That 16-ounce steak is definitely not good for your heart – but the culprit might be microbes in your gut, not the saturated fat. A new study found that L-carnitine – a nutrient in red meat and added to some energy drinks – interacts with bacteria in your gut to release a compound that slows the removal of cholesterol, clogging artery walls and increasing the risk of a heart attack. The idea that bacteria is a culprit in heart disease is relatively new, and possibly game changing. The first nutrient proved to clog arteries was choline, also found in meat and dairy products. We can now add L-carnitine to that list. "Heart disease is the number one killer in the United States," says the study's co-author Dr. Stanley Hazen, from the Cleveland Clinic. "So we are looking for other pathways, like gut flora, that are contributing to heart disease. Now that we know a specific nutrient is contributing to heart disease, we can go after this to try and block it."