The Human Gut Microbiota
The human microbiota is defined as all microbes associated with human beings, these include bacteria, eukaryotes, viruses, fungi, protozoa and archaea. The microbiome denotes the collection of these microbes and their genes. Remarkably, it has been discovered that there is a great diversity regarding the microbiomes found in two different individuals, which is not apparent in the human genome. Humans are approximately 99.9% identical with regard to their genome, however their gut microbiome can vary up to 80–90%.
It has recently been estimated that there are 3 × 1013 (trillion) microbes harboured by each human being. The gut microbiota alone contains more than 1,000 microbial species. It is difficult to identify all bacterial species present in the large intestine as anaerobic species often attach to the gut mucosa. As such, stool samples are used to provide am estimation of the gut microbiota and overall, the genera Bacteroides, Prevotella, Bifidobacterium, Eubacterium, Clostridium, Streptococcus, and Enterobacteriaceae are most commonly found.
A plethora of research has been conducted investigating the role of the gut microbiota in both health and disease. As such, it has been suggested that the microbiome functions as a separate ‘organ’. Emerging research suggests that plant-based diets foster different microbiota profiles, with only small differences between vegetarians and vegans. Here we discuss…
The Effect of Diet on the Gut Microbiota
Differences in microbiota observed in a vegetarian or vegan diet may be due to differences in bacteria directly consumed through food, differences in substrates consumed, variations in transit time through the gastrointestinal system, pH, host secretion influenced by dietary patterns, and regulation of gene expression of the host and their microbiota. It appears that a plant-based diet may be beneficial for human health by promoting the development of a more diverse gut microbial system, or even by the distribution of different species.
It has recently been discovered that larger food particles, intact plant cell walls and food without thermal treatment could result in more nutrients reaching lower in the gastrointestinal system, thus enriching nutrient delivery to the gut microbiota. Thus, whole plant foods are thought to support the growth of beneficial fibre-degrading bacteria in the colon.
Carbohydrates – Digestible carbohydrates from fruits have been shown to reduce Bacteroides and Clostridia. Whereas, non-digestible carbohydrates (fibre), found exclusively in plants, most consistently increase lactic acid bacteria, Ruminococcus, E. rectale, and Roseburia, and reduce Clostridium and Enterococcus species.
Both digestible and non-digestible carbohydrates have been shown to increase Bifidobacteria, although this has been shown to be greatly affected by alcohol intake. High fibre intake also boosts the growth of species that ferment fibre into metabolites as short-chain fatty acids (SCFAs), including acetate, propionate and butyrate.
Proteins – Many studies have shown that animal and plant-proteins influence the gut microbiota differently. A high animal protein diet tends to result in lower abundances of bacteria, such as Roseburia, Eubacterium rectale and Ruminococcus bromii, which metabolize dietary plant polysaccharides. In turn, bile-tolerant microorganisms, such as Bacteroides and Clostridia, are increased in response to a high animal protein diet.
Fats – The gut microbiota composition is also impacted by the quantity and the quality of fats in the diet. A plant-based diet is generally naturally low in fat, with predominantly mono and polyunsaturated fats, which favours beneficial Bifidobacteria in human gut microbiota.
Polyphenols – Polyphenols are naturally occurring plant metabolites and have been shown to increase Bifidobacterium and Lactobacillus, which provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection.
Current research shows that diet is a key factor in influencing human gut microbiota composition. Up to date scientific research suggests that a vegetarian or vegan diet is effective in encouraging a diverse ecosystem of beneficial bacteria to support both the human gut microbiome and overall health. However, due to the complexity of the gut microbiota and the differences encountered in individuals, further research is required to fully understand these complex interactions.
Tomova A et al. (2019) The Effects of Vegetarian and Vegan Diets on Gut Microbiota. Front Nutr. Apr 17;6:47.