TMAO and Heart Health: What Gut Bacteria Have to Do With Your Heart

TMAO stands for trimethylamine N-oxide.

It is a small compound that forms in two steps.

First, gut bacteria break down nutrients like choline, carnitine, and lecithin into a gas called trimethylamine, or TMA.

Choline is found in eggs, red meat, and dairy.

Carnitine is found mostly in red meat.

Then TMA travels to the liver, where an enzyme converts it into TMAO.

TMAO then circulates in the blood and can be measured with a simple blood test.

Not everyone makes the same amount of TMAO from the same meal.

This depends on the mix of bacteria living in your gut, sometimes called your gut microbiome.

Scientists have studied TMAO for over a decade because it keeps showing up in people with heart disease.

People with higher blood TMAO tend to have more plaque buildup in their arteries.

A large new study looked at more than 5,000 people getting heart procedures.

It found that people with higher TMAO levels were significantly more likely to already have atrial fibrillation, even after accounting for other risk factors like age and blood pressure.^[1]

Atrial fibrillation, often shortened to AF or afib, is a condition where the heart beats irregularly and often too fast.

It raises the risk of stroke, heart failure, and other complications.

This makes TMAO one of the clearer examples of how gut bacteria can influence a disease far outside the gut.

Researchers have tested TMAO directly in animal models to understand cause and effect, not just association.

In mice prone to atrial fibrillation, adding TMAO to the diet made the condition appear sooner.^[1]

Feeding mice choline, which raises TMAO levels in the body, also sped up the onset of abnormal heart rhythms.^[1]

When researchers blocked the gut bacterial enzyme that makes TMA, using a compound called iodomethylcholine, TMAO levels dropped and the heart rhythm problems were reduced.^[1]

This is strong evidence that the gut bacterial pathway itself, not just choline in general, is driving the harm.

On a tissue level, mice given extra choline developed changes in the heart's upper left chamber, including dilation, or stretching, of the left atrium.

Detailed heart mapping showed the electrical signals in the heart moved more slowly and less predictably after choline supplementation.^[1]

The study also found that TMAO blocks a receptor called muscarinic receptor 2.^[1]

This receptor helps the autonomic nervous system, the part of the nervous system that runs in the background, keep heart rhythm steady.

When TMAO interferes with this receptor, it may throw off the delicate nervous system control that normally prevents the heart from slipping into chaotic rhythms.^[1]

TMAO production depends heavily on diet, because the raw materials come from food.

• Red meat, a major source of L-carnitine
• Egg yolks, rich in choline and lecithin
• Liver and other organ meats
• Full-fat dairy products
• Certain fish, though fish also contain TMAO directly rather than just its building blocks
• Energy drinks and supplements containing added choline or carnitine

Interestingly, fish naturally contains some preformed TMAO, yet fish consumption is generally linked to better heart health in large population studies.

This suggests that the source and context of TMAO exposure matters, and that fish's other nutrients, like omega-3 fatty acids, likely offset any downside.

Plant foods contain very little choline or carnitine in the forms that gut bacteria convert efficiently into TMA.

This is one reason plant-based eating patterns tend to produce lower TMAO levels overall.

A related dietary pattern, sometimes called the sulfur microbial diet, is also being studied for its effect on gut bacterial metabolism and body weight.^[2] This eating pattern is high in red and processed meat, refined grains, and eggs, many of the same foods that supply choline and carnitine for TMAO production.^[2] In adults with overweight or obesity, following this pattern more closely was linked to a less favorable metabolic profile and higher adiposity, adding to the case that these foods affect the body through more than one gut related pathway.^[2]

No official guidelines yet recommend testing or targeting TMAO directly.

But the diet patterns already known to protect the heart also happen to lower TMAO, which adds another reason to follow them.

• Shift the balance of your plate toward vegetables, fruits, whole grains, and legumes rather than red meat
• Choose fish and poultry over red meat as your main animal protein most days
• Limit processed meats like bacon, sausage, and deli meat, which are high in both carnitine and sodium
• Support a diverse gut microbiome by eating plenty of fiber from varied plant foods
• Include fermented foods like yogurt or kefir, which may support a healthier gut bacterial balance
• Talk to a doctor before starting any choline or carnitine supplement, especially if you have a family history of heart disease

This approach lines up closely with the Mediterranean diet, which is already well studied for lowering heart disease risk.

Reducing red meat and processed meat intake is one of the most consistent, low risk changes a person can make for long term heart health.

• People with a personal or family history of atrial fibrillation
• People with existing heart disease or risk factors like high blood pressure
• People who eat a diet very high in red meat and eggs
• People with chronic kidney disease, since the kidneys clear TMAO from the body
• Anyone interested in supporting long term cardiovascular and gut health through diet

People with kidney disease deserve special mention, because TMAO builds up when the kidneys cannot filter it out efficiently.

This may partly explain why chronic kidney disease and heart disease so often occur together.

Much of the strongest evidence linking TMAO to atrial fibrillation currently comes from animal studies and a large observational human study, rather than human clinical trials.^[1]

Observational studies can show a strong association, but they cannot fully prove that TMAO itself causes disease in people, since other factors often travel alongside it.

Mouse models help fill this gap by testing cause and effect directly, but mice are not people, and results do not always translate perfectly.^[1]

No drug that lowers TMAO has yet been proven in large human trials to reduce heart attacks, strokes, or atrial fibrillation.

For now, TMAO is best viewed as a promising marker and possible contributor, not a fully confirmed cause that can be targeted with treatment.