As you have likely heard, assuming you weren't grazing on another planet over the past couple of weeks, there may be a new reason to eat less meat. While we were still digesting the study about meat, we were served the news that there may be a new reason to eat fewer eggs, as well. But before we wind up overcooking this potential new beef with meat and eggs, we'd best carve up the particulars and chew carefully.
The new studies address the generation of a compound called trimethylamine-N-oxide, or TMAO. TMAO has been shown to induce atherosclerosis in mice, and levels of TMAO in the blood of humans correlate with rates of cardiovascular disease, although that does not prove causality.
TMAO is potentially generated in the body following intake of l-carnitine, a protein found in meat, and following ingestion of choline found in lecithin, also known as phosphatidylcholine, a fat-like compound present in eggs among other foods. But this part of the story is anything but straightforward.
The first complicating consideration is that the generation of TMAO from foods containing either l-carnitine or lecithin depends on the presence of specific bacteria in the GI tract. It is the actions of intestinal bacteria that metabolize nutrient precursors into TMAO. But we also know that dietary pattern influences those same bacterial populations. So food intake influences what happens to our intestinal microbes, and our intestinal microbes influence what happens to our food. Stated differently, we may not yet know whether chicken and eggs are chicken, or egg, in this scenario.
The study linking meat intake to TMAO production demonstrated that more carnitine was converted to TMAO by omnivores than by vegans. This suggests that eating meat routinely affects the balance of intestinal bacteria, which in turn affects what happens when meat is eaten. Vegans, it seems, have gut microbes less prone to make TMAO even when they do encounter carnitine, although they do make some.
The second consideration is that both l-carnitine and choline are found in diverse foods, including plants. Carnitine is most concentrated in meat, but it is found in wheat and asparagus, along with dairy and some fish. Choline is most concentrated in eggs, and to a lesser extent, meat, but it is also found in fish, grains, vegetables, and fruits. And so, whatever the new research means, it does not mean we should aim to avoid either carnitine or choline completely, as no healthful, balanced diet would allow for that. Nor can we avoid making some TMAO. So this is a story about gradations, not either/or choices.
The third consideration is that the study of meat compared the metabolic responses of vegans to those of omnivores. But there are many ways to be an omnivore. In our culture, the most common kind of omnivore is a devotee of the typical American diet. The typical American diet isn't just a mix of plant and animal foods, however; it's a mix of real food and junk. A third to half the calories in the prevailing version of modern omnivorousness come from junk food.
In contrast, vegans tend to be among the most conscious and careful eaters in modern society. Being vegan generally indicates a commitment to some blend of health and ethics, and both require thoughtful choices. A balanced, prudent vegan diet isn't just totally plant-based, it is substantially junk-free. Vegans eating junk exist, of course, but they are few and far between, and they are doing veganism wrong.
We Homo sapiens are constitutional omnivores. We have in fact been omnivorous far longer than we have been sapiens. But in modern context, an omnivore is generally not someone who eats mostly plants along with some wild salmon, organic eggs, and a bit of bison. In our culture, an omnivore is someone who eats mac n' cheese, salami, pepperoni pizza, and Big Macs. In contrast, vegans are, generally, not just people who only eat plants. The differences here are not subtle, and go well beyond the inclusion of some meat or eggs in the diet. A comparison between a group of omnivores and a group of vegans is likely to be a comparison between one group eating badly, and another eating well. That such differences would influence metabolism, intestinal flora, and health outcomes is far from shocking.
Yet another consideration is that our Stone Age ancestors, while reliably omnivores, didn't just eat mammoth rather than meatloaf; they did so in a pristinely germy world, free of antibiotics, food chemicals, or processing other than applications of heat and cold. We know that antibiotics can wreak havoc with our intestinal microbes, as well as propagate antimicrobial resistance -- and that antibiotics in feed animals may be a more important source for most of us than antibiotics we take on purpose. Perhaps some of the TMAO story is not all about carnitine or choline, but also the company they keep in the flesh of modern feed animals.
A fifth consideration is the tendency to conflate new research about potential health risks, with new potential health risks. When new studies are published, we may tend to think we face new threats. But meat and eggs are not suddenly more dangerous than they were yesterday; we just have new insight into the mechanisms of any such danger.
And yesterday, meat and eggs per se may not have been dangerous at all. We have evidence from large cohort studies that processed meat is bad for health, but pure meats are less so, and in moderation may not be harmful. Health benefits associated with the Paleo diet suggest that game intake might even confer health benefit. Health benefit has consistently been associated with fish intake, even despite the worry about contaminants. The weight of evidence suggests that egg intake is largely or entirely free of harmful effects for most people, and might be associated with some benefit depending on how foods are used to replace one another. And studies of healthful dietary patterns that include eggs and certain meats, the Mediterranean diet noteworthy among them, show health benefits at least as decisive as any linked to veganism. New insights about TMAO do not undo any of the epidemiologic science that came before.
That said, there are excellent arguments for veganism. Eating only plants, done well, figures among contenders for the most healthful diet, although it's not the clear winner. In addition, veganism is much about ethics, and avoids the abuse of animals practiced on behalf of often unknowing omnivores. It makes more efficient use of food energy as well, and is a kinder, gentler approach to the planet. In a world of 7 billion Homo sapiens, the case for veganism beyond individual health is strong.
Getting back to TMAO, a sixth and final consideration is salient. This new research highlights the dependence of our health on the bacterial populations within. This is a fascinating, profoundly important area we are only beginning to understand, though some relevant insights have been germinating for decades. I commend to all interested in this burgeoning field the book An Epidemic of Absence. We and our bacterial tenants are in this together, and are together influenced by genes, diet, and environmental exposures.
Which leads to my conclusions. We had compelling reasons to eat mostly plants and close to nature before, and TMAO may be a new entry on that list. We knew before that junk food, processed meat, and random antibiotic exposures were doing us no favors. We had no such clear indictments, however, of fish, lean meats, and eggs. And for those inclined to include these in their diets, the TMAO studies do not as yet provide a clear argument to do otherwise. We should all eat mostly plants, but arguments for eating only plants derive from considerations other than individual health.