The Mysterious Reason Behind the Mediterranean Diet's Benefits: GUT MICROBIOTA

The human body hosts trillions of microorganisms, including bacteria, archaea, protozoa, and viruses, collectively called the microbiota. Research today examines how the microbiota, particularly the gut microbiota, is affected and its effects on health. Diet is one of the main factors influencing the gut microbiota.

The Mediterranean diet is a dietary pattern recommended for preventing numerous diseases, including cardiovascular diseases, type 2 diabetes, cancer, and age-related frailty and loss of cognitive function, and its beneficial effects have long been known. New studies in humans reveal beneficial effects of the gut microbiota's composition and functions on the Mediterranean diet.

Mediterranean-style eating is rich in fresh vegetables and fruits, whole grains, dried legumes, and oilseeds, while limiting red meat and saturated fat. Olive oil is preferred as the main fat. Fish consumption is emphasized.

The Relationship Between the Mediterranean Diet, the World's Healthiest Dietary Pattern, and the Microbiota

In this article, we discuss advances in understanding the gut microbiome's role in the protective effects of the Mediterranean diet through two new studies in humans.

In the first randomized controlled study, 82 obese participants with low vegetable and fruit intake and a sedentary lifestyle were followed for 8 weeks. Of these, 39 continued their usual habits as the control group, while 43 followed a Mediterranean diet. Participants in the Mediterranean diet group were asked to improve their diet quality without changing their energy or macronutrient intake or physical activity.

A reduction in total plasma cholesterol was observed after 4 weeks in the group following the Mediterranean diet. Low HDL and LDL plasma cholesterol levels also changed in relation to following the Mediterranean diet. Metabolomic analyses of stool, urine, and blood showed that biomarkers associated with plant-based food intake were higher in the Mediterranean diet, while animal-derived biomarkers were lower.

When the study examined bacteria in the gut microbiota, the butyrate-producing bacteria Faecalibacterium prausnitzii and Roseburia increased. The diversity of the pro-inflammatory Ruminococcus gnavus and R. torkl decreased. Urinary levels of urolithin glucuronides produced by urolithin-producing gut bacteria increased. Although stool levels of bile acids and branched-chain fatty acids decreased in the Mediterranean diet group, high fiber intake did not affect short-chain fatty acid (SCFA) levels. Participants with high levels of Bacteroides uniformis and B. vulgatus and lower levels of Prevotella copri in their gut microbiota were found to have lower Homeostatic Model Assessment for Insulin Resistance values. In addition, individuals with high gene diversity in their microbiome showed a decline in CRP (C-reactive protein) levels following the dietary intervention.

The second randomized controlled study, NU-AGE, included participants from France, Italy, the Netherlands, Poland, and the United Kingdom: 612 individuals aged 65-79.

Over one year, effects on the gut microbiota were profiled in 323 people following a Mediterranean diet and 289 in a control group. To understand the diet's effects on older people's health, the researchers suggested that people living in care facilities tended to have fewer beneficial bacteria in their gut microbiota, probably because they were exposed to more restrictive diets. An unhealthy gut corresponds to poorer overall health and a faster onset of frailty in older people.

Significant differences were found when the compositions of the two groups' gut microbiomes were compared at the end of one year. Body mass index (BMI) and age notwithstanding, participants who adhered to the Mediterranean diet throughout the study showed better bacterial diversity in the gut. An analysis of 75 operational taxonomic units (OTUs) predicting the microbiome's response to diet found positive effects on reduced frailty, walking speed, cognitive function, and episodic memory. Increased bacterial diversity influencing reductions in inflammatory markers such as CRP and IL-17 led to lower-grade inflammation markers. Changes in beneficial bacteria in the gut microbiome are the key link between dietary adherence and health.

Based on fecal microbial metabolites associated with gut microbiota species, the Mediterranean diet was suggested to increase SCFA production while reducing harmful metabolites, including bile acids and "krizoller".

Why Is the Mediterranean Diet Beneficial?

By examining participants' gut microbiomes to understand the Mediterranean diet's health effects, researchers found an association with rich populations of bacteria producing beneficial SCFAs and reduced populations of bile-acid-producing bacteria.

They suggest that when bacteria secrete too much of certain bile acids, this is associated with insulin resistance, fat accumulation in the liver, cell damage, and even an increased risk of bowel cancer.

The researchers believe that the positive changes result from the Mediterranean diet, which includes balanced nutrition with high fiber and vitamins B6, B9, and C, as well as minerals such as copper, potassium, iron, manganese, and magnesium.

They considered differences in microbiome changes depending on participants' countries of residence. They emphasize that, regardless of age, BMI, and environmental factors, all people following the Mediterranean diet showed the same general improvements in gut and systemic health.

Although the researchers caution that their research is observational and therefore cannot establish a direct causal relationship, they say:

"Maintaining a Mediterranean-style diet may provide balance and resilience by preserving the 'core' of the gut microbial community and may protect against shifts toward the alternative states found in unhealthy individuals."

Further studies will need to focus on which key nutrients in the Mediterranean diet are responsible for the positive changes in the gut microbiome.

These studies show that the Mediterranean diet's positive effects on metabolic health and healthy aging are linked to favorable changes in the gut microbiome.