Despite being overlooked for years, the gut microbiome is one of the largest components of our body. The commensal gut microbiota maintains a crucial host-bacteria balance that plays a key role in health and disease. The composition of the human gut microbiota is altered by various situations, such as cancer, diabetes, obesity, eating disorders, anxiety and depression.
There is increasing evidence that shows that the gut microbiota has an effect on inflammation, insulin and glucose metabolism and hepatic lipid metabolism. Also, gut bacteria interact directly with the central nervous system in the microbiota-gut-brain-axis, altering endocrine signaling pathways and thus affecting appetite.
Role of microbiota in metabolism
The gut microbiota is thought to be a contributing and even causal factor for obesity and metabolic syndrome, although the mechanism has not been exactly defined. Intestinal bacteria produce active metabolites in a diet-dependent manner, including short-chain fatty acids and conjugated fatty acids. These metabolites have peripheral effects but also modulate the brain via direct or indirect mechanisms, thus altering central regulation of appetite and food intake.
Effect of diet on gut microbiota
Diet is responsible for 57% of the changes in microbiome variations. Western diets (low fiber) have profoundly affected microbiome diversity. A high-fat and high-sugar diet is associated with a higher proportion of Firmicutes, that can process simple sugars, and lower abundance of Bacteroidetes.
Different components of our diet are responsible for the change in gut microbiome composition and associated health effects:
- Fiber: high-fiber diets are associated with a diverse and healthy microbiota.
- Fat: indirectly modulates intestinal microbiota through bile acid secretion.
- Saturated fats: associated with increase in Firmicutes and decreased Bacteroidetes. Health effects include endotoxemia, increase in body weight, adiposity and lower insulin sensitivity.
- Unsaturated fats: associated with an increase in Bifidobacterium and Akkermansia. Health effects include reduced inflammation and adiposity.
- Proteins: they are a major source of nitrogen, essential for fermentation of carbohydrates and production of short-chain fatty acids.
Effect of bariatric surgery on gut microbiota
Bariatric surgery has dramatically increased worldwide and due to the decrease in cardiovascular risk and diabetes, it is considered as a “metabolic procedure”. Gut dysbiosis plays a key role in the pathophysiology of obesity and it has been identified in overweight, moderately and severely obese patients. This dysbiosis consists of changes in bacterial composition and low microbial gene richness (MGR). In addition to its restrictive and malabsorptive properties, it has been suggested that bariatric surgery has an effect on regulating the microbiome towards a healthier phenotype.
Bariatric surgery leads to specific changes in host metabolism. The effect might be the key of the success of RYGB in the treatment of obesity, metabolic syndrome, and diabetes. Moreover, obesity entails an alteration of the food intake behavior after a modification in brain process.
To this day, the Roux-en-Y gastric bypass (RYGB) is the gold standard of bariatric surgery. Results from human and animal studies show that RYGB reduces adiposity and improves hormonal and inflammatory status, increases bile acids pool and improves insulin sensitivity. RYGB affects the composition of gut microbiota, increasing diversity. In general, patients have an increased abundance of Gammaproteobacteria and Verrucomicrobia (Akkermansia) and decreased abundance of Firmicutes.
Fecal microbiota transplantation from mice that received a RYGB into germ-free mice resulted in weight less and decreased fat mass, potentially due to altered microbial production of short-chain fatty acids, increased propionate levels and decreased acetate levels.
Gut microbiome as therapeutic strategy
Given the important role of gut microbiota in host’s metabolism regulation, recent studies are focusing on the modulation of intestinal microbiota with prebiotics, probiotics or fecal transplant as anti-obesity strategies.
- Probiotics: live microorganisms that offer beneficial effects to the host. The most common probiotic species are Bifidobacterium and Lactobacillus and more recently Akkermansia mucinophila, that has shown the potential to reverse high-fat diet induced metabolic effects (ie. endotoxemia, adipose tissue inflammation and insulin resistance).
- Prebiotics: are non-digestible compounds that, through their metabolism by gut bacteria, modulate the composition and/or activity of the gut microbiome.
- Fecal matter transplant: evidence is increasing on its potential to treat obesity and associated comorbidities such as type 2 diabetes. Transfer of gut microbiota through a duodenal tube from healthy donors with a normal BMI to obese patients increased insulin resistance, faecal microbiota diversity and decreased faecal short-chain fatty acids.