Ok, so this is a long one, but still I am barely skimming the surface about gut bacteria in this blog. I wanted to put together some of the most relevant and interesting information I could find with regards to optimising your gut health to benefit your training and exploiting your microbes to your benefit. In this blog I will discuss the links between gut microbiota and both mental and physical health as well as discussing what you can do to improve your own gut health (and, conversely, what to avoid for the sake of your microbes). Finally there will be a list of scientific papers and books that I used to compile this blog at the end so you can always read more if what you see here sparks an interest.

We have, on average, just under 2kg of bacteria inhabiting our guts, the majority of which inhabit the start of the large intestine (sometimes known as the colon). In fact, we have so many that these bacteria cells make up 90% of the total cells in our bodies, we are effectively just 10% human (Collen 2015)! These bacteria are involved in a variety of processes from helping us break down our food to producing vital hormones, vitamins (our microbes are the sole source of the active form of vitamin B12 our bodies need to function) and brain messenger chemicals (known as neurotransmitters) as well as producing chemicals that help instruct immune and inflammatory signalling pathways. Part of the bacteria present is due to our genes but a huge amount is down to our environment and what we put in our bodies – all of which we have the power to change.

Healthy guts for healthy minds – the impact of gut microbes on our mental health

The notion that the gut governs the mind dates back around 100 years when some scientists believed waste products from the colon built up and produced anxiety, depression and psychosis. However, these views were quickly dismissed by mainstream scientists of the time (Schmidt, 2015). But now, newer reports have since revisited these ideas, and a good job that is too. An incredible 90% of the feel-good brain chemical serotonin in our bodies is produced by certain strains of gut microbes during periods where we’re not eating (DeVadder et.al. 2018) from the amino acid tryptophan. Decreased serotonin levels have long-been associated with depressive states and, to evidence this, reduced gut microbiota diversity and species variation have been found in patients with depression (reviewed in Dinan and Cryan 2017). This suggests a strong link between gut and mental health and potentially also helps to explain reports of euphoria after periods of fasting.

Other bacterial species are responsible for the production of other key brain chemicals including dopamine (known as the reward chemical) and acetylcholine which is used to activate muscles. Although it is unlikely that any of the neuro-chemicals produced in the gut are able to cross the barrier into the brain, they have been shown to affect the nervous system within the gut, which then signals to the brain via the vagus nerve (Dinan and Cryan 2017).  The vagus nerve is a direct link between the gut and the brain, its role is to keep the gastrointestinal tract in good working order, regulating inflammatory and digestive processes as well as carrying signals about satiety (fullness) and immune responses back to the brain. The gut microbiota-brain signalling axis relies heavily on this physical connection as nutrients, microbes and their metabolites can all trigger vagal nerve responses both good and bad (Bauer et.al. 2019). We will talk more about the vagus nerve throughout later on.

A lot of the studies into gut microbes utilise germ-free mice, which are born and raised in completely sterile conditions and therefore have no gut bacteria. These mice display altered stress responses and anxiety levels as wells as augmented levels of the stress hormones ACTH and cortisol (reviewed in Breit et.al. 2018), making a pretty big case for the role of our gut microbes in mental health.

A weighty issue – the impact of gut microbes on our weight and waistlines

There are really strong links between gut bacteria and host (read: our) weight. Although in recent years there has been lots of research into gut microbes, health and disease there is still no hallmark signature as to what makes up a healthy gut. Generally, greater diversity of microbe species is known to be more beneficial for health and weight regulation, where obese individuals demonstrate a decreased gut microbiome diversity (Haro et.al. 2016).  Levels and/or presence of certain species of bacteria have been associated with obesity too. For example, obese individuals tend to have increased Firmicuites levels and decreased Bacteroidetes levels compared to those of a healthy weight (Haro et.al. 2016). It is also known that women tend to have a decreased gut microbiome diversity compared to men, potentially due to the production of certain female hormones (Spector, 2015).

Certain types of gut microbes are actually able to alter body composition and some harvest more energy from food, these are often found in obese individuals and may contribute to the reason why many obese people find it difficult to lose weight (Turnbaugh et.al. 2006). Back to our germ-free mice – these mice are unable to get fat even when overfed! However, introducing microbes from obese mice into these sterile mice made them gain weight showing us that the wrong types of bacteria in our gut can have huge impacts on our waistlines (Turnbaugh et.al. 2006). Furthermore, other researchers have shown a positive correlation between microbe diversity and response to various diet regimes (LeChatelier et.al. 2013) – our guts even dictate how well we are able to diet!

Finally, several studies have shown use of broad-spectrum antibiotics in infants, which wipe out both good and bad bacteria in the gut, have been linked with 22% increase in fat in follow up studies as well as predisposing them to obesity (reviewed in Spector, 2015). Just as scarily, babies born via C-section are 20% more likely to be obese, something which is thought to be due to not being exposed to the mother’s microbes in the birth canal as would occur with a natural birth, even if they are breast-fed as newborns (Darmasseelane et.al. 2014). It likely doesn’t help that mothers undergoing planned C-sections are often pre-treated with antibiotics to help prevent a small number of infections that occur in 1-3% of cases. Obviously a huge number of C-sections only occur in emergency situations to preserve the life of the mum and/or baby therefore are unavoidable, luckily it is quite easy to begin to alter our gut microbiome through diet as I will discuss a little later on.

No guts, no glory – the impact of gut microbes on physical health and sporting performance

So earlier I mentioned the vagus nerve and how important it was in digestions and the gut-brain axis. Well, physical activity can help modulate vagal tone, playing an important part in the gut-microbiome-brain axis (O’Mahony et.al. 2015). Moreover, exercise stimulates your gut microbes to produce butyrate, a short chain fatty acid that is key for regulating satiety (feelings of fullness), preventing leaky gut syndrome, regulating the immune system and is even thought to be protective against a host of diseases including obesity, IBS, diabetes and colorectal cancer (reviewed in Baxter et.al. 2019).

Overall exercise appears very beneficial for our gut microbes – there is a clear correlation between exercise and gut diversity as well as healthier levels of inflammation markers (Spector, 2015) so a regular gym, pole, aerial class or run could really benefit your mind, body and your gut. Another paper backed this up showing that athletes have greater microbial diversity than control subjects (Clareke et.al. 2014) with some reports linking this to increased protein intake (O’Sullivan et.al. 2015). There is also evidence that the right types of microbes can benefit our sporting performance, even boosting endurance (Turnbaugh et.al. 2006) and that a high microbial diversity and the presence of certain species of microbes correlates to better cardiovascular fitness (Estaki et.al. 2016). Although this is only correlation so we cannot say out-right that one causes the other.

Athletes should be wary of overdoing it however, as periods of energy deficiency or periods of enhanced physiological or psychological stress  (e.g. overtraining or lots of stress at work or in your personal life) can lead to non-beneficial changes in the gut microbiome (Bermon et.al. 2015 and Leclercq et.al. 2016).  Because of this, it has been suggested that athletes increase their consumption of fibre-rich foods such as vegetables, wholegrains, beans and pulses to help support their GI tract and their microbes from the stress of increased exercise load (Clark and Mach 2016).

Food for thought – what to feed your gut

  1. The good old Mediterranean diet

Several reports have shown the Mediterranean diet to be best when it comes to beneficial gut composition (De Filippis et.al. 2016). This is thought to be due to the large quantities of vegetables, legumes and pulses consumed which provide great food for your gut bacteria, as opposed to the high calorie, high processed foods and trans-fat-packed delicacies of the Western diet. The red wine polyphenols and omega-3 from the plentiful fish consumed as part of the Mediterranean diet appear to be beneficial to our microbes and help to restore the good-to- bad bacteria ratio (Lau et.al. 2018). A well varied is also key to ensure a good microbial diversity and an easy way to boost your own gut and general health.

  1. Polyphenols and antioxidants

Polyphenols and flavanoids are chemicals found in certain plants that are packed with antioxidants. They can be found in things like berries coca powder and red wine, with cocoa powder having the highest concentration of polyphenols and flavonoids of any food! Just to reiterate that’s cocoa powder not the product we like in huge quantities that’s been mixed with large doses of fats and sugar often called chocolate, sorry. These plant chemicals have been shown to increase levels of ‘good’ bacteria whilst decreasing levels of ‘bad’ bacteria and markers of inflammation (Tzounis et.al. 2011). In moderation red wine may be beneficial but be aware that an excess of alcohol can also cause a dysregulation in gut microbe levels (Engen et.al. 2015).

  1. Probiotics

Probiotics are the little microbes themselves. They are found in lots of different foods but are particularly enriched in fermented foods such as kefir, kombucha, kimchi, unpasteurised cheeses and sauerkraut or in specially formulated yoghurt drinks from your local supermarket. These foods, when eaten cold and not heated, can help introduce a range of beneficial bacteria to your gut. But, as soon as they are heated the bacteria are killed off and the product is no longer contains those beneficial microbes.

As with everything that’s supposedly good for us, it now also comes in pill form but there are questions over the effectiveness of these pills and whether we are actually better off just eating the correct foods instead. In order to ensure at least some of the bacteria you are eating actually survive the strong stomach acid and reach the colon you need a large number of them to start with (reviewed in Spector 2015). A few cases have however shown these supplements to be beneficial. One example demonstrates the benefit of probiotic supplements in pregnancy on glucose (sugar) metabolism (Zheng et.al. 2018) and thus may be able to play a role in preventing gestational diabetes. Of course, if you are pregnant, speak with a health professional first before you try anything new – particularly anything that is knowingly teeming with little bacteria(!) – and stay away from unpasteurised cheeses.

  1. Prebiotics

Prebiotics are food for our microbes, this includes lots of foods high in indigestible fibre such as raw garlic, onions, chicory root, raw asparagus, under-ripe bananas and acacia gum (sourced from Dr. Axe). A good source of these foods, eaten regularly, alongside some probiotics is the most effective way to increase growth of ‘good’ bacteria within your gut. However, just be aware that these foods tend to cause gas and bloating in some people and are likely to be avoided by those on a low-FODMAP diet. To help avoid this increase your intake of these foods slowly to allow your body to adjust.

If you’re looking for a prebiotic supplement then try inulin (NOT inSulin) powder – you can add it to your shakes, soups, smoothies, baking – pretty much anything to help give a good dose of microbe food.

Not so sweet – why sweeteners and antibiotics may not be all they’ve cracked up to be

  1. Sweeteners

So after many years of controversy, reports of sweeteners potentially causing cancer were finally rubbished and the sweetener scene has been quiet for a little while. The sugar tax in the UK has now been passed therefore cheaper varieties of our favourite fizzy drinks packed with calorie free sweeteners instead of sugar to avoid the tax. However, recent reports have shown that non-nutritive, low calorie sweeteners like sucralose-containing Splenda® and saccharin can lead to decreased gut microbe diversity (Abou-Donia et.al. 2008). Plant-based sweeteners such as Stevia may not be much better either. The active ingredient of stevia, rebaudioside A, prevents the growth of bacteria, including the good microbes, in the gut (Wang et.al. 2018) as well as altering the composition of the gut microbiome, even at low doses (Denina et.al. 2014, Nettleton et.al. 2019).

Alcohol sugars/ polyols (xylitol, malitol, isomaltase) are often avoided in low-FODMAP diets due to causing bloating, however, these sugar alternatives are able to reach the large bowel and increase numbers of beneficial bifidobacteria in humans as well as acting like a prebiotic for bacteria to feed off. Therefore these polyol sweeteners are probably your best alternative if you want to look after your gut health as well as your waistline and your teeth. A really comprehensive review of various sweeteners on gut health was compiled by Ruiz-Ojeda et.al. (2019), which you can read more of if you are interested.

  1. Antibiotics

Our second gut microbiome devil is antibiotics as I mentioned earlier on. As antibiotics kill bacteria (including the ones making us unwell), this means they do a good job of destroying the good microbes in your gut too. Sometimes a dose of antibiotics is essential but for minor infections like colds, coughs and sore throats just ask your doctor if antibiotics are really necessary before you take that prescription to your pharmacy. A huge number of the prescriptions given out each and every day are for unnecessary antibiotics where a virus in the cause and not a strain of bacteria. Even if it is a bacterial infection, a course of antibiotics often only speeds up recovery by a day or so (reviewed in Spector, 2015). As well as being harmful to your gut bacteria, excessive antibiotic consumption is leading to the global issue of antibiotic-resistance so the more we cut down, the better. If you can’t avoid an antibiotic prescription then make sure you dose up on pro and prebiotics to try to restore your gut bacteria and (unless otherwise directed) take the tablets alongside food.

In Conclusion….

So there are many more benefits to eating a wide variety of fruit, vegetables and wholegrains far beyond that of just improving our gut health. A healthy, balanced diet with lots of variety and little processed food provides the body with vitamins, minerals and other key nutrients to build muscle, survive and thrive on a day-to-day basis. However, there is a lot to be said for looking after your gut bacteria with benefits to both your physical and mental health. Although some of the work in this area thus far has been carried out on animal models like the germ-free mice (it’s pretty unethical to have babies born and raised in sterilised facilities so likely these kinds of experiments will never extend to humans), there is some really interesting evidence emerging for the benefits of looking after our gut microbes. This is a really exciting and fast-paced area of research which means there is more being done all the time so I will try to keep this blog updated as new research emerges.

Whilst gut bacteria aren’t the be-all and end-all to human health and diseases they sure seem to play a larger part than anyone ever imagined. It seems if you look after your gut bacteria they will very much look after you.

References and Further Reading

Abou-Donia. MB, El-Masry. EM, Abdel-Rahman. AA, McLendon. RE, Schiffman. SS, 2008. Splenda alters gut microflora and increases intestinal p-glycoprotein and cytochrome p-450 in male rats. Jornal of Toxicology and Environmental Health. 71(21):1415-1429. https://www.ncbi.nlm.nih.gov/pubmed/18800291

Bermon. S, Petriz. B, Kajėnienė A, Prestes. J, Castell. L, Franco. OL, 2015. The microbiota: an exercise immunology perspective. Exercise Immunology Review. 21:70-79. http://eir-isei.de/2015/eir-2015-070-article.pdf

Bauer. KC, Rees. T, Finlay. BB, 2019. The Gut Microbiota-Brain Axis Expands Neurologic Function: A Nervous Rapport. Bioessays. 17: e1800268. https://onlinelibrary.wiley.com/doi/epdf/10.1002/bies.201800268

Baxter. NT, Schmidt. AW, Venkataraman. A, Kim. KS, Waldron. C, Schmidt. TM, 2019. Dynamics of Human Gut Microbiota and Short-Chain Fatty Acids in Response to Dietary Interventions with Three Fermentable Fibers. MBio. 10(1). Pii: e02566-18. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6355990/

Breit. S, Kupferberg. A, Rogler. G, Hasler. G, 2018. Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in Psychiatry. 9:44. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5859128/

Clark. A, Mach. N, 2016. Exercise-induced stress behaviour, gut-microbiota-brain axis and diet: a systematic review for athletes. Journal of the International Society of Sports Nutrition. 13:43. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5121944/

Clarke. SF, Murphy. EE, O’Sullivan. O, Lucey. AJ, Humphreys. M, Hogan A, Hayes. P, O’Reilly. M, Jeffery. IB, Wood-Martin. R, Kerins. DM, Quigley. E, Ross. RP, O’Toole. PW, Molloy. MG, Falvey. E, Shanahan. F, Cotter. PD, 2014.  Exercise and associated dietary extremes impact on gut microbial diversity. Gut. 63(12): 1913-1920. https://gut.bmj.com/content/gutjnl/63/12/1913.full.pdf

Collen. A, 2015. 10% human. How your body’s microbes hold the key to health and happiness. William Collins, Nycteris Ltd. ISBN 978 0 00 758405 5.

Darmasseelane. K, Hyde. MJ, Santhakumaran. S, Gale. C, Modi. N, 2014.  Mode of delivery and offspring body mass index, overweight and obesity in adult life: a systematic review and meta-analysis. PLoS One. 9(2): e87896. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3935836/

De Filippis. F, Pellegrini. N, Vannini. L, Jeffery. IB, La Storia. A, Laghi. L, Serrazanetti. DI, Di Cagno. R, Ferrocino. I, Lazzi. C, Turroni. S, Cocolin. L, Brigidi. P, Neviani. E, Gobbetti. M, O’Toole. PW, Ercolini. D, 2016. High-level adherence to a Mediterranean diet beneficially impacts the gut microbiota and associated metabolome. Gut. 65(11): 1812-1821. https://gut.bmj.com/content/gutjnl/65/11/1812.full.pdf

Denina. I, Semjonovs. P, Fomina. A, Treimane. R, Linde. R, 2014. The influence of stevia glycosides on the growth of Lactobacillus reuteri strains. Letters in Applied Microbiology. 58(3): 278-284. https://www.ncbi.nlm.nih.gov/pubmed/24251876

De Vadder. F, Grasset. E, Manneras Holm. L, Karsenty. G, Macpherson. AJ, Olofsson. LE, Backhed. F, 2018. Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks. Proceedings of the National Academy of Sciences USA. 115(25):6458-6463. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6016808/

Dinan. TG and Cryan. JF, 2017. Brain-Gut-Microbiota Axis and Mental Health. Psychosomatic Medicine. 79(8):920-926.  https://www.ncbi.nlm.nih.gov/pubmed/28806201

Engen. PA, Green. SJ, Voigt. RM, Forsyth. CB, Keshavarzian. A, 2015. The Gastrointestinal Microbiome: Alcohol Effects of the Composition of Intestinal Microbiota. Alcohol Research. 37(2): 223-236. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590619/

Estaki. M, Pither. J, Baumeister. P, Little. JP, Gill. SK, Ghosh. S, Ahmadi-Vand. Z, Marsden. KR, Gibson. DL, 2016. Cardiorespiratory fitness as a predictor of intestinal microbial diversity and distinct metagenomic functions. Microbiome. 4(1): 42. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4976518/

Haro. C, Rangel-Zuniga. OA, Alcala-Diaz. JF et.al. 2016. Intestinal Microbiota is Influenced by Gender and Body Mass Index. PLoS One. 11(5): e0154090. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4881937/

Lau. LHS and Wong. SH, 2018. Microbiota, Obesity and NAFLD.  Advances in Experimental Medicine and Biology. 1061:111-125. https://www.ncbi.nlm.nih.gov/pubmed/29956210

Le Chatelier. E, Nielsen. T, Qin. J et.al. 2013. Richness of human gut microbiome correlates with metabolic markers. Nature. 500(7464): 541-546. https://www.ncbi.nlm.nih.gov/pubmed/23985870

Leclercq. S, Forsythe. P, Bienenstock. J, 2016. Posttraumatic Stress Disorder: Does the Gut Microbiome Hold the Key? Canadian Journal of Psychiatry. 61(4): 204-213. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4794957/

Nettleton. JE, Klanic. T, Schic. A, Choo. AC, Shearer. J, Borgland. SL, Chleilat. F, Mayengbam. S, Reimer. RA, 2019. Low-dose stevia (Rebaudioside A) consumption perturbs gut microbiota and the mesolimbic dopamine reward system. Nutrients. 11(6). Pii:E1248. https://www.ncbi.nlm.nih.gov/pubmed/31159256

O’Mahony. SM, Clarke. G, Dinan. TG, Cryan. JF, 2017. Early-life adversity and brain development: Is the microbiome a missing piece of the puzzle? Neuroscience. 342: 37-54. https://www.ncbi.nlm.nih.gov/pubmed/26432952

O’Sullivan. O, Cronin. O, Clarke. SF, Murphy. EF, Molloy. MG, Shanahan. F, Cotter. PD, 2015. Exercise and the microbiota. Gut Microbes. 6(2): 131-136 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4615660/

Ruiz-Ojeda. FJ, Plaza-Diaz. J, Saez-Lara. MJ, Gil. A, 2019. Effects of sweeteners on the gut microbiota: A review of experimental studies and clinical trials. Advances in Nutrition. 10: S31-48. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6363527/

Schmidt. C, 2015. Mental health: thinking from the gut. Nature. 518(7540): S12-5.  https://www.nature.com/articles/518S13a

Spector. T, 2015. The Diet Myth. Weidenfeld and Nicolson. ISBN 978 1 780 22900 3.

Turnbaugh. PJ, Ley. RE, Mahowald. MA, Magrini. V, Mardis. ER, Gordon. JI, 2006.  An obesity-associated gut microbiome with increased capacity for energy harvest. Nature. 444(7122): 1027-1031. https://www.ncbi.nlm.nih.gov/pubmed/17183312

Tzounis. X, Rodriguez-Mateos. A, Vulevic. J, Gibson. GR, Kwik-Uribe. C, Spencer. JP, 2011. Prebiotic evaluation of cocoa-derived flavanols in healthy humans by using a randomised, controlled, double-blind, crossover intervention study. The American Journal of Clinical Nutrition. 93(1): 62-72. https://academic.oup.com/ajcn/article/93/1/62/4597700

Wang. QP, Browman. D, Herzog. H, Neely. GG, 2018. Non-nutritive sweeteners possess a bacteriostatic effect and alter gut microbiota in mice. PLiS One. 13(7): e0199080. https://www.ncbi.nlm.nih.gov/pubmed/29975731

Zheng. J, Feng. Q, Zheng. S, Xiao. X, 2018. The effects of probiotics supplementation on metabolic health in pregnant women: An evidence based meta-analysis. PLoS One. 13(5): e0197771. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5962059/