Preventive Medicine Column
Dr. David L. Katz
Two new studies address the generation of a compound called trimethylamine-N-oxide, or TMAO, when we ingest meat or eggs. 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.
The generation of TMAO from foods containing either l-carnitine or lecithin depends on the presence of specific bacteria in the GI tract. 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.
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.
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.
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. 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.
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.
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. The weight of evidence suggests that egg intake is largely or entirely free of harmful effects for most people. 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.
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.
We should all eat mostly plants for many reasons. But the TMAO studies, though provocative, do not prove, as some related hype might suggest, that eating only plants is the only way to be healthy. And avoiding dogma about diet is the best hope we have to all wind up loving food that loves us back.
Dr. David L. Katz; www.davidkatzmd.com