Wednesday 24/5/2023

PHOTO: Daniel Oberg

Text: Nicoleta Casangiu and Alejandra Misiolek 

The gut environment is home to the gut microbiota, a complex community of bacteria, archaea, fungi, viruses and protozoa that provide the host organism with a larger endowment of cells and genes than its own. Over the past 10 years, increasing evidence has highlighted the influence of bacterial communities on human health.

Microbial colonization occurs in parallel with the development of the immune system and plays a role in human health. The gut microbiota community does not fluctuate significantly during adult life. However, bacterial infections, antibiotic treatment, lifestyle, surgery and diet can lead to changes in the microbiota.

The composition of the microbiome is unique to each individual, developing rapidly in childhood to become established in adulthood. Changes in microbial composition depend on both genetic and environmental factors, such as diet, geographical location, exposure to toxins/carcinogens and hormones.

In this blog we will discuss the relationship between gut microbiota and diet, especially the effects of vegetarian and vegan diets.

It is not yet clear what defines a “healthy” microbiota. We know that 30-40% of the adult gut microbiota can be modified throughout life, and diet is one of the most powerful tools for doing so.

Several studies have suggested that there are three basic bacterial enterotypes: 1) genus Prevotella (considered primarily anti-inflammatory and protective), 2) genus Bacteroides (more pro-inflammatory and possibly related to increased risk of metabolic syndrome and other pathological conditions), and 3) genus Ruminococcus (whose biological importance is less clear).

An imbalance of these three components of the gut microbiota has been linked to gastrointestinal conditions such as reflux, peptic ulcers, irritable bowel syndrome and inflammatory bowel disease. In addition, systemic conditions such as obesity, atherosclerosis, type 2 diabetes, cancer, Alzheimer’s and Parkinson’s disease, amyotrophic lateral sclerosis, autism spectrum disorder also appear to be linked to unfavorable changes in the composition of the gut microbiota.

There is increasing evidence for the microbiota as a mediator of the impact of diet. According to several studies, diet composition appears to have long-term and acute effects on the gut microbiota ecosystem. Different long-term patterns, such as vegetarian/vegan versus omnivorous diets, significantly influence the gut microbiota.

How plant food components influence the gut microbiota

Below we will discuss recent studies and how different components of plant foods (protein, fat, polyphenols, short-chain fatty acids, or phytoestrogens) influence the gut microbiota.

The importance of consuming nutrients with low bioavailability has recently been discovered. The lower bioavailability of nutrients, found in larger particle sizes, intact cell walls and non-heat-treated foods means that more nutrients reach the lower part of the gastrointestinal system, thus enriching the nutrient supply to the gut microbiota. This contributes to gut development and function.

Modern Western diets contain more ultra-processed foods and acellular nutrients, or nutrients that do not contain cells. These components are absorbed in the small intestine, depriving the colon of important nutrients, which can alter the composition and metabolism of the gut microbiota. Acellular foods, for example sugar, have been shown to induce inflammation in infants, adolescents, women of childbearing age and older adults. Whole plant foods, however, have protective effects, favoring the growth of beneficial fibre-degrading bacteria in the colon.

Bifidobacterium is a genus known to play a protective role in the human intestinal barrier, defending against pathogens and disease. Studies have shown that a high intake of cholesterol from animal products is strongly associated with a lower abundance of Bifidobacteria. Studies comparing Bifidobacteria levels in vegan vegetarians and controls found that Bifidobacteria were significantly lower in vegan samples than in those on a standard omnivorous diet. The relative decrease of Bifidobacterium in vegetarians and vegans may be explained by a relative abundance of other protective bacterial species such as Prevotella. Prevotella has been found to confer anti-inflammatory effects and may decrease the growth of other bacteria by competing for fibre as an energy substrate.

With regard to protein intake, most studies have found that protein intake correlates positively with microbial diversity. However, animal and plant proteins influence the microbiota differently. For example, individuals consuming a diet rich in animal protein from beef, which is also rich in fat, showed a lower abundance of bacteria, such as Roseburia, Eubacterium rectale, and Ruminococcus bromii, which metabolize plant polysaccharides from the diet.

In addition, a high-protein diet tends to limit carbohydrate intake, which may lead to a decrease in butyrate-producing bacteria and thus to a pro-inflammatory state and an increased risk of colorectal cancer. Similarly, plant-based proteins have been associated with lower mortality than animal-based proteins.

In terms of fat intake, recent studies suggest that both the quantity and quality of fat consumed significantly influence the composition of the gut microbiota. A plant-based diet is often naturally low in fat, which supports beneficial Bifidobacteria in the human gut microbiota. The fat in a vegan/vegetarian diet is composed of predominantly monounsaturated and polyunsaturated fats, which increase the Bacteroidetes:Firmicutes ratio, and at the genus level, increase lactic acid bacteria, Bifidobacteria and Akkermansia muciniphila. Nuts, particularly walnuts, have been shown to improve gut microbiota by increasing Ruminococcaceae and Bifidobacteria, and decreasing Clostridium sp. cluster XIVa Species.

Conversely, high consumption of saturated and trans fats, predominant in the Western diet, increases the risk of cardiovascular disease and decreases Bacteroidetes, Prevotella, Lactobacillus ssp. and Bifidobacterium spp. and increases Firmicutes.

Polyphenols, or natural plant metabolites in plant foods, increase the abundance of Bifidobacterium and Lactobacillus, which provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection. Among the most common polyphenol-rich foods are fruits, seeds, vegetables, tea, cocoa products and wine. The interaction of polyphenols and gut microbiota is bidirectional. Gut bacteria produce microbial metabolites from polyphenols, which in turn serve as prebiotics for gut bacteria. These metabolites, in particular the urolithins

promote the growth of Lactobacillus and Bifidobacterium.

Short-chain fatty acids act as a substrate to maintain the colonic epithelium and are correlated with the consumption of plant-based foods. Maintenance of the intestinal barrier prevents endotoxemia (presence of endotoxins in the bloodstream) and subsequent inflammatory effects. Thus, their levels increase in people who start a plant-based diet. Interestingly, an increase in SCFAs is observed when omnivores eat a Mediterranean diet rich in fruit, legumes and vegetables. SCFA promote immunity against pathogens, and are important components for the function and maturation of microglia and the control of blood-brain barrier integrity.

Another important effect of SCFA is to increase thermogenesis, preventing/treating obesity, and they are involved in the regulation of critical gut functions such as intestinal motility, visceral sensitivity, the epithelial barrier, immune homeostasis and the mucosal oxygen gradient.

Phytoestrogens are plant-derived polyphenols that interact with estrogen receptors with agonist or antagonist actions.  The vast majority of polyphenols reach the gut, given their bioavailability of 1%. The protective effects of plant polyphenols, in particular their anticarcinogenic, anti-inflammatory and antioxidant effects, and their association with reduced risks of cardiovascular disease, obesity, diabetes, osteoporosis and amyloid formation in humans are highlighted.

Most research suggests that a vegetarian or vegan, plant-based diet promotes greater microbial diversity. Microbiota diversity appears to have an important relationship with BMI and obesity. Studies have observed lower microbial diversity in obese versus non-obese people, implying that a vegetarian or vegan diet, associated with lower body weight, may benefit microbial diversity and protect against gut inflammation.

Current knowledge suggests that a diet can be an effective way to promote a diverse ecosystem of beneficial microbes that support overall health. However, due to complexity and individual differences, further research into the interactions between diet, the microbiome and health is needed.

References: 

1. Tomova, A et all The Effects of Vegetarian and Vegan Diets on Gut Microbiota, doi: 10.3389/fnut.2019.00047
2. Asadi et all, Obesity and gut–microbiota–brain axis: A narrative review DOI: 10.1002/jcla.24420