The gut microbiota is the term given to the vast and diverse microbial population present in our gastrointestinal tract. This microbiome consists of around 10¹⁴ resident commensal microorganisms, including bacteria, viruses, fungi, and protozoa living in harmony within the intestine. Overall, the predominant bacterial groups in the microbiome are gram-positive Firmicutes and gram-negative Bacteroidetes. This microbial population is essential for the proper functioning of the gut and digestion, immunity, and mental health.
The internal ‘ecosystem’ is unique to every individual and can undergo alterations as one ages. The proper working of the gut microbiota is determined by the kind of diet we have been given since infancy. The entire setup for a stable gut microbiota takes place when the infant transitions from a liquid diet to a more solid food diet between the years of 2 to 3. Exposure to antibiotics and diet during this age lays down the foundation of the gut ecosystem which then stabilises and is resistant to dietary changes.
It is also observed that between 6 to 12 years of age there is greater microbial diversity whereas as an adult the microbiome is comparatively stable. Hence after any kind of disruption such as malnutrition or a gastric infection, gut health can be easily restored at a very young age. As a person ages, dietary habits are established based on lifestyle and personal preferences leading to a more stabilized microbiota.
The gut microbiome is heavily influenced by circadian ebb and flow despite the lack of dark and light exposure. For example, jet lag promotes glucose intolerance and obesity and shift workers are prone to increased dietary intake, hence more inflammatory issues. Timely food intake also plays an important role in the stability of the microbial ecosystem. Eating a little later than normal can alter the microbial population to become more proinflammatory and affect body weight, cortisol levels, basal metabolic rate, glucose tolerance and body temperature.
Short term changes in the gut microbiome are seen with extreme dietary interventions. A person who consumes exclusively animal-based products (like a traditional keto diet) have increased protein fermentation levels and much lower carbohydrate fermented metabolites as compared to a person who consumes a baseline (equal quantities of both animal and plant-based products) or a solely plant-based diet. However, the microbiota did return to normal pre-diet state three days after extreme diet alterations.
Prebiotics play an important role in priming the gut ecosystem. Prebiotics are substances that are selectively acted on by the host microorganisms conferring health benefits. Prebiotic consumption helps with the growth of Bifidobacterium, Lactobacillus and lactic acid bacteria. The most common prebiotic is fibre. Fibre is resistant to digestion and breakdown by host enzymes and hence is a substrate for gut microorganisms.
These fibres are digested down to short-chain fatty acids which are further broken down into butyrate, propionate and acetate which in turn strengthen the inner lining of the intestine. Consuming whole grain fibre has been associated with lower inflammation. However, for people suffering from ulcerative colitis or Crohn’s Disease, the gut microbiome is compromised and they are unable to digest fibre, hence a refined grain diet is prescribed to them.
A study conducted by a research team at the University of California, San Francisco found that caloric restriction disrupted gut microbial composition. They observed that the microbial composition deteriorated despite there being improved metabolic health in mice. Disrupted microbiota is associated with several inflammatory disorders. When the mice were replenished post the extreme diet, their microbial health returned to pre-diet. They stated that calorie-restricted weight loss leads to impaired nutrient uptake and decreased bile production. These findings could explain the imbalance seen in people who suffer from eating disorders such as anorexia and people who undertake extreme fad diets.
Many studies have gone on to prove that diet heavily impacts the gut ecosystem which in turn impacts overall health. Understanding how diet influences microbial composition can be an excellent therapeutic point for many gastrointestinal disorders like Irritable Bowel Syndrome, Ulcerative colitis, Crohn’s disorder etc. It can also help with recovery from dysentery, and help people who suffer from eating disorders to proper health. This can pave the way for personalised nutrition which is extremely beneficial and supports precision medicine.