As a well-credentialed physician who advocates for vegetarian diets, Dr. David L. Katz, the founding director of Yale University's Prevention Research Center, would reasonably be expected to pile onto the generally negative media coverage of stories last month surrounding new research implicating meat-eating as a risk factor in cardiovascular disease.
Only for new and different reasons.
His recent Huffington Post column began exactly as you’d suspect:
“As you have likely heard, assuming you weren’t grazing on another planet the past couple of weeks, there may be a new reason to eat less meat: A compound called trimethylamine-N-oxide (TMAO), which seems to induce atherosclerosis in mice; in humans, blood levels of TMAO correlate with rates of cardiovascular disease.”
But to his credit, Dr. Katz quickly amended that statement by adding, “Although that does not prove causality.”
You didn’t find such a qualifier until deep into most mainstream media stories about this “shocking” new finding, so I give the good doctor props for at least attempting to clarify that an association between two factors—no matter how closely they appear linked—does not mean that the former causes the latter.
The study to which he referred was published last month by a well-regarded group of researchers at the Cleveland Clinic, considered to be a cardiac research institution second only to the Mayo Clinic. They discovered that certain intestinal bacteria transform a compound known as l-carnitine, which is formed from amino acids found in meat and helps transport lipids within cells, into TMAO. Again, there is no cause-and-effect, merely a strong association.
TMAO is potentially generated after eating foods containing carnitine along with choline, which is found in the lecithin present in eggs and other foods.
At this point, most media simply acted as if there was a straight line from meat to carnitine to TMAO to fatal heart attacks. Thankfully, Dr. Katz wrote that “this part of the story is anything but straightforward.”
Complications on the theory
He noted two complications. First, TMAO cannot be formed in the intestinal tract unless there are some very specific bacteria present. To be sure, it seems that people who enjoy regular servings of red meat in their diets are more likely to have those bacteria in their systems, but it’s not a given. Vegans seem to have fewer of the intestinal bacteria capable of converting carnitine into TMAO, but it’s not known if that’s because the bacteria simply aren’t present or because their diet suppresses the bacteria living in the intestines.