The answer may not be the fat in the steak.

The researchers had come to believe that what damaged hearts was not just the thick edge of fat on steaks, or the delectable marbling of their tender interiors. In fact, these scientists suspected that saturated fat and cholesterol made only a minor contribution to the increased amount of heart disease seen in red-meat eaters. The real culprit, they proposed, was a little-studied chemical that is burped out by bacteria in the intestines after people eat red meat. It is quickly converted by the liver into yet another little-studied chemical called TMAO that gets into the blood and increases the risk of heart disease.

The study is not conclusive, but it may cast a pall over some energy drinks that contain carnitine, a substance that is found in red meat. Gut bacteria metabolizes the carnitine to produce TMAO in the blood.

Many energy drinks, like Red Bull, Monster, and Rockstar, contain l-carnitine with the idea that it helps to metabolize fat more quickly, releasing energy.