What is the relationship between a plant-based diet and the gut microbiome?

By Dr. Priyom Bose, Ph.D.Reviewed by Lily Ramsey, LLMMar 28 2023

A recent study published in the Nutrients journal conducted a systematic review of the impact of various plant-based diets on gut microbiota.

Study: Effect of Plant-Based Diets on Gut Microbiota: A Systematic Review of Interventional Studies Image Credit: FOTOGRIN/Shuttershock.com

Background

Across the world, people are accustomed to different types of diets, such as vegetarian, semi-vegetarian, lacto-ovo vegetarian, vegan, pescatarian, and many more. Several studies have identified the association between plant-based diets and decreased risk of obesity, cardiovascular diseases, and other health issues.

In this study, researchers have reviewed the existing literature to understand how certain diets lead to gut dysbiosis, negatively impacting disease progression.

Introduction

A typical plant-based or vegetarian diet includes foods that are derived from plants. Individuals who adhere to a semi-vegetarian diet mostly consume plant-based foods with a small proportion of meat, poultry, dairy, and seafood. Pescatarians are those who consume fish and seafood but avoid both meat and poultry. People who follow lacto-ovo vegetarian diets consume eggs and dairy but avoid meat, fish, poultry, and seafood. Vegans do not consume any animal-based foods.

Plant-based diets are gaining popularity due to their associated health and environmental benefits. Around 1.5 billion individuals worldwide follow vegetarianism, of which Asians are the highest practitioners. Around 40% of the Indian population is vegetarian due to religious beliefs, such as Hinduism, Sikhism, Jainism, and Buddhism. 

Increased awareness of animal rights and animal cruelty linked to animal-based food production has also enhanced the number of plant-based diet consumers. Some other factors that have increased adherence to a vegetarian diet are food allergies, weight maintenance, mental well-being, and family influences.

Furthermore, plant-based diets significantly decrease global carbon emissions, particularly by avoiding the emissions linked to rearing animals and livestock. Overall, this diet decreases the risk of developing obesity, certain cancers, cardiovascular diseases, and diabetes.

A plant-based diet is also believed to be beneficial for the gut microbiome because of the production of important phytochemicals. The human gut microbiome comprises around three trillion microbes, mostly bacteria. However, it also consists of archaea, viruses, fungi, and protozoa. Gut microbes play a crucial role in the overall health of an individual and disease manifestations, particularly gastrointestinal diseases.

About the study

The current review included pre–post-intervention studies, randomized controlled trials, and non-randomized trials associated with the impact of plant-based diets on gut microbiota. It included studies on adults who were on a plant-based diet. Their gut microbiota was quantified from stool samples using a molecular sequencing technique targeting the 16s ribosomal ribonucleic acid (RNA) gene. In addition, the authors considered research detecting the microbial metabolites using liquid gas chromatography technique and analyzed their role in the gut microbiome.

Relevant peer-reviewed articles were obtained from electronic bibliographic data sources, such as Scopus, Ovid Medline, Embase, and PubMed. A total of 203 articles were obtained, and after excluding duplicate studies, 101 titles and abstracts were evaluated. Twelve studies were finally considered as they fulfilled the eligibility criteria.

Key insights

Typically, vegan/vegetarian diets are richer in fiber and carbohydrates than other diets. These diets have also been associated with increased short-chain fatty acids (SCFAs), negatively correlated with pro-inflammatory cytokines.

SCFAs are known to improve immunity in the host. They are also associated with neuron function, maturation, and blood–brain barrier function maintenance. SCFAs promote metabolism and prevent obesity.

A decreased level in the plasma L-carnitine was observed in individuals with ischemic heart disease who practice vegan diet intervention. In contrast, those who consumed red meat showed a high level of plasma L-carnitine. This group was at a higher risk of developing inflammatory bowel and cardiovascular diseases.

The current study elucidated how different diet types influence an individual's microbiota composition. Research has shown that the number of gut bacteria that can break down fibers from vegan/vegetarian diets is much greater than the number of fat-protein metabolizing bacteria required to digest an animal-based diet.

A healthy human gut comprises many butyrate-producing bacteria, such as Roseburia, Ruminococcaceae, Lachnospiraceae, Blautia, and Faecalibacterium prausnitzii. Butyrate is a metabolite with many physiological activities, such as improvement in the intestinal barrier through upregulation of the tight junctions.

It also decreases systemic inflammation by restricting lipopolysaccharide (LPS) from crossing the intestinal wall and reaching the bloodstream. Several studies have documented evidence that butyrate, acetate, and propionate can prevent many diseases, such as immunological diseases, inflammatory bowel disease, and type two diabetes.

Conclusions

The authors claim this systemic review is the first to evaluate the link between vegan/vegetarian diets and gut microbiota composition and human health outcomes. The high level of SCFAs and trimethylamine N-oxide (TMAO) metabolites in the plasma of individuals under vegan/vegetarian diets indicate the role of the gut microbe as a modulator of the diet–host interaction.

In the future, more research is required regarding the interaction of gut microbiota with other metabolites, such as vitamins, polyphenols, intestinal aryl-hydrocarbon receptor ligands, and isothiocyanates.