The human body is host to trillions of gut bacteria, also known as microbiome, that collectively control many of the bodies functions including the immune system and energy balance. It has now become apparent that a certain balance of gut bacteria is optimal for human health. In fact, an association has been made to dyslipidemia, insulin resistance and type 2 diabetes when an individual develops an imbalance in the gut bacteria. It is thought that the changes in bacterial diversity can promote inflammation, insulin resistance and metabolic syndrome.

It has been well established that dietary interventions can change the bacterial diversity and provide a therapeutic tool for treating conditions that may be influenced by gut bacteria. Two recent studies have explored how the Mediterranean diet (MedDiet) with olive oil influences gut bacteria, insulin sensitivity and metabolic syndrome.

The first study, published last month in the Journal of Clinical Endocrinology and Metabolism, was conducted over a period of one year in 20 obese participants within the Coronary Diet Intervention With Olive Oil and Cardiovascular Prevention (CORDIOPREV) study.

The study compared a MedDiet (35 percent fat, 22 percent monounsaturated) to a low-fat, high complex carbohydrate (LFHCC) diet (28 percent fat, 12 percent monounsaturated) to changes in bacterial diversity related to insulin sensitivity and type 2 diabetes. The main fat source for the MedDiet group was olive oil, the research team distributing olive oil to all participants to ensure this was the case.

Both diets showed changes in bacterial diversity, but in different ways. The authors saying: “Our results suggest that long-term consumption of the Med and LFHCC diets exerts a protective effect on the development of type 2 diabetes by different specific changes in the gut microbiota, increasing the abundance of the Roseburia genus and F. prausnitzii, respectively.”

The role of gut bacteria, and the large diversity of their nature means this is a fairly complex area that is not yet fully understood. However, a study published in PLoS ONE, 2013, suggests that Roseburia and F. prausnitzii are both buryrate-producing bacteria, that also supply other short chain fatty acids that help reduce oxidative stress and inflammation.

A second study, also conducted within the CORDIOPREV study, published last month in the Journal of Nutritional Biochemistry, followed 239 participants: 138 participants with metabolic syndrome and 101 participants without.

At baseline there were noticeable differences in the gut bacteria of both groups. The relative abundance of Bacteroides, Eubacterium and Lactobacillus genera was higher in the participants with metabolic syndrome. The relative abundance of 18 other bacterial species were lower in the participants with metabolic syndrome at baseline.

To assess whether the dietary interventions had impact on the bacterial profile, the bacteria were examined again after 2 years. In the MedDiet group the abundance of P. distasonis, B. thetaiotaomicron, F. prausnitzii, B. adolescentis and B. longum were significantly increased in those with metabolic syndrome but not in those without. On the other hand, the abundance of E. rectale increased and P. disansonis decreased in the non-metabolic syndrome group only.

This study revealed that despite the metabolic syndrome still being present in participants, significant changes to gut bacteria do occur. Most notably, the gut bacteria that improved in abundance in the metabolic syndrome group show a negative correlation with levels of glucose, triglycerides, and HDL cholesterol, supporting the idea that gut bacteria can indeed influence insulin resistance and the development of metabolic syndrome, whereas these changes did not occur in the non-metabolic syndrome group.

Researchers suggest that it is a combination of fiber and the phenolic-compound-rich foods such as olive oil, vegetables, fruits, wine and so forth, that contribute to these positive changes in gut bacteria.

Since it is now well established that metabolic syndrome is associated with chronic low-grade inflammation, another important observation was that “the consumption of a Mediterranean diet increased the abundance of the Bacteroides genus member B. thetaiotaomicron and F. prausnitzii, which suggest that the consumption of this diet may increase or maintain a microbiota with anti-inflammatory capability.”

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