Just as the question, “what is it to be human?”, has troubled humans from the beginning of history, the question, “what is the human microbiome?” has troubled researchers since the term was first coined in 2001. Rapidly developing sequencing methods and analytical techniques are now increasing our ability to understand the human microbiome, and, indeed, how the microbiome and its constituents are defined. The human microbiota consists of between 10-100 trillion microbial cells which are found in every individual, primarily through bacteria in the gut; the human microbiome consists of the genes found within these microbial cells. It has been estimated that the gut microbiota contains over 3 million genes, which would make it about 150 times more genetically varied than the human body.
Evidence is now mounting for an inextricable link between an individual’s microbiota, digestion, and metabolism. Dietary changes can lead to significant changes in bacterial metabolism, especially small chain fatty acids and amino acids. In fact, the genetic diversity found within the gut microbiota allows us to digest compounds via metabolic pathways not specifically coded to us, which greatly increases our ability to extract energy from diverse diets. It is now thought that the microbes we ingest with our food may in fact be providing our individual microbiome with new genes to digest new foods, eg a porphyran-digesting gene found in Japanese individuals has not been found in the gut microbiome of American individuals which is possibly due to the consumption of seaweed, common in a Japanese diet but not in an American diet.
We take antibiotics to get rid of a bacterial infection. However, the current generation of antibiotics known as “broad spectrum” antibiotics means that they can also target our gut microbiota. Three to four days after starting treatment with a “broad-spectrum” antibiotic the gut microbiota appears to experience a decrease in richness, diversity, and evenness. The actual differences an antibiotic can make to the gut microbiota varies from person to person, but while the gut microbiota is able to resemble its pre-treatment state a week after completing treatment, differences have again been seen between individuals as to how closely the post-treatment gut microbiota resembles the pre-treatment gut microbiota. In fact, it has been estimated that re-establishment of some species of gut microbiota can take up to four years following the antibiotic treatment.
The gut microbiota of each individual is therefore unique, and it can heavily contribute to how a person fights disease, digests food, and even their mood and psychological processes.
It has been said by some researchers that up to 90% of all medical conditions can be traced back in some way to the gut and health of the microbiome. Currently the medical conditions that may be linked to gut microbiota include:
- Coeliac disease
- Heart disease
- Multiple sclerosis
- Parkinson’s disease
- Rheumatoid arthritis