What’s TMAO, and What Does It Have to Do With My Health?

With the last few weeks’ definitive guide and follow-up on fish, a reader asked me about trimethylamine N-oxide, or TMAO. What is it?

TMAO is the latest justification given for why eating meat just has to be bad for you. Saturated fat didn’t take. Animal protein didn’t work. Iron was a dud. IGF-1 hasn’t panned out. Methionine isn’t enough. So now they’re using TMAO to convince you not to eat that steak.

How’s it supposed to work?

How TMAO Happens

When certain gut bacteria encounter choline (found in eggs and liver) or carnitine (found in meat, especially red meat), some of it is converted to trimethylamine, or TMA. TMA is the compound that gives fish its “fishy odor.” Fish is actually extremely high in TMA, which I’ll discuss later on. Then, the liver converts a portion of the TMA to TMAO. Studies have shown that elevated serum levels of TMAO are linked to heart disease, type 2 diabetes, kidney disease, and even all-cause mortality. There’s definitely some heterogeneity among the studies, but enough have found a strong connection between TMAO and all manner of poor health conditions that researchers have focused on this compound.

Okay, so anything that contains choline or carnitine will increase TMAO, which should in theory increase your risk of heart disease. Right? Let’s go down the list.

Dietary TMAO Precursors and Their Effects On Health

Eggs. The best source of TMAO-precursor choline in our diet—eggs—should absolutely skyrocket TMAO levels. Except it doesn’t happen.

Three eggs a day has no effect on TMAO levels, even as it increases choline levels and HDL cholesterol.

Okay, so maybe the choline slipped past the TMA-producing gut bacteria in that study, but what about if you quickly switch people from eating oatmeal for breakfast to eating eggs. Surely bad things will happen, right?

No. Eating eggs instead of oatmeal has no effect on TMAO levels. It increases carotenoid and choline levels, though.

Liver. Okay, liver has to do the trick. It has high levels of both carnitine and choline. But no: feeding liver (among other foods) to men fails to increase TMAO levels above control.

Carnitine. Forget meat. What if you go straight to the offensive precursor itself and give actual human women a big daily dose of carnitine for, I don’t know, 24 weeks? Surely it will do something bad.

Nope. TMAO skyrockets, an indication that these ladies’ gut bacteria are converting carnitine to TMA and TMAO, but serum C-reactive protein, interleukin-6, L-selectin, P-selectin, vascular cell adhesion molecule-1, intercellular adhesion molecule-1 and lipid profile markers are completely unaffected. If gut bacterial conversion of carnitine to TMAO is the preeminent risk factor for heart disease, you’d think some of these ladies’ cardiovascular risk factors would have responded. They had half a year to respond. They did not.

Okay, but maybe there’s lag time between TMAO increases and deleterious changes to health. Nope. They followed those same ladies after cessation of carnitine supplementation. Their TMAO levels dropped, but their health markers stayed the same. No change.

And here’s a study where they used carnitine to increase TMAO levels in patients on dialysis. Not only did nothing bad happen, but the carnitine even reduced markers of vascular injuries. Higher TMAO, better health.

Seafood. As I mentioned earlier, fish and shellfish come pre-contaminated with the TMAO precursor TMA. It’s what gives the characteristic fishy odor, and it definitely gets converted to TMAO. In fact, a human study from a few years ago found that feeding people fish spiked TMAO levels by 60 times. A more recent study even concluded that elevated TMAO levels are a reliable marker for cod intake. The more fish you eat, the more TMAO your body will process.

If you’re going to claim that TMAO is dangerous and causes heart disease, you’ll have to make the case that fish is dangerous and causes heart disease. All the evidence we have points in the opposite direction—that fish and shellfish are protective against heart disease.

So, Why Is TMAO Linked To Poor Health Then?

How do we explain the connection between increased TMAO and poor heart health?

Here it is linked to atrial fibrillation.

Here it is linked to stroke.

Here it is predicting heart events.

The connection is there. And in animal models, TMAO even appears to mechanistically increase atherosclerosis. The mice they dosed with TMAO to increase atherosclerosis were genetically engineered to be ApoE knockouts, a strain of lab mouse that gets heart disease from almost everything, but still.

The connection isn’t causal. It’s an observation. There are no controlled studies giving people foods (or even supplements) that raise TMAO and increase disease or death. There aren’t even prospective observational studies where they track a group’s food intake, TMAO levels, and death/disease over time.

You know what I think (and have always thought)?

High TMAO can be a marker for metabolic disease. It could indicate inhibited kidney function, as the kidneys are response for disposing of excess TMAO. It could indicate poor health in general.

The latest evidence is confirming what I’ve long suspected: the reason high TMAO levels are linked to cardiovascular disease and overall mortality is that both type 2 diabetes and chronic kidney disease cause elevated TMAO levels. The causality is reversed.

What’s one of the kidney’s primary jobs? Excreting waste materials and toxins. What’s going to happen if the kidney begins to fail or lose its functioning? The stuff that used to be excreted starts backing up. TMAO is supposed to be excreted in the urine via the kidneys. If the kidneys aren’t working, TMAO levels skyrocket.

But even then, high TMAO isn’t even necessarily a bad thing. Check out that study I linked to earlier where women were given carnitine every day. Their TMAO levels skyrocketed but nothing bad happened. No health markers worsened. In one study, they even improved.

The Takeaways…

That’s the thing with biology. There are dozens of reasons TMAO could be elevated, some of them bad, some of them harmless, some of them good.

There simply exists no credible evidence that increased TMAO because you’re eating fish, or eggs, or liver, or meat, does anything untoward to your health. I’m not ruling it out. But the evidence just isn’t there. There’s far more evidence that eating fish, eggs, liver, and meat improve your health.

That’s it for today, folks. If you have any further questions about TMAO, leave them down below. Until then, enjoy your choline and carnitine!

Thanks for reading.


Schiattarella GG, Sannino A, Toscano E, et al. Gut microbe-generated metabolite trimethylamine-N-oxide as cardiovascular risk biomarker: a systematic review and dose-response meta-analysis. Eur Heart J. 2017;38(39):2948-2956.

Dimarco DM, Missimer A, Murillo AG, et al. Intake of up to 3 Eggs/Day Increases HDL Cholesterol and Plasma Choline While Plasma Trimethylamine-N-oxide is Unchanged in a Healthy Population. Lipids. 2017;52(3):255-263.

Missimer A, Fernandez ML, Dimarco DM, et al. Compared to an Oatmeal Breakfast, Two Eggs/Day Increased Plasma Carotenoids and Choline without Increasing Trimethyl Amine N-Oxide Concentrations. J Am Coll Nutr. 2018;37(2):140-148.

Zhang AQ, Mitchell SC, Smith RL. Dietary precursors of trimethylamine in man: a pilot study. Food Chem Toxicol. 1999;37(5):515-20.

Samulak JJ, Sawicka AK, Hartmane D, et al. L-Carnitine Supplementation Increases Trimethylamine-N-Oxide but not Markers of Atherosclerosis in Healthy Aged Women. Ann Nutr Metab. 2019;74(1):11-17.

Samulak JJ, Sawicka AK, Samborowska E, Olek RA. Plasma Trimethylamine-N-oxide following Cessation of L-carnitine Supplementation in Healthy Aged Women. Nutrients. 2019;11(6)

Fukami K, Yamagishi S, Sakai K, et al. Oral L-carnitine supplementation increases trimethylamine-N-oxide but reduces markers of vascular injury in hemodialysis patients. J Cardiovasc Pharmacol. 2015;65(3):289-95.

Svingen GFT, Zuo H, Ueland PM, et al. Increased plasma trimethylamine-N-oxide is associated with incident atrial fibrillation. Int J Cardiol. 2018;267:100-106.

Liang Z, Dong Z, Guo M, et al. Trimethylamine N-oxide as a risk marker for ischemic stroke in patients with atrial fibrillation. J Biochem Mol Toxicol. 2019;33(2):e22246.

Haghikia A, Li XS, Liman TG, et al. Gut Microbiota-Dependent Trimethylamine N-Oxide Predicts Risk of Cardiovascular Events in Patients With Stroke and Is Related to Proinflammatory Monocytes. Arterioscler Thromb Vasc Biol. 2018;38(9):2225-2235.

Jia J, Dou P, Gao M, et al. Assessment of Causal Direction Between Gut Microbiota-Dependent Metabolites and Cardiometabolic Health: A Bidirectional Mendelian Randomization Analysis. Diabetes. 2019;68(9):1747-1755.

TAGS:  Hype, omega 3s, toxins

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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15 thoughts on “What’s TMAO, and What Does It Have to Do With My Health?”

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  1. So wonderful to see you are still doing these excellent reviews of the latest ‘scare’ tactic, long may you continue to speak sense into the nonsense 🙂

  2. Good work Mark – Why is TMAO linked to kidney and heart disease?
    The TMAO that is soi linked is being made from trimethyl-lysine, TML.
    TML is formed in the body when cells are trying to make carnitine,
    If this doesn’t happen, the TML can be converted to TMAO.
    TML seems to be a better predictor of heart disease as TMAO.
    It looks to me that TMAO can be a sign of diseases that lead to carnitine insufficiency, rather than the other way round

    1. This is really helpful, thanks. In strict keto (for migraines, epileptic children, etc), carnitine is often taken as a supplement as the body can’t do this exact thing fast enough. Note that’s keto the medical diet, not primal keto the health diet.

  3. Yeah, these tests are just so strange. For instance using cholesterol to predict heart disease is ridiculous because one person might be eating grass fed meat, olive oil, and grass fed butter to contribute to that number while someone else is eating donuts with transfats to get the same cholesterol reading. So it is more likely WHAT you are eating than the high number itself causing problems. Not to mention cholesterol is not even raised in the majority of people with events. Same kind of logic here and not enough specific testing. which is hard. My experiments with diet have been interesting. I have very high cholesterol–sometimes sky high. No other risk factors. Very low blood pressure (96-110 over 60 something), low blood sugar, very very low triglycerides, very high HDL (good cholesterol), good ratios. I just had full body scan and no plaque in any arteries–abdominal, carotid, peripheral, nothing. I’m 64, too. Of course, I’m not eating garbage. And I exercise a lot. Follow along on my journey. lekimball.com

    1. Lynn, I have noticed similar results as you from my paleo diet. I have very high total cholesterol (high hdl and ldl) but very low triglycerides (36 last time). I have always had low bp and blood glucose levels and am quite thin (104 lbs 5’5”). My PCP recommended a diet low in fat and a medication to lower my cholesterol! I had to laugh. I mentioned this to my GI doctor (who is very smart) and he said to eat all of the cholesterol and fatty foods my body wants.

  4. I’m a big fan of neurotransmitter precursors so I take a combination product containing L-tyrosine and 5-htp in a ratio of 10 to 1 and some choline every day. I assume based on the above that I don’t need to be concerned about supplementing with choline.

  5. Great post! Just a shout out to your graphics team, can you find some better pictures to represent paleo diet besides the holy trinity plus one of red meat, sweet potatoes, avocado and kale? What about bamboo shoots, green papaya, and shrimp? Or morning-glory shoots, ostrich meat and okra? (yum that sounds like a great curry) or Bitter melon, chayote and mole sauce? Many moons ago Mark mentioned studying vegans for the way they eat vegetables and it has opened a whole new world of dietary choices for me. Otherwise, I love most of the images here, just need to represent more diversity overall.

  6. I have had hflc diet (cyclical now) for 5+yrs and have low triglycerides, high LDL, very high HDL, low blood pressure, eat mostly vegetables, grass-fed beef, 4 eggs/day, & coconut oil. My CAC score is still zero. But my TMAO was 23 with “normal” below 6. Should I be looking for kidney disease markers?

    1. You don’t normally get kidney markers done? (These are usually part of a basic blood panel.) Mine are fine, low carb/keto 5.5 years, but I don’t know what my TMAO is. (Hmm…maybe I have to recheck my blood tests to see if it’s there.) My CAC score is also zero, after 5.5 years of low carb/keto. I also have very high Lp(a).

      I eat a lot of meat, eggs sometimes, some dairy, some vegetables.

      Do you have high total cholesterol too? Are you lean? Do you exercise? If so, you might be a “lean mass hyper-responder”. See Dave Feldman, CholesterolCode.

  7. Mark, Great writing. You left one out. Studies are based on the notion that meat eater. Ketogenic and carnivore specifically, are eating meat none stop. Little they know (or perhaps they are just playing dumb), that most if not all who follow these lifestyles, are eating in a small window or OMAD. Plenty of time for TMAO to dissipate. Do you agree?

  8. Thank you for these studies and the insight here, Mark. You always make digesting things easier (pun intended.)

  9. I think since it’s involved in “liver flavin monooxygenase activity” another question that needs asking is, how many medications are people with a high level of TMAO taking? Because we know that all these health negatives are also associated with polypharmacy. And every drug is some kind of chemical the liver has to clean up after.

    Ref for quote: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6213249/

    If you overwhelm the body’s FMO pathway, all sorts of nasty things happen, you could even have fishy body odor and eat NO fish!

    Since we have no idea why we are getting cardiovascular disease, we’re acting without scientific basis to vilify any specific food. Actually we’re acting harmfully if that food isn’t toxic and is nutritious. I don’t accept the cholesterol theory, when my cholesterol numbers are checked I look at them to see how my liver is doing. That’s its job to make those packages. I worry about triglycerides because I see them as a marker for fatty liver, not heart disease. But I realize I’m in the minority.

  10. Hi and thanks for your good service. Sorry for this post not being theme related.
    Regarding Google’s black-out on alternative health issues, why don’t you “health gurus” come together and promote other search tools like DuckDuckGo, I use it and get to where I want.