People following a Mediterranean diet (MedDiet)—characterized by low intake of refined cereal products, dairy and meat products and higher amounts of vegetables, fruits, legumes, whole grains, fish and a daily serving of nuts—show the best overall health and appear to be protected from metabolic-related diseases, cancer and even mental health-related disorders.

Two new human intervention studies have recently made progress towards understanding the gut microbiome’s involvement in the protective effects of the MedDiet.

In the first randomized controlled study, sedentary subjects who were overweight or obese were assigned to follow either a MedDiet (n = 43) or their regular diet (n = 39), characterized by low amounts of fruits and vegetables, for a period 8 weeks. It should be acknowledged that participants in the MedDiet group were instructed to improve the quality of their diet without changing their habitual energy intake, macronutrient intake or physical activity.

In the MedDiet subjects, a reduction in total plasma cholesterol was reported as early as 4 weeks after the beginning of the diet. In addition, lower total high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol was observed at the end of the intervention, which was proportional to MedDiet adherence rates.

Metabolomic analyses of feces, urine and blood showed that the MedDiet was characterized by higher levels of the biomarkers of whole grains, legumescommonly used as biomarkers of plant food intake—vegetables and nuts, together with reduced concentrations of the biomarkers of meat and protein degradation products.

Specific changes in the gut microbiota’s composition and functions were also found, including an abundance of butyrate-producing bacteria Faecalibacterium prausnitzii and Roseburia; a lower abundance of the potentially proinflammatory Ruminococcus gnavus and R. torques; and increased urinary levels of urolithin glucuronides produced by urolithin-producing gut bacteria. Although MedDiet consumption was related to a reduction in fecal levels of bile acids and branched-chain fatty acids when compared with the conventional diet group, the high fiber intake did not affect fecal levels of short-chain fatty acids.

These studies show that one of the ways by which the MedDiet is linked to better metabolic health and healthy ageing is through positive changes in the gut microbiome

Baseline gut microbiota is known to be a major factor that influences how people respond to dietary intervention. The authors found that the participants with higher baseline levels of Bacteroides uniformis and B. vulgatus and lower levels of Prevotella copri reduced their Homeostatic Model Assessment for Insulin Resistance. Furthermore, subjects showing increased gene richness displayed lower levels of C-reactive protein after dietary intervention.

In the second randomized controlled trial, elderly non-frail and pre-frail subjects across five European countries followed either a MedDiet (n = 323) or a control diet (n = 289) for 1 year.

Regardless of body mass index and age, adherence to a MedDiet led to a higher abundance of different taxa that were positively linked to markers of lower frailty and better cognitive function and negatively associated with inflammatory markers such as C-reactive protein and interleukin-17.

Although diet and gut microbiome profiles differed between countries at baseline, gut microbiome diversity remained almost unaffected by the MedDiet across different countries. Interestingly, high levels of adherence to the MedDiet attenuated the loss of gut microbiome diversity across the study, with 75 operational taxonomic units (OTUs) predicting the microbiome’s response to the diet. On the other hand, machine learning methods allowed for diet-responsive taxa to be identified. While 44 OTUs showed a positive association with adherence to the diet, 45 OTUs were negatively associated.

A prediction of fecal microbial metabolites based on gut microbiota species suggested that the MedDiet promoted short-chain fatty acid production while decreasing detrimental metabolites, including bile acids and cresols.

On the whole, these studies show that one of the ways by which the MedDiet is linked to better metabolic health and healthy ageing in older adults is through positive changes in the gut microbiome at both the composition and functional level. Further studies will need to focus on what key nutrients and food staples in a MedDiet are responsible for these positive gut microbiome changes and who will benefit the most.



Meslier V, Laiola M, Roager HM, et al. Mediterranean diet intervention in overweight and obese subjects lowers plasma cholesterol and causes changes in the gut microbiome and metabolome independently of energy intake. Gut Epub ahead of print: 2 April 2020. doi: 10.1136/gutjnl-2019-320438.

Ghosh TS, Rampelli S, Jeffery IB, et al. Mediterranean diet intervention alters the gut microbiome in older people reducing frailty and improving health status: the NU-AGE 1-year dietary intervention across five European countries. Gut Epub ahead of print: 2 April 2020. doi: 10.1136/gutjnl-2019-319654.