A vast number of microbes inhabit our gastrointestinal tracts – and are collectively termed the gut microbiome. The gut microbiome profoundly orchestrates multiple domains of our normal human physiology. An average adult will harbour roughly 10 trillion human cells.
In contrast, the same adult will host approximately 100 trillion human cells, which is a staggering 10-fold increase! This enigmatic gut microbiome also weighs approximately 4.4 pounds! In essence, each human body (and specifically, each human gut) is a limitless, highly varied and thriving ecosystem which has the means to influence our normal physiology, metabolism, immune system and even mood.
Who are the Players in our Gut Microbiome?
Bacteria are fascinating creatures! Despite all the bad reputation these diverse creatures have earned throughout human history, most bacteria are harmless, and some are even fruitful to overall health – not just limited to the gut as we shall shortly see. So, who are the main players in our gut microbiome? The average healthy human adult will host approximately 1000 distinct species of bacteria, amongst which certain phyla dominate. These are the Bacteroidetes and Firmicutes bacteria. Of course, the composition of the gut microbiome is susceptible to environmental changes, just as the composition of oil and sweat on our skin is amenable to change by the weather. Nutrition, the use of antibiotics, certain diseases such as Celiac Disease or Inflammatory Bowel Disease, one’s physical activity, and even the mode of delivery can influence the composition of the gut microbiome.
What is the Gut Microbiome responsible for?
The Gut Microbiome has many functions apart from digestion and metabolism. The gut microbiome has been implicated in regulating the central nervous system (chiefly, the brain) and chronic inflammatory processes (such as asthma, eczema, allergies, and celiac disease, amongst others).
Functionally, the gut microbiome is chiefly responsible for digestion and nutrition. For example, the capacity to digest xyloglucans which are found in certain vegetables is undertaken by a species of Bacteroides. This ability is a rare trait in Bacteroides – yet, 92% of human individuals harbour at least one of these rare Bacteroides species, conferring themselves with the ability to digest xyloglucans.
The Gut Microbiome & Mood
However, one often overlooked role of the gut microbiome is in regulating mood. The microbial diversity and specific bacterial compositions of one’s gut has recently been validated as an important determinant of one’s risk of developing mood disorders such as Major Depressive Disorder (MDD) and Generalized Anxiety Disorder (GAD) among others. Indeed, patients with MDD have consistently reduced Faecalibacterium and consistently increased Actinobacteria and Enterobacteriaceae according to a study published this year. In fact, the role of the gut microbiome in eliciting mood disorders such as MDD has been a central focus of research and academic centres worldwide.
Although there isn’t a causal relationship that has been clearly defined (yet), researchers postulate that the gut microbiome affects mood by one of three pathways – modulating the hypothalamic-pituitary-adrenal axis, producing neurologically active substances such as GABA and influencing the neural firing rate of the hippocampus. Always ahead of his time, the eminent musician and lyricist Kurt Cobain was plagued with MDD and struggled with mental illness and alcoholism. He had intractable Irritable Bowel Syndrome and could not remedy his symptoms despite seeing physicians for six years.
Just last year, light was shed on the association between an altered gut microbiome and irritable bowel syndrome – changes in the gut microbiome resulted in dysmotility, increased gut leakiness, and a hyperactive gut immune system. Could Kurt’s depression have been a result of an altered gut microbiome which manifested as irritable bowel syndrome? The latest evidence seems to suggest so.
1. Menees, S. and W. Chey, The gut microbiome and irritable bowel syndrome. F1000Research, 2018. 7: p. F1000 Faculty Rev-1029.
2. Winter, G., et al., Gut microbiome and depression: what we know and what we need to know. Rev Neurosci, 2018. 29(6): p. 629-643.
3. Lucas, G., Gut thinking: the gut microbiome and mental health beyond the head. Microbial ecology in health and disease, 2018. 29(2): p. 1548250-1548250.
4. Liu, L. and G. Zhu, Gut-Brain Axis and Mood Disorder. Frontiers in psychiatry, 2018. 9: p. 223-223.
5. Huang, T.-T., et al., Current Understanding of Gut Microbiota in Mood Disorders: An Update of Human Studies. Frontiers in genetics, 2019. 10: p. 98-98.
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