You’ve probably all heard of the the gut microbiome (the collection of microorganisms living in the digestive tract) and you may be aware of the effects that it can have on the health of your digestive system. But are you aware of how it can influence the health of many other bodily systems?
In a 70 kg human, there are approximately 38 trillion bacteria within our gut – that’s 38,000,000,000,000 (38 with twelve zeros after it) (1). This means that there are more bacterial cells in the body than human cells. It’s starting to come to light that these bacteria not only play a vital role in the maintenance of a healthy digestive tract, but that dysbiosis (an imbalance or maladaptation of the bacteria) can cause a vast array of other health conditions.
Let’s start by looking how they influence the health of the digestive tract.
What makes up the gut microbiome?
The microorganisms that make up the microbiome include bacteria, viruses, fungi and, sorry to say it, parasites. Most of these organisms inhabit the large intestine, with a much smaller amount making their home in the small intestine. A normal gut microbiome is hard to define, as it varies from population to population around the world. The microbiome of indigenous hunter-gatherer populations share bacterial families with other traditional societies, that are all but missing from the microbiomes of industrialised populations. There are observed seasonal shifts in bacterial species and diversity (2). The microbiome also varies with sex and age. What we do know however is that a greater diversity in the microbiome is linked to better health. Lower diversity is considered a marker of dysbiosis and has been found in obesity, autoimmune diseases and cardiometabolic conditions.
However, there is a common “core” of bacteria which inhabit most people’s digestive system. Bacteroides and Firmicutes make up approximately 90% of the categories present in the distal gut (4). Bacteria can be:
- Commensal – those that normally live in the gut
- Pathogenic – those that cause disease, such as Salmonella.
- Opportunistic – those usually present at low levels, but multiply to potentially cause problems when beneficial bacteria levels are low
Viruses can live in harmony with the bacteria of the gut (5) or cause infection (such as norovirus, the “winter vomiting bug”). Fungi are usually present in small amounts within the digestive tract, but can proliferate to cause problems with antibiotic use or if people consume a high amount of carbohydrates.
Functions of the gut microbiome – within the gut
A healthy gut microbiome plays important roles in producing important fatty acids, vitamin synthesis, gastrointestinal motility and maintaining the barrier function of the gut.
Short chain fatty acids (SFCAs)
These are produced by gut bacteria and control the pH of the bowel. An optimal pH controls the growth of pathogenic organisms and helps with the absorption of nutrients. Other roles of SFCAs include:
- Maintaining the barrier function of the tight junctions.
- Participation in lipid and glucose metabolism in the liver.
- Production of gut hormones.
- Appetite control.
- Controlling inflammation.
- Protection against the formation of colorectal cancer (6).
A healthy gut microbiome plays a role in Vitamin K2, folate and thiamine synthesis. Vitamin K2 is important in maintaining cardiovascular health, skin appearance and strengthening bones. Folate is important for DNA synthesis and cell division, while thiamine helps carbohydrate metabolism, muscle contractions and the conduction of nerve signals.
This is a two way relationship here, with a disrupted microbiome having an effect on motility and motility issues having an effect on the microbiome. Dysmotility predisposes to an increase in colonic bacteria in the small intestine and the development of SIBO (Small Intestinal Bacterial Overgrowth) as a complication.
Barrier function of the gut
The microbes of the gut help maintain a healthy intestinal lining and help maintain the barrier function of the gut. This is by targeting and changing the expression and distribution of tight junction proteins (7). Tight junctions regulate the intestinal barrier function, keeping out the things that shouldn’t normally be absorbed. When the barrier system malfunctions and permeability becomes inappropriate, this is called leaky gut , or intestinal permeability.
Functions of the gut microbiome – outside the gut
Research has steadily been emerging that the gut microbiome has an effect on many organ systems and tissues elsewhere in the body.
The gut-brain axis is no longer the realm of alternative health practitioners. Mainstream medicine is starting to recognise the very real associations between gut microbiome disruptions and neurological or psychiatric disorders. These include:
- Autism (8, 9, 10, 11, 12)
- Parkinson’s disease (13)
- Alzheimer’s disease (14)
- Anxiety and depression (15)
Importantly for the future, the gut microbiota may be a modifiable factor modulating the development or pathogenesis of neuropsychiatric conditions (16). Future research is expected to assess whether modulation of the gut microbiota and its functions has any effect in improving mental health.
The Cardiovascular System
Heart disease is still one of the leading causes of death worldwide and is the leading cause of death of males in the UK. There is emerging evidence that the gut microbiome may play a significant role in atherosclerosis, coronary artery disease and myocardial infarction (17). Structural component of microbiota such as lipopolysaccharides (LPS) and pro-inflammatory peptidoglycans can trigger numerous downstream signalling processes within blood vessels, particularly when gut wall barrier function is impaired.
Gut microbiota can also have an impact via bioactive metabolites that can affect distal organs directly or indirectly, through a number of pathways, including the trimethylamine N-oxide (TMAO) pathway and the short chain fatty acid (SFCA) pathway. Trimethylamine production from gut microbes and subsequent modification to TMAO has emerged as a strong microbiome-mediated risk factor for cardiovascular disease. TMAO is associated with enhanced atherosclerosis and thrombosis in vitro and in vivo. SCFAs can play an important role in regulating metabolic health and reducing cardiovascular disease risk.
Atherosclerotic plaques contain bacterial DNA, and the bacterial taxa observed in these plaques were also present in the gut of the same individuals.
Various factors are involved in the development of Type 1 Diabetes, including diet, genome, and gut microbiota. Some have reported reduced microbial diversity in children with islet autoimmunity before progression to diabetes, compared with healthy controls (18). The gut microbiome has also been implicated in the development in Type 2 Diabetes (19), with a reduction in butyrate producing species in people with Type 2 Diabetes.
An elevation in the ratio of Firmicutes to Bacteroidetes has been linked with a greater risk of being obese or overweight (20). Dysbiosis can lead to obesity by increasing the amount of calories that are absorbed from a person’s food intake. Other mechanisms dysbiosis promotes obesity are by the development of leaky gut which can cause systemic inflammation, by increasing appetite and by impairing mechanisms of satiety or fullness after eating. (21)
Changes in the gut microbiome have been associated with hyperthyroidism (22). The mechanism through which this can occur is by the sustained release of LPS into the bloodstream. LPS has been demonstrated to inhibit the enzyme iodothyronine deiodinase, which converts thyroxine (T4) into the more active thyroid hormone triiodothyronine (T3). LPS has also been shown to reduce the expression of thyroid hormone receptors, which have effects on all cells in the body.
As you can see from this article, a healthy gut microbiome is not just essential for a healthy digestive system, but also a crucial component for good overall health too. Hippocrates was on the right track by saying “all disease begins in the gut”. Diet, managing your stress and exercise all play vital roles in maintaining a healthy gut microbiome. Evidence is starting to emerge of the effects that the gut microbiome composition has on organs systems distant to the gut and the list is growing all the time. It will be interesting to see how future research addresses modulation of the gut microbiome and how this impacts the development of diseases, that are becoming increasingly common in the industrialised world.
If you would like to know more about your own microbiome, get tested for dysbiosis, or improve your overall gut health, you can make an appointment to see me by clicking here.
- Sender R, Fuchs S, Milo R (2016) Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biol 14(8): e1002533. doi:10.1371/journal.pbio.1002533. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4991899/pdf/pbio.1002533.pdf
- Gabriela K. Fragiadakis, Samuel A. Smits, Erica D. Sonnenburg, William Van Treuren, Gregor Reid, Rob Knight, Alphaxard Manjurano, John Changalucha, Maria Gloria Dominguez-Bello5, Jeff Leach, Justin L. Sonnenburg. Links between environment, diet, and the hunter-gatherer microbiome. Gut Microbes. 2019;10(2):216-227. doi: 10.1080/19490976.2018.1494103. Epub 2018 Aug 17.https://www.biorxiv.org/content/biorxiv/early/2018/05/15/319673.full.pdf
- Valdes A et al. Role of the gut microbiota in nutrition and health. BMJ 2018;361:k2179. https://www.bmj.com/content/bmj/361/bmj.k2179.full.pdf
- Qin J et al. A human gut microbial gene catalog established by metagenomic sequencing. Nature. 2010 March 4; 464(7285): 59–65. doi:10.1038/nature08821. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3779803/pdf/emss-54210.pdf
- Ríos-Covián D et al. Intestinal Short Chain Fatty Acids and their Link with Diet and Human Health. Front. Microbiol., 17 February 2016 | https://doi.org/10.3389/fmicb.2016.00185
- Dulantha Ulluwishewa Rachel C. Anderson Warren C. McNabb Paul J. Moughan Jerry M. Wells Nicole C. Roy Regulation of Tight Junction Permeability by Intestinal Bacteria and Dietary Components. The Journal of Nutrition, Volume 141, Issue 5, May 2011, Pages 769–776, https://doi.org/10.3945/jn.110.135657
- Finegold, S. M. et al. Pyrosequencing study of fecal microflora of autistic and control children. Anaerobe 16, 444–453, https://doi.org/10.1016/j.anaerobe.2010.06.008 (2010).
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- Williams, B. L., Hornig, M., Parekh, T. & Lipkin, W. I. Application of novel PCR-based methods for detection, quantification, and phylogenetic characterization of Sutterella species in intestinal biopsy samples from children with autism and gastrointestinal disturbances. MBio 3, e00261–11, https://doi.org/10.1128/mBio.00261-11 (2012).
- Gondalia, S. V. et al. Molecular characterisation of gastrointestinal microbiota of children with autism (with and without gastrointestinal dysfunction) and their neurotypical siblings. Autism Research 5, 419–427, https://doi.org/10.1002/aur.1253 (2012).
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- Filip Scheperjans MD et al. Gut microbiota are related to Parkinson’s disease and clinical phenotype. Mov Disord., 30: 350-358. https://doi.org/10.1002/mds.26069
- Nicholas M. Vogt et al. Gut microbiome alterations in Alzheimer’s disease. Scientific Reports 7, Article number: 13537 (2017). https://www.nature.com/articles/s41598-017-13601-y
- Jane A.Foster et al. Gut–brain axis: how the microbiome influences anxiety and depression. Trends in Neurosciences. Volume 36, Issue 5, May 2013, Pages 305-312. https://doi.org/10.1016/j.tins.2013.01.005
- Cenit MC, Sanz Y, Codoñer-Franch P. Influence of gut microbiota on neuropsychiatric disorders. World J Gastroenterol 2017; 23(30): 5486-5498. https://www.wjgnet.com/1007-9327/full/v23/i30/5486.htm
- W.H. Wilson Tang, MD, Takeshi Kitai, MD, PhD, and Stanley L Hazen, MD, PhD. Gut Microbiota in Cardiovascular Health and Disease. Circ Res. 2017 Mar 31; 120(7): 1183–1196. doi: 10.1161/CIRCRESAHA.117.309715
- de Goffau MC, Luopajärvi K, Knip M, et al. Fecal microbiota composition differs between children with β-cell autoimmunity and those without. Diabetes. 2013;62:1238–44. [PMC free article]
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- Lei ZhouXinli Li, Ayaz Ahmed, Dachang Wu, Liang Liu, Juanjuan Qiu, Yao Yan, Meilan Jin, Yi Xin. Gut Microbe Analysis Between Hyperthyroid and Healthy Individuals. Current Microbiology November 2014, Volume 69, Issue 5, pp 675–680.
Asim is a medical doctor (Consultant Anaesthetist) with 18 years experience in the NHS. He currently specialises in providing anaesthesia for Hepato-pancreato-biliary (liver, pancreas and bile duct) procedures and Liver transplantation. In his current post, he is part of a cohesive team of highly skilled multidisciplinary clinicians, forming the largest liver transplant unit in Europe. Hepato-pancreato-biliary anaesthesia comprises 30 per of the work that he undertakes. The remaining 70 per cent is split across many different specialties, such as anaesthesia for Vascular, Renal, General, Urological, ENT and Emergency surgery, amongst many.
Asim has undergone an extensive Functional Medicine Certification programme, with over 1000 hours of training at the Kresser Institute, run by Chris Kresser M.S., L.Ac, who is the co-director of the California Center for Functional Medicine and founder of Kresser Institute. The ADAPT (Advanced Diagnostics and Personalised Treatment) programme is one of the first and only training programmes to fully integrate functional medicine with the better questions and new insights of an ancestral, evolutionary perspective. Asim is also a member of the Institute of Functional Medicine (IFM). As a hospital Consultant and a Functional Medicine practitioner, he has learnt how to effectively combine his knowledge and skills to treat of a range of health issues.