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Is red meat bad for your heart? A new study suggests it is, but not for the reasons you might expect.

There鈥檚 long been a perception鈥攏ot necessarily backed by strong evidence鈥攖hat eating steak, hamburger, lamb, and other red meat ups the risk of heart disease. The saturated fat and cholesterol they deliver have been cited as key culprits. A team from a half dozen U.S. medical centers says the offending ingredient is L-carnitine, a compound that is abundant in red meat.

According to this work, published online in the journal , eating red meat delivers L-carnitine to bacteria that live in the human gut. These bacteria digest L-carnitine and turn it into a compound called trimethylamine-N-oxide (TMAO). In studies in mice, TMAO has been shown to cause atherosclerosis, the disease process that leads to cholesterol-clogged arteries. We know that clogged coronary arteries can lead to heart attacks.

So, case closed鈥攄on鈥檛 eat red meat? Sorry, nutritional science isn鈥檛 that simple.

鈥淭he studies of red meat and heart disease in humans are conflicting,鈥� says Dr. Dariush Mozaffarian, associate professor of medicine at Harvard-affiliated Brigham and Women鈥檚 Hospital. 鈥淭his new research was well-done and compelling, but it鈥檚 too early to decide that this molecule, TMAO, causes atherosclerosis in humans or that this is responsible for some of the associations of meat intake and risk.鈥�

Dr. Mozaffarian, a cardiologist and epidemiologist, studies the health effects of dietary habits and other lifestyle factors in large populations. His team has previously and found that people who eat unprocessed red meat regularly have, at worst, only a slightly higher risk of developing heart disease. Unprocessed red meat includes virtually all fresh cuts of beef, pork, lamb, and the like.

鈥淚f you look at people who eat unprocessed red meat, there is a relatively weak association with heart disease,鈥� Dr. Mozaffarian says. 鈥淚t鈥檚 not protective鈥攁nd healthier dietary choices exist鈥攂ut major harms are also not seen.鈥�

In the bigger picture, we do have pretty damning evidence about the harms of eating a particular type of meat. 鈥淧rocessed red meats鈥攂acon, sausage, salami, deli meats鈥攁re associated with much higher risk of heart disease,鈥� Dr. Mozaffarian says.

Research at the Harvard School of Public Health has shown that . The ultimate reason for this is not yet clear, says Dr. Mozaffarian, but it may be the huge doses of sodium delivered by all those low-fat deli sandwiches and salami-festooned platters.

And here comes other spoilers against the L-carnitine study: Processed meats generally contain less L-carnitine than does fresh red meat. Heart-healthy fish and chicken also contain L-carnitine, Dr. Mozaffarian points out鈥攁lthough five to 10 times less of it than red meat. 鈥淭MAO needs to be studied more in humans to understand the implications for public health,鈥� Dr. Mozaffarian says. 鈥淭his new research is very interesting but is not yet the final word.鈥�

To further complicate matters, a study published online today in the suggests that supplements of L-carnitine may help heart attack survivors reduce the chances of dying prematurely or reduce symptoms of angina (chest pain with exertion or stress).

L-carnitine supplements: 鈥淭hink three times before taking鈥�

There鈥檚 still a long way to go before we know the full story about L-carnitine and heart disease. Even so, the Nature Medicine report is very important, Mozaffarian says. It suggests that regularly eating red meat boosts the number of L-carnitine-loving bacteria in your gut. 鈥淚t鈥檚 the best demonstration so far of two-way communication between ourselves and the bacteria in out gut: what we eat affects the bacteria, and what they do with what we eat can influence health.鈥�

鈥淏ased on the Nature Medicine study, I鈥檇 be concerned about taking L-carnitine supplements,鈥� Dr. Mozaffarian says. 鈥淭here was no strong reason to take such supplements before the study, and now this well-done study suggests there may be harm. I would definitely think three times before taking an L-carnitine supplement.鈥� The studies in the Mayo report were mostly small with short follow-up, and included only heart attack survivors.

Of course, there are reasons to avoid eating red meat that aren鈥檛 directly related to individual health. Cattle farming has devastating environmental effects, including production of greenhouse gases, water pollution, and deforestation. 鈥淗ealth effects in humans aside, red meat consumption is clearly bad for the health of our planet,鈥� says Mozaffarian.