How plant-based, Mediterranean, and Western diets affect gut microbiota and disease

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Sep 20 2024

Current scientific literature consistently highlights the superiority of plant-based and Mediterranean diets over Western diets in promoting gut health and preventing non-communicable diseases. 

Study: The Effect of Dietary Types on Gut Microbiota Composition and Development of Non-Communicable Diseases: A Narrative Review. Image Credit: Shutterstock AI Generator / Shutterstock.com

A recent study published in Nutrients examines how different dietary components affect the gut microbiome and the development of non-communicable diseases (NCDs).

Diet and gut microbiome

To date, over 2,000 species of bacteria, viruses, Archaea, and protists have been identified within the human microbiome.

Nearly 94% of gut bacteria are represented by Firmicutes, Bacteroidetes, and Actinobacteria, with prevalences of 65%, 23%, and 5%, respectively, in addition to Proteobacteria. These microorganisms engage in a symbiotic relationship with humans by absorbing and extracting nutrients, protecting against infection, maintaining homeostasis, and regulating appetite.

Fiber is broken down by a healthy and diverse gut microbiome through fermentation, which leads to the production of anti-inflammatory metabolites that promote a longer lifespan. Compared to traditional rural diets that are high in fiber, the low fiber and high fat content of Westernized diets reduce microbiome diversity and promote inflammation.

The microbiome at various levels of the gut

The abundance of microorganisms varies based on different locations throughout the gut microbiome. For example, biofilm-forming organisms predominate in the oral cavity, which has hard dental surfaces, whereas Streptococcus is dominant in the esophagus, and Prevotella and Veillonella are primarily found in the stomach microbiota.

The small intestine, which has a relatively less diverse and abundant microbiome than the colon, is dominated by Firmicutes and Proteobacteria that can survive rapid transit through the acidic digestive enzyme-rich chyme. In the colon, anaerobic flora, including Bacteroidetes and Firmicutes, comprise 90% of the total microbiota and contribute to 10⁹ bacteria/gram of colonic tissue.

Additional pathogens in the colon include Escherichia coli, Vibrio cholerae, Bacteroides fragilis, and Campylobacter jejuni, which comprise about 0.1% of the total bacteria.

Functions of the gut microbiota

Dietary fiber is fermented by gut microorganisms to yield various products, including short-chain fatty acids (SCFAs) like acetate, butyrate, and propionate. SCFAs are absorbed by the gut epithelium and used as energy sources.

SCFAs also affect host metabolism by regulating transcription, the cell cycle, epithelial integrity, and appetite regulation. These fatty acids are also involved in mucosal immunity through immunoglobulin A (IgA) production and reduce blood pressure.

Gut bacteria synthesize Vitamin K, folate, and other B-group vitamins. These vitamins support the reabsorption of bile acids by producing secondary from primary bile acids, thereby promoting lipid digestion and absorption. They also activate polyphenols to increase their absorption.

Gut microorganisms also break down lecithin and choline from fats to trimethylamine-N-oxide (TMAO). TMAO promotes atherogenesis by increasing cholesterol absorption, reducing cholesterol levels, and activating platelets. Phenylacetylglutamine (PAG), which is also a metabolite derived from the breakdown of dietary protein and amino acids, regulates cardiovascular function and blood pressure.

The gut-brain axis is a bidirectional network that connects the central, autonomic, and enteric nervous systems to the hypothalamo-pituitary-adrenal axis. Crosstalk in the gut-brain axis is mediated by bacterial metabolites, including neurotransmitters like serotonin and dopamine. Gram-negative bacteria can also induce lipopolysaccharide synthesis, which triggers inflammatory cytokines that can have adverse effects on the central nervous system (CNS).

Factors affecting the gut microbiota

The infant gastrointestinal tract is seeded from the mother’s, with subsequent alterations in the microbiome composition derived from infections, antibiotics, diet, immune responses, and genetic influences.

By 65 years of age, bacterial diversity declines, and facultative anaerobes increase in abundance. Drinking, smoking, and exercise also affect the composition of the gut microbiota, thereby suppressing nutrient absorption, changing the gut pH and oxygen levels, and reducing immunity.

Dietary patterns in different nations also significantly impact the composition of the microbiome. For example, the ratio of Bacteroidetes to Firmicutes is often higher in individuals residing in industrialized nations. Anaerobic bacteria are also more abundant in the microbiomes of individuals from higher altitudes and colder regions.

Gut microbiota and disease

In irritable bowel syndrome (IBS), gut-brain signaling is altered, which affects gut motility, visceral signaling, and epithelial barrier function, along with reduced abundance of Bifidobacterium and Fecalibacterium. Colorectal cancer (CRC) may also arise following chronic inflammation caused by Bacteroides fragilis and Escherichia coli.  

Other medical conditions related to dysbiosis include diabetes mellitus, neuroinflammatory conditions like Parkinson's disease and Alzheimer’s disease, and psychological disorders like depression, anxiety, and autism spectrum disorder (ASD).

Diets and gut microbiota

Plant-based and Mediterranean diets are rich in fiber, which supports SCFA production and suppresses harmful metabolites like TMAO. These diets reduce the risk of cancer, diabetes, and metabolic disorders

The Mediterranean diet provides polyunsaturated fatty acids (PUFAs), which, like SCFAs, have anti-inflammatory and cardioprotective properties. This dietary pattern also provides iron and zinc, which activate the immune system.

Both plant-based and Mediterranean diets increase the abundance of certain species, like Ruminococcaceae, while reducing Bacteroidaceae levels. These microorganisms have been shown to have beneficial effects on rheumatoid arthritis (RA) symptoms and cardiovascular disease.

The health consequences of the Western diet are extensive, such as dyslipidemia, insulin resistance, systemic inflammation, overactivation of sympathetic and renin-angiotensin systems, as well as alterations in gut microbiota.”

Conclusions

Numerous studies have established the nutritional and health outcome-related superiority of plant-based and Mediterranean diets as compared to the Western diet. These dietary patterns support healthy gut microbiome growth while also preventing NCDs. Nevertheless, additional research is needed to identify novel therapeutic approaches that personalize diets to prevent and manage chronic disease.