What Is the Main Idea?

The Mediterranean diet is a plant-based diet that has multiple benefits for healthy living. Based on the review article “Influence of Mediterranean Diet on Human Gut Microbiota”, published in the journal Kompass Nutrition & Dietetics, this blog post describes how through its interaction with the microorganisms in the intestine, the diet helps in reducing cardiovascular risks and other inflammatory diseases.

What Else Can You Learn?

The composition of the Mediterranean diet compared to the Western diet is described. Further, details of how the diet and microorganisms influence the health of the individual are explained.

How Was the Importance of the Mediterranean Diet Identified?

The Mediterranean diet was conceptualized by the American physiologist Ancel Keys. In the Seven Countries Study, led by him, the authors compared the local diet and correlated it to the health of people in 7 countries including the USA, Finland, Netherlands, Italy, Yugoslavia, Japan and Greece (Crete). The results showed that the Cretan diet was the least associated with cardiovascular diseases. This is despite the diet containing high fat, primarily olive oil.

Similarly, in another study called the Lyon Diet Heart Study, 605 patients who had previously had a heart attack were split into 2 groups. One group consumed the diet recommended for their heart condition (low in fat and cholesterol) and the other group followed the Mediterranean diet which included omega-3 fats. Surprisingly, the group following the Mediterranean diet had 70% less chance of mortality.

Finally, the Mediterranean diet was tested in the largest prevention trial called PREDIMED involving more than 7,000 participants. Those following the diet showed a reduction in cardiovascular issues, in type 2 diabetes, and in the development of Alzheimer’s disease, as well as an increase in life expectancy.

What Is the Mediterranean Diet and How Does It Differ from the Western Diet?

The Mediterranean diet is essentially based on the traditional food cooked along the Mediterranean Sea. To make it easier to understand how to consume the diet, a food pyramid has been envisioned. The bottom of the pyramid consists of exercise followed by the consumption of whole grain cereals, legumes, nuts, and seeds. Along with fresh fruits, vegetables, and fats, specifically extra virgin olive oil, these are the food groups that need to be consumed every day. Followed by this, consuming seafood like fish is recommended a few times per week, dairy, eggs, and poultry in small portions are recommended once a week and red meat and sweets should be limited to occasional consumption. A moderate amount of red wine consumption is allowed. Therefore, the diet heavily focuses on plant-based components. It is made flavorful with the use of herbs and spices, which also provides additional health benefits.

In contrast, the Western diet contains saturated fat, refined carbohydrates, and salt. It typically consists of a high amount of red meat, high-fat dairy products, high-sugar foods, and processed and packaged food.

Characteristics of Mediterranean diet (Merra G, Noce A, Marrone G, Cintoni M, Tarsitano MG, Capacci A, De Lorenzo A. Influence of Mediterranean Diet on Human Gut Microbiota. Nutrients. 2020;13(1):7 (DOI: 10.3390/nu13010007) © 2020 by the authors (reprint, publisher’s note removed) licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed)).

How Do These Differences in Diet Affect the Gut Microbiome?

The gut microbiome is composed of microorganisms that reside in the digestive tract of humans and animals. The present organisms and the diet correlate with each other. Based on the diet and the fiber consumed, the concentration of certain species of organisms thrives by feeding on this fiber. In turn, based on the composition of the organisms present, certain by-products of their metabolism are released that may or may not be beneficial for the health.

With a primarily plant-based Mediterranean diet, the percentage of fiber-degrading bacteria and short-chain fatty acids (SCFA) increases. Short-chain fatty acids are a by-product of gut bacteria metabolism. They help the gut in multiple ways, including maintaining the intestinal barrier and mucus production as well as protecting against inflammation. These help with regulating immunity, lipid metabolism, blood pressure, and glucose. Primarily, with this diet, there is an increase in Bifidobacteriaceae, Bacteroidaceae, Lactobacillaceae, and Prevotellaceae families of bacteria.

On the other hand, if the Mediterranean diet is poorly adhered to or if a Western diet is followed, then there is a higher concentration of bacteria that create a by-product of bacteria metabolism called trimethylamine N-oxide in the urine. The higher concentration of this has been associated with cardiovascular events with alteration of cholesterol and activation of inflammatory pathways.

Gut Dysbiosis and Diseases

Dysbiosis occurs when there is a structural or functional change in the gut microbial composition. This causes adverse health issues like triggering inflammation and diseases. Dysbiosis can occur due to stress, infections, and bad eating habits. In chronic kidney disease, the gut microbiome is altered with bacteria that cause dysbiosis. With an Italian Organic Mediterranean Diet, the authors of the above paper could show that various parameters of chronic kidney disease patients improved.

Dysbiosis is also directly correlated to colorectal cancer. There is an increase in bacterial species that promote tumor cells to multiply and induce an inflammatory state. With a Mediterranean diet and higher amounts of short-chain fatty acid production, there is shown to be a reduced risk of colorectal cancer.

To rectify dysbiosis, a short-term modification of 6 months was not enough to change the microbiome composition. However, in a study where the participants had a Mediterranean diet for 2 years, there were an increase in healthy bacteria and possible anti-cancer effects.

Benefits of Extra Virgin Olive Oil, Polyunsaturated Fats, and Fiber in the Mediterranean Diet

Extra Virgin Olive Oil

The main components of extra virgin olive oil are oleic acid and polyphenols. These are antioxidants and anti-inflammatory compounds that work against oxidative stress. Oxidative stress is associated with heart diseases, cancer, and inflammatory diseases.

In a study with cells, it was shown that polyphenols present in olive oil were able to mix with gastric juices and survive the acidic condition. They also had a bactericidal effect on 8 strains of Helicobacter pylori, which are bacteria that can cause ulcers and lead to stomach cancers. In another study with patients with high cholesterol, adding olive oil to the diet was able to stimulate the immune system. While these are preliminary results, they are promising and need to be further tested.

Polyunsaturated Fats

Fish and other marine-based diets as recommended in the Mediterranean diet contain high quantities of polyunsaturated fatty acids, especially omega-3. As described in a previous blog post, omega-3 has been shown to help in cardiovascular health and reduce inflammation which is the basis of diseases like cancer, diabetes, renal failure, and more. One of the mechanisms of the anti-inflammatory effect is found to be through reducing the unhealthy bacteria in the gut and improving the intestinal barrier.


Dietary fibers are prebiotics which are compounds required for the growth of healthy bacteria in the intestines. The bacteria feed on these fibers and release the short-chain fatty acids. With higher dietary fibers, healthier bacteria occupy the gut and also help reduce the level of cholesterol and insulin. Whereas, with low dietary fibers, the bacteria use mucoglycoproteins from the intestinal lining of the host which can lead to damage to the intestinal barrier, allowing disease-causing bacteria to pass through and resulting in colitis (inflammation of the intestine). The Mediterranean diet provides 14 g of dietary fiber per 1,000 kcal of food. This is double the amount of fiber compared to that provided by the Western diet. Hence, it has a direct effect on the prebiotic composition and in supporting healthy gut bacteria.

In Short

The Mediterranean diet has huge health benefits, especially by increasing the antioxidants, anti-inflammatory compounds, dietary fiber, and healthy fats. These help with promoting the good bacteria in the gut which in turn release short-chain fatty acids which have multiple benefits. It also helps avoid dysbiosis of the gut which can be a cause of chronic diseases. A healthy Mediterranean diet consists of daily consumption of a healthy dose of whole grain cereals, legumes, nuts, fruits, vegetables, and olive oil, with fish and dairy products consumed on a weekly basis. Red meat and sweets are meant to be consumed only occasionally.

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