount of caffeine in energy drinks or coffee may cause a slight and temporary increase in blood pressure, it would have no greater effect than walking up a flight of steps," the American Beverage Association industry trade group said in a statement responding to the findings.

"So singling out energy drinks in a unique manner, particularly when compared to a more commonly consumed caffeinated beverage like coffee, does not provide a full and proper context for consumers."