TMAO (trimethylamine N-oxide) is produced by gut bacteria from choline, lecithin, and L-carnitine-rich foods (mainly fish, meat, egg, and dairy). High levels are associated with heart disease, hardening of the arteries, diabetes, and colon cancer. However, the role of TMAO in these diseases is still controversial. Read more to find out how TMAO levels may affect health and what factors may lower levels.

What Is TMAO?

TMAO (trimethylamine N-oxide) is an organic compound produced from TMA (trimethylamine), a common product of plant and animal decomposition [1].

TMA is generated by gut bacteria when they metabolize choline, lecithin (phosphatidylcholine), and L-carnitine [2, 3].

TMA is then transferred via the bloodstream to the liver where it becomes oxidized into TMAO, primarily by the enzyme FMO3 [4].

Potential foods that increase TMAO are those rich in choline, lecithin, and L-carnitine, like red meat, eggs, dairy, and saltwater fish [5].

TMAO Blood Test

You can check your TMAO levels with a blood test. However, the test is relatively new and the referent values are still being established.

TMAO levels tend to increase with age [6].


1) Associated With Heart Disease

In a meta-analysis of 19 studies with 19,256 subjects, high TMAO levels were associated with increased rates of cardiovascular diseases, such as heart failure and stroke. Furthermore, people consuming higher levels of TMAO precursors, like L-carnitine and choline, were 1.4 times more likely to suffer from heart failure, stroke, or other heart diseases [7].

In another meta-analysis of 11 studies with around 7700 subjects, those with elevated levels of oxidized TMA had a 23% higher risk of heart diseases and a 55% higher risk of dying (from all causes) [8].

According to a study of 81k women and 39k men, a higher intake of dietary phosphatidylcholine (source of TMAO) resulted in a higher all-cause death, especially among diabetics [9]. However, the study is limited because it did not assess the gut microbiota and gut health among diabetics versus healthy subjects which is typically poor in comparison.   In general, phosphatidyl choline is known to be a beneficial compound to consume, so more research is needed in the opinion of our medical experts. 

High TMAO levels were associated with blood clots, heart attack, and stroke in over 4,000 subjects [10].

Similarly, high levels of this compound were associated with increased mortality in 720 patients with heart failure [11].

In rats, TMAO increases with age and causes oxidative stress and inflammation in blood vessels, leading to endothelial (blood vessel lining) dysfunction, which increases the risk of heart disease [12].

2) May Cause Hardening of the Arteries

High levels of TMAO were associated with increased cIMT (carotid intima-media thickness), an early marker for hardening of the arteries (atherosclerosis), in 220 patients irrespective of other factors, such as insulin resistance, obesity, and fatty liver [13].

Elevated levels of this compound were also associated with higher rates of hardening of the arteries in 220 subjects with chronic kidney disease [14].

Dietary TMAO precursors such as L-carnitine and choline increased the hardening of the arteries in atherosclerosis-prone mice. The effect was blocked by antibiotics, which decreased the activity of gut bacteria [15, 16].

3) Is Associated with Diabetes

In 2,977 participants, high TMAO levels were associated with increased rates of type 2 diabetes [17, 18].

Of 191 heart disease patients, those with high TMAO were more likely to have diabetes [19].

Dietary TMAO decreased glucose tolerance, increased insulin levels and insulin resistance (HOMA-IR), and increased inflammation in mice on a high-fat diet [20].

All of this hint that it may increase the risk of diabetes.

4) Impairs Kidney Function

Chronic kidney disease patients had higher TMAO levels compared to controls (521 patients and 3,166 controls). Furthermore, higher TMAO in chronic kidney disease patients was associated with a 2.8-fold increase in mortality risk [3].

In 179 chronic kidney disease patients, high TMAO was associated with faulty kidney function, chronic inflammation (high IL-6 and CRP), and increased mortality. After kidney transplantation, TMAO levels decreased back to normal [21].

High levels of this compound were also associated with worse kidney function and advanced chronic kidney disease in 227 patients with heart disease [22].

In animals, high dietary TMAO or choline levels caused kidney damage and dysfunction [3].

In another study, high TMAO levels, caused by a high-fat diet, increased oxidative stress and inflammation in the kidneys and lead to kidney damage and dysfunction in mice [23].

5) Increases Inflammation

High TMAO levels were associated with higher levels of TNF-alpha, an inflammatory cytokine, but not with IL-6 or CRP in 271 adults [24].

A diet higher in cow’s dairy was also associated with higher TMAO and higher inflammation [24].

However, high TMAO was associated with chronic inflammation and high IL-6 and CRP levels in 179 patients with kidney disease [21].

This compound increased inflammation by increasing NF-κB (a master regulator of inflammation) in mice [25].

It also increased inflammation and oxidative stress in human blood vessel cells [26].

6) Cancer

TMAO contributes to the production of N-Nitroso compounds (NOCs). These products can cause DNA-damage and potentially cancer [27, 28].

In 1,670 women, there was a link between higher TMAO and colon cancer. Women with the highest levels of choline, the source of TMAO, were at a 2.4-times greater risk of developing the disease [29].

In 138 subjects, those who had colon cancer had higher TMAO levels. Elevated TMAO levels also predicted lower disease-free survival among colon cancer patients [30].

However, in another study, only elevated choline but not TMAO was associated with an increased risk of colon cancer in 1,288 men [31].

TMAO was associated with elevated rates of aggressive prostate cancer in 200 prostate cancer patients [32].

7) Liver Dysfunction

In 1,628 adults, high TMAO levels were associated with more severe liver damage and non-alcoholic fatty liver disease [33].

Also, high levels were associated with worse outcomes in patients with primary sclerosing cholangitis, an immune-related disease of the liver [34].

A high conversion rate of choline into methylamines, such as TMAO, decreases choline levels and causes fatty liver disease in mice [35].

Also, dietary TMAO causes liver damage in mice [36].

8) May Cause Obesity

Higher FMO3 enzyme activity, responsible for producing TMAO, was associated with higher body mass index (BMI), waist-to-hip ratio, more body fat, and lower insulin sensitivity in 770 men [37].

Blocking FMO3 activity protects mice from becoming obese [37].

Factors That May Increase TMAO Levels

1) Animal Products

Eggs, red meat, dairy products, and fish are rich in choline, carnitine, and lecithin, which then get converted to TMAO by gut bacteria [5].

In 297 subjects, fish and meat consumption increased blood TMAO levels [38].

In 40 healthy young men, fish consumption increased TMAO levels more than beef or eggs [39]. However, this response correlated with the presence of specific gut microbes that are found in people with supposedly less healthy gut microbiotia [39].

Fish protein increased TMAO in mice more than other protein sources such as soy or casein [40].

In rats, a beef diet increased TMAO levels more than chicken [41].

Eggs, and especially egg yolks, are rich in choline. Eating more than two eggs per day increased TMAO levels in six healthy volunteers [42, 43].

However, another study showed that the intake of up to three eggs per day did not increase TMAO levels, but had beneficial effects such as increasing HDL-cholesterol and choline [44].

Dairy consumption was associated with higher TMAO levels in 271 adults [24].

On the contrary, in 38 overweight or obese women, increased consumption of dairy resulted in lower TMAO levels [45].

Should you stay away from fish and animal products because these can increase TMAO? The answer is not simple.

Many studies show that consuming fish lowers the risk of heart disease. Fish is also an excellent source of omega-3 fatty acids [46, 47].

It seems that the benefits of other healthy ingredients found in fish outweigh the effect of higher TMAO, at least when it comes to heart disease prevention.

Diets rich in choline or carnitine also have beneficial effects on human health. These foods are important sources of nutrients, such as iron and vitamin B12 [47].

Therefore, you should take the studies above with a grain of salt.

2) High-Fat Diet

A short-term high-fat diet increased TMAO levels during or after meals but did not have long-term effects in 10 non-obese men [48].

3) L-Carnitine Supplements

Oral L-carnitine supplements increased TMAO levels in 31 patients on hemodialysis [49].

L-carnitine supplements can also increase TMAO levels in people with organic acidemias (these are rare inherited inborn errors of metabolism), irrespective of the meat consumption [50]. But these studies are small and need much more follow up work before any conclusions are drawn from it.

4) Gastric Bypass Surgery

In 34 obese people, TMAO levels increased after gastric bypass surgery, likely due to alterations in gut microbiota [51].

However, gastric bypass surgery decreases overall heart disease risk, so the impact of high TMAO in these cases is still not understood [52].

5) Pollutants

In mice, dioxin-like pollutants, which are highly toxic environmental pollutants, increased the FMO3 enzyme production, which elevated TMAO levels [53].

6) Sleep Deprivation

Sleep deprivation increased TMAO levels in 15 healthy men [54].

7) Pregnancy

Sex hormones during pregnancy increase FMO3 enzyme activity, which can increase TMAO [55].

Factors That May Decrease TMAO levels

1) Vegetarian/Vegan/Mediterranean Diet

Among 74 subjects given L-carnitine, meat-eaters had greater increases in TMAO levels compared to vegans or vegetarians. This means that vegans and vegetarians have gut bacteria that produce less TMAO [56].

Among 153 people, vegetarians, vegans, and those who adhered to a Mediterranean diet (rich in fruit, legumes, and vegetables) had lower TMAO levels [57].

2) Prebiotics and Potential Probiotics

Probiotics are bacteria that, when digested, have beneficial effects. Prebiotics are plant fibers, which act as fertilizers for the growth of the good bacteria in the gut.

Soluble dietary fiber, acting as a prebiotic, reduced TMAO by over 60% in mice fed red meat [58].

In another study, a single type of prebiotics increased, whereas a prebiotic mixture decreased TMAO levels [59].

This suggests that having a high diversity of gut bacteria is important for controlling TMAO levels.

In human studies, conventional probiotics were unsuccessful in decreasing TMAO [60, 61].

However, the bacterium Enterobacter aerogenes, given as a probiotic, decreased TMAO levels in mice [62].

3) Vitamins D and B

In a small study on 52 adults, supplementation with vitamins B and D decreased TMAO levels [63].

4) Resveratrol

Resveratrol, found in berries and grapes, decreased blood TMAO levels in atherosclerosis-prone mice [64]. Future clinical studies are needed before any conclusions can be drawn from this.

5) Pistachios

A pistachio-supplemented diet decreased TMAO levels in 39 prediabetic subjects [65].

6) Brussels Sprouts

Daily consumption of Brussels sprouts decreased FMO3 enzyme activity in 10 healthy volunteers. This can result in lower TMAO levels [66].

7) DMB Can Decrease TMAO Levels

DMB (3,3-dimethyl-1-butanol) is found in red wine, balsamic vinegar, cold-pressed extra virgin olive oil, and grapeseed oil [67].

It decreases TMAO levels in mice fed high-choline or high-carnitine diets [67].

8) Meldonium

Meldonium is an anti-ischemic medication that increases blood flow and oxygen supply to the body.

This drug decreased blood TMAO levels in eight healthy adults on TMA-rich diets by increasing the elimination of TMAO through urine [68].

In rats, meldonium reduces the production of TMA from L-carnitine and, thus, decreases TMAO levels. Twenty-four hours after meldonium treatment, the TMAO levels in urine were 3.6 times lower [69].

Irregular TMAO Levels?

LabTestAnalyzer, a sister company of SelfHacked, helps you make sense of your lab results. It informs you which labs are not in the optimal range and gives you guidance about how to get them to optimal. It also allows you to track your labs over time. No need to do thousands of hours of research on what to make of your lab tests.

About the Author

Anastasia Naoum, MS (health informatics)

MS (Health Informatics)

Anastasia holds an MSc in Health Informatics from the Sheffield University, an MSc in Health Economics from the Erasmus University of Rotterdam and a BSc in Economics from the University of Macedonia.

Anastasia grew up in a medical environment, as both her parents are doctors and developed from a young age a passion for medicine and health. She has worked in several institutions and associations which promoted healthy living and sustainable healthcare systems. Currently, she is leading a green life, sailing with her boyfriend across Europe, living in their sailboat with the help of solar and wind power, minimizing CO2 production.

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