Red Meat, TMAO, and Heart Disease: Summary of Critical Reviews
It's Friday, so there must be another study/article "proving" that red meat is the scourge of human health. Meat eaters will cringe nervously, vegetarians will cry out triumphantly, and everybody in between will wonder what-the-heck to believe. This time, the culprit is a little-known compound called TMAO (trimethylamine N-oxide), which has been associated with heart disease, mostly recently through this particular study.
The basic hypothesis goes something like this: TMAO is found in larger quantities in people who have heart disease. And TMAO levels are elevated via increased amounts of carnitine consumption through a diet full of red meat. Thus, avoid red meat, avoid TMAO, avoid heart disease.
Oh, if only life and the human body were so simple. Haven't we learned by now that vililfying one particular compound or hormone is overly simplistic and error-prone? The body is too interconnected to simply cut out one chemical and not get multiple effects. Thankfully a bunch of great independent researchers have taken a closer look at the study (rather than just glossing over with a catchy headline like the main-stream media does) and provided some excellent context.
First of all, lots of foods increase TMAO in humans and red meat isn't really the most significant one. Chris Masterjohn explains:
"... high-dose choline and carnitine, but not lecithin, generated TMAO. Eight ounces of carnitine-rich foods, however, like red meat, produced no more TMAO than common fruits and vegetables. Seafoods, by contrast, led to large increases in TMAO."
All of the review articles out there could have easily (and maybe more accurately) been titled: "Another Reason to Avoid Squid Meat, New Heart Disease Risk Found."
Secondly, nothing has been conclusively proven with regard to causitive effects. The excellent health resource Examine.com breaks it down:
What cannot be concluded from this study?
- TMAO in humans causes heart disease. ApoE-/- mice are used since it is easy to research pro-atherogenic cholesterol metabolism, but these are mice with a specific genetic fault. This research would have to be replicated in regular mice (and then in humans). Additionally, the human correlational research does suggest there is a link but cannot state that it is a causative link, and past research on Choline does suggest that TMAO is fairly rapidly excreted via the kidneys (suggesting some effects that are differential dependent on species).
- Carnitine causes heart disease, as the link between TMAO and heart disease is not fully established in humans.
- TMAO is bad. To nip this in the bud, most molecules have good and bad associated with them (good example: Cortisol) and it would be improper to conclude that TMAO must be avoided at all cost. To reiterate as well, the correlation between TMAO and cardiovascular disease could very well be a biomarker of some other lifestyle habits.
To be fair, it is possible that TMAO is a cause of heart disease, but randomized controlled trials with humans, rather than genetically engineered mice, would be needed to prove that.
Finally, the epidemiological evidence on the matter of meat-eating and health is rather inconsistent. Chris Kresser demonstrates:
If eating meat increases heart disease risk we might expect lower rates in vegans and vegetarians. Early studies suggested this was true, but later, better-controlled studies suggest it’s not. The early studies were poorly designed and subject to confounding factors (i.e. vegetarians tend to be more health conscious on average than general population, so there could be other factors explaining their longevity, such as more exercise, less smoking, etc.). Newer, higher quality studies that have attempted to control for these confounding factors haven’t found any survival advantage in vegetarians.
For example, one study compared the mortality of people who shopped in health food stores (both vegetarians and omnivores) to people in the general population. They found that both vegetarians and omnivores in the health food store group lived longer than people in the general population. This suggests, of course, that eating meat in the context of a healthy diet does not have the same effect as eating meat in the context of an unhealthy diet.
Ultimately, I think people need to calm down and not jump to conclusions and make radical changes in their diet and lifestyle when every new study comes out. Maybe one day it will be conclusively proven that meat consumption is generally bad for human health. I personally doubt it, but am open to new information. The important thing here, is to not let personal ideology and bias cloud your judgement, and to seek out critical examinations of the data as much as possible.