Doctors have long assumed that saturated fat and cholesterol in red meat are what raise the risk of heart disease. But a study in the journal Nature Medicine fingers another culprit: carnitine, a compound abundant in red meat that also is sold as a dietary supplement and found in some energy drinks.

Carnitine typically helps the body transport fatty acids into cells to be used as energy. But researchers at the Cleveland Clinic found that in both humans and mice, certain bacteria in the digestive tract convert carnitine to...