New research adds to the list of the Mediterranean diet’s potential cardiovascular benefits. The results suggest that fish and vegetables trigger the production of a gut-derived metabolite that reduces signs of hypertension and heart abnormalities in rats.
mediterranean food on a plate
A diet rich in fish and vegetables triggers a heart-healthy compound, new research indicates.

Various recent studies have hailed the health benefits of the Mediterranean diet, which mainly consists of vegetables, fish, and whole grains.

Results of some studies have linked the diet with good cardiovascular health and a longer lifespan, and some research has suggested that it helps protect against health issues, such as diabetes and stroke.

Researchers believe that the heart-healthy benefits of the Mediterranean diet result from the quantity of monounsaturated fats, which increase levels of “good” cholesterol and improve its functioning.

New research adds to the list of reasons why the Mediterranean diet may be good for the heart.

The results indicate that a compound called trimethylamine N-oxide (TMAO) reduced cardiac fibrosis, or the thickening of the heart, and signs of heart failure in rodents. Other studies have associated the compound with eating seafood and vegetables.

Tomasz Huć, of the Medical University of Warsaw in Poland, is the first author of the paper, which was just published in the American Journal of Physiology — Heart and Circulatory Physiology.

Studying TMAO and heart health in rats

Blood levels of TMAO increase after consuming foods that are high in the compound, such as fish, seafood, and vegetables. The liver also produces TMAO with the help of gut bacteria, explain the researchers.

As Huć and his colleagues note, the role of TMAO in the cardiovascular system has been unclear, with several studies yielding contradictory results.

Some studies suggest that TMAO has a harmful effect on the cardiovascular system, while others indicate that the compound has protective effects in animal models.

Huć and his team set out to study the effects, using a model of rats that were predisposed to hypertension. One group of rodents received a low dosage of TMAO, which was added to their drinking water, while another group received no supplemental TMAO.

The dosage was high enough, however, to increase the amount of TMAO in the blood to four or five times normal levels.

The rats received supplementation for either 12 weeks or 56 weeks. The researchers compared them with a control group of rodents that were not genetically predisposed to developing high blood pressure.

Between weeks 7 and 16 of the rats’ lives, the team measured their blood pressure and left ventricular end-diastolic pressure.

The researchers also used echocardiography to assess the structure and functioning of the rodents’ hearts and electrocardiography to monitor their heartbeats.

Because previous studies point to an inverse correlation between TMAO levels and kidney function, the researchers also examined the rats for signs of kidney damage.

How TMAO protects the heart

The dosage of TMAO did not appear to affect the development of hypertension in rodents predisposed to the condition.

In fact, the condition of the rats’ hearts improved, even after taking TMAO supplementation for 56 weeks. Huć and his colleagues explain:

[A] four- to fivefold increase in plasma TMAO does not exert negative effects on the circulatory system. In contrast, a low-dose TMAO treatment is associated with reduced cardiac fibrosis and [markers of] failing heart in spontaneously hypertensive rats.”

“Our study provides new evidence for a potential beneficial effect of a moderate increase in plasma TMAO on pressure-overloaded heart,” the authors continue.

The researchers note that fully understanding the effects of TMAO on the cardiovascular system will require further studies.

However, a person could indirectly conclude from these findings that a Mediterranean diet rich in fish and vegetables may benefit the heart.

While some older studies indicate that the body makes TMAO in response to the intake of beef and eggs and that the compound has a harmful effect, Huć and his team pay heed to the evidence that TMAO is produced after the ingestion of fish, seafood, and vegetables.

They acknowledge that “it seems that a fish-rich and vegetarian diet, which is beneficial or at least neutral for cardiovascular risk, is associated with a significantly higher plasma TMAO than red meat- and egg-rich diets, which are considered to increase the cardiovascular risk.”