Research Blog

December 26, 2023

Cardiovascular Biomarkers: Trimethylamine N-oxide (TMAO)

Optimal Takeaways

Elevated blood levels of trimethylamine N-oxide (TMAO) have been associated with the development of atherosclerosis, cardiovascular disease, diabetes, and other metabolic dysfunctions, including kidney disease. It is produced by the liver from TMA, a compound made from dietary precursors by an unhealthy balance of gastrointestinal bacteria. TMAO is also found in some foods, though this source is not closely associated with cardiometabolic disease.

Standard Range: 0.00 – 6 uM

The ODX Range: 0.00 – 3.7 uM

Low TMAO levels are associated with vegetarian diets (Thomas 2021) and a healthy gastrointestinal microbiota balance (Tacconi 2023).

High TMAO levels are associated with endothelial dysfunction, foam cell formation, vascular damage, decreased reverse cholesterol transport (Thomas 2021), coronary artery disease, cardiac events (Tang 2021), hypertension, atrial fibrillation (Gatarek 2021), stroke, myocardial infarction, peripheral artery disease, acute coronary syndrome, atherosclerosis, systemic inflammation, dysbiosis, insulin resistance (Tacconi 2023), increased HOMA-IR values (Krishnan 2021), diabetes mellitus (Li 2015), renal insufficiency, and loop diuretics (Gungor 2023)


Trimethylamine N-oxide (TMAO) is a compound produced in the liver and excreted by the kidney. It is formed when liver enzymes convert the bacterial metabolite trimethylamine (TMA) to TMAO. Certain GI bacteria form TMA from dietary precursors such as choline, phosphatidylcholine, betaine, and L-carnitine.

Choline is an essential nutrient for cell membrane structure and function and a precursor to the neurotransmitter acetylcholine. Choline in the gut can be converted to TMA by bacteria or oxidized to betaine, a cell stabilizer and methyl donor involved in homocysteine detoxification. Betaine balance is vital to cardiovascular health. Carnitine participates in lipid metabolism and energy generation in the mitochondria. It can be synthesized endogenously or obtained exogenously from meat, fish, dairy, or supplements. In the gut, dietary L-carnitine is primarily converted to gamma-butyrobetaine, which can then be converted to TMA by gut bacteria. TMA is released into circulation and then travels to the liver, which converts it to TMAO (Li 2015).

TMAO may contribute to atherosclerosis by promoting foam cell formation, triggering inflammation, altering cholesterol and bile acid metabolism, and affecting reverse cholesterol transport and platelet aggregation (Velasquez 2016).

Various factors influence blood levels of TMAO, including diet, GI microbial composition, and liver enzyme activity. Conversion of TMA to TMAO may be increased by genetic polymorphisms, low-grade inflammation, poor diet, dysbiosis, and chronic disease, including diabetes and cardiovascular disease. Dysbiosis appears to be a significant factor. Antibiotics can lower TMAO levels but kill off beneficial bacteria and promote antibiotic-resistant strains (Tacconi 2023).

Elevated blood levels of TMAO may be a significant predictor of major adverse cardiac events. A nested case-control study of initially healthy participants from the prospective EPIC-Norfolk study observed a dose-dependent increase in coronary artery disease risk with increasing TMAO levels over time. A TMAO of 6.2 uM or higher was associated with a 27% increased relative risk compared with a TMAO below 6.2 uM. Interestingly, baseline TMAO was significantly higher in those who eventually developed cardiovascular disease, i.e., 3.70 uM compared to 3.25 uM in those who did not (Tang 2021). Researchers note that increased CVD risk is significantly associated with elevated TMAO levels even in those traditionally not considered high risk, e.g., non-smokers, less than 65 years old, LDL-C below 100 mg/dL, and normotensive (Tang 2013).

Diet can affect circulating TMAO directly and indirectly. Preformed TMAO is found naturally in some foods, especially saltwater fish, mollusks, and crustaceans (Kelly 1999, Treberg 2002, Kruger 2017). Consumption of these foods can falsely elevate blood TMAO, especially if consumed within 24 hours of testing. However, cardiovascular risk is associated with the TMAO produced by gut bacteria (Wang 2011, Tang 2013, Zhu 2017, Koeth 2013, Cleveland Heart Lab). Ongoing research is investigating which GI microbial composition may significantly contribute to blood levels of TMAO (Tacconi 2023, Thomas 2021).

Restricting TMAO precursors in those with elevated bacterial TMAO production may be prudent. Precursors include carnitine, mainly in red meat and dairy products, and choline, primarily in red meat, dairy, liver, egg yolks, wheat germ, soybeans, and peanuts. Free choline is absorbed earlier in the small intestine, and only excess choline is thought to travel to the colon, where GI bacteria can convert it to TMA.

It is important to note that increased TMAO levels associated with choline intake may not be related to markers of CVD risk, such as oxidized LDL, homocysteine, uric acid, myeloperoxidase, and protein carbonyls. It is still unknown whether TMAO causes adverse metabolic changes associated with CVD or whether it is an “innocent bystander” (Thomas 2021).

Further research is needed to determine the association between TMAO levels and choline sources. One study of 82 healthy subjects consuming 1 of 5 sources of choline found that supplemental choline bitartrate significantly increased circulating TMAO, while consuming phosphatidylcholine supplements or four egg yolks per day did not (Wilcox 2021).

The amount of meat consumed may influence circulating TMAO. Secondary analysis of data from a randomized, crossover, controlled feeding study of 39 overweight or obese individuals found that a Mediterranean-style diet with higher amounts of unprocessed lean red meat was associated with significantly higher fasting levels of TMAO, i.e., 5.0 uM with 500 grams (18 ounces) of meat per week versus 3.0 uM with 200 grams (7 ounces) per week. Circulating TMAO decreased significantly when intake was changed from 500 to 200 grams of meat per week. HOMA markers of insulin resistance also decreased with the reduced amount of meat. Researchers acknowledge that additional factors that could influence circulating TMAO but were not investigated include GI microbial composition, genetic factors, and other dietary factors (Krishnan 2021).

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Cleveland Heart Lab. TMA O

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Tag(s): Biomarkers

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