By Gina Kolata updated 4/7/2013 3:36:56 PM ET 2013-04-07T19:36:56 It was breakfast time and the people participating in a study of red meat and its consequences had hot, sizzling sirloin steaks plopped down in front of them. The researcher himself bought a George Foreman grill for the occasion and the nurse assisting him did the cooking. For the sake of science, these six men and women ate every last juicy bite of the 8-ounce steaks. Then they waited to have their blood drawn. Dr. Stanley Hazen of the Cleveland Clinic, who led the study, and his colleagues had accumulated evidence for a surprising new explanation of why red meat may contribute to heart disease. And they were testing it with this early morning experiment. 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 stomach 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. That, at least, was the theory. So the question that morning was: Would a burst of TMAO show up in peoples’ blood after they ate steak? And would the same thing happen to a vegan who had not had meat for at least a year and who consumed the same meal? The answers were: yes, there was a TMAO burst in the five meat eaters and no, the vegan did not have it. And TMAO levels turned out to predict heart attack risk in humans, the researchers found. The researchers also found that TMAO actually caused heart disease in mice. Additional studies with 23 vegetarians and vegans and 51 meat eaters showed that meat eaters normally had more TMAO in their blood and that they, unlike those who spurned meat, readily made TMAO after swallowing pills with carnitine. “It’s really a beautiful combination of mouse studies and human studies to tell a story I find quite plausible,” said Dr. Daniel J. Rader, a heart disease researcher at the University of Pennsylvania School of Medicine, who was not involved in the research. .