Exercise and Gut Microbiota
Kirstie Lamb – Researcher and Trainer in Public Health/ 3 July 2017
Our body contains tens of trillions of microbes (bacteria). There are more than a thousand different known species of these microbes, both ‘good’ and ‘bad’, that could be present in our gut (our gastrointestinal (GI) tract). We all have different compositions, amounts and types (referred to as the diversity) of these microbes. These make up our own unique microbiota profile. Our gut microbiota assists with the digestion and absorption of our food in order to produce the energy we need to function. Gut microbes are also required to maintain a healthy immune system and to reduce inflammation. Therefore, a diverse microbiota is essential for all aspects of our health. In this blog we’ll discuss exercise and gut microbiota.
Gut microbes and our health
This is a relatively new but expanding area of research, with an increasing amount of evidence showing that our gut microbiota plays a key role in the development of many diseases. For example, microbe imbalance (a reduction in beneficial ‘good’ bacteria and an increase in disease-causing ‘bad’ bacteria), potentially as a result of increased inflammation in the gut, has been linked to allergies, colon cancer, heart disease, digestive disorders, metabolic syndrome and diabetes . This imbalance is known as gut microbiota dysbiosis and defined by Gut Microbiota for Health (www.gutmicrobiotaforhealth.com) as ‘a disruption in the beneficial relationship between the body and its microbiota’. Loss of bacterial diversity has also been linked to GI diseases, obesity and obesity-associated disorders. For example, the abundance of a specific microbe called Akkermansia muciniphilla has been shown to inversely correlate with obesity (i.e. more of this microbe means a lower risk of obesity).
Our microbiota can be influenced by both internal and external factors including our diet, lifestyle (e.g. stress, antibiotic use and alcohol consumption) and our genetics. Our physical activity levels have also recently been suggested to affect our microbiota.
Physical activity and gut microbes
Sedentary behaviour (i.e. lack of physical activity) has been linked to several health conditions including heart disease, cancer and diabetes, which are also associated with poor microbiota diversity. It has therefore been proposed that one of the ways in which physical activity influences our health is through the modification of our microbiota. The influence of diet on our microbiota is more well-known than the effect of exercise. As this has been the focus of most existing studies. However, there is an increasing number of studies investigating physical activity and fitness. Initial findings suggest that we may be able to increase the number of ‘good’ bacteria in our gut by increasing our physical activity levels.
As this area of research is in its infancy, there is limited clinical evidence available and most existing studies have used mice or rats rather than humans! Studies in rats and mice have suggested that specific microbiota compositions help protect against obesity . Exercise (wheel running) has been demonstrated to induce changes in the gut microbiota of rats, increasing the presence of some ‘good’ bacteria . However, it has been acknowledged that diet may also play a role in these observed effects .
Unlike animal studies, human studies tend to be observational, rather than clinical trials, as there are greater ethical considerations required. One study conducted on professional rugby players showed them to have a greater diversity of gut microbes in contrast to healthy non-athletes [6, 7]. Within this study, non-athletic individuals with a lower BMI (body mass index) were shown to have a greater abundance of the Akkermansia muciniphilla bacteria than those with a high BMI, reinforcing the potential relationship that this bacteria has with our body mass.
However, as the diversity also correlated to protein intake, individuals’ diets may have confounded results. It is difficult to isolate the effects of physical activity on our microbiota from the effects of diet, as we know that the impact of the diet can be great. Nevertheless, one study found that, regardless of diet, cardiorespiratory fitness (CRF) correlated with increased microbial diversity (i.e. greater diversity of microbes found in fitter individuals). CRF is considered a high predictor of mortality (death), so greater physical activity-induced diversity may therefore reduce risk of mortality. Further research is needed to separate the effects of diet and exercise.
But I’m not a professional rugby player, you may be thinking, so how does this apply to me?
Well fear not, as there is good news for those of us who are not professional athletes! It has been observed that reductions in sedentary (inactive) behaviours may also help to increase microbiota diversity . This suggests that simply avoiding long bouts of inactivity, i.e. taking regular breaks from sitting, could help to increase the abundance of ‘good’ bacteria or bring about positive changes in our gut. By meeting the minimum World Health Organisation recommendations (150 minutes of moderate-intensity aerobic physical activity, or 75 minutes of vigorous activity, per week, in bouts of at least 10 minutes), we could induce additional benefits to our microbiota.
Additionally, physical activity may be indirectly beneficial for other health conditions via gut bacteria alterations. For example, low levels of a bacteria known as Coprococcus have been linked to inflammatory bowel disease (IBD), but have also been found to be more abundant in active versus sedentary women. This suggests a possible link between IBD, physical activity levels and gut health .
Potential mechanisms for the effect of exercise on our microbiota
There are several proposed mechanisms for the way in which physical activity can benefit our gut, although these are currently hypothetical owing to limited research[7, 11]:
Modification of the profile of bile acids.
Bile acids, produced by the liver, assist with the digestion of fats. However, they also have an anti-microbial effect, meaning that if too much acid is produced, the number of microbes in the gut may fall. The profile of our bile acids can therefore determine the diversity of bacteria present in our gut. An inverse relationship has been observed between the amount of bile acids produced and physical activity levels, i.e. as the amount of physical activity increases, the amount of acid decreases. As such, it is plausible that physical activity may indirectly alter our microbiota profile by regulating the production of bile acids.
Physical activity can be associated with weight loss. The diversity and composition of microbiota have both been shown to differ in obese vs non-obese individuals; although the mechanism for this is as yet unknown. For example, as previously mentioned, the Akkermansia muciniphilla bacteria is often present in greater abundance in obese individuals. It is therefore possible that weight loss could lead to a reduction in this species of bacteria. Additionally, this could mean that we could help promote weight loss by making other lifestyle changes that reduce the abundance of this particular bacteria. Further research is required to fully understand the mechanisms behind this.
Reduced gut transit time.
Intestinal transit time shown to reduce with both low and moderate intensity exercise (i.e. dietary components pass through the intestine quicker). It is suggested that this may lead to changes in microbiota composition, as the species of microbes required to aid digestion and absorption of our food will have to cope with this faster transit. There is also reduced contact time between pathogens (microorganisms that cause disease) and the mucus layer of the gut. Moderate exercise has also been associated with a lower rate of bacterial translocation, i.e. there is less movement of bacteria in our gut. This increases the opportunity for microbes to perform health-aiding functions.
Release of cytokines and other peptides.
During physical activity, molecules called cytokines are released from our muscles. An increased level of inflammatory cytokines in the blood triggers the secretion of anti-inflammatory cytokines. Anti-inflammatory cytokines help reduce inflammation and help us to heal following injury (either internal or external). The release of anti-inflammatory molecules has been shown following both aerobic and resistance training. Our microbiota is affected by inflammation and so it is plausible that exercise-induced release of anti-inflammatory cytokines could reduce or prevent inflammation of the gut, positively impacting our gut microbiota and health.
Suppression of proinflammatory cytokines.
In contrast, proinflammatory cytokines are signalling molecules excreted from immune cells which promote inflammation and damage. Excessive production of proinflammatory cytokines may contribute to the development of diseases such as cancer, atherosclerosis and neurological diseases. Habitual exercise and high fitness levels have been demonstrated to suppress pro-inflammatory cytokines. This suppression has also been observed following both aerobic and resistance training in individuals with Type 2 diabetes. Professional athletes have been shown to have lower levels of proinflammatory cytokines and an increased microbial diversity . It is therefore possible that anti-inflammatory responses to physical activity help promote positive changes to our gut microbiota.
Changes to our SCFA profile.
Physical activity may also incite changes to our short chain fatty acid (SCFA) profile as a result of altering our microbiota. An increase in the production of SCFA can have health-promoting effects through regulating lipid and glucose metabolism, as well as in-turn helping to increase the diversity of microbes in our gut. This was observed in rugby union players, who were shown to have greater microbial-produced SCFA than sedentary individuals .
This is an exciting area of research, but there is a requirement for further studies (especially in humans!) to separate the effects induced by physical activity and our diet. However, the evidence so far is positive and just reinforces the need for us to be more active and reduce our sedentary behaviours.
Therefore, whilst the exact mechanisms of the health benefits are currently unknown, the main message to take away from this blog is that there are benefits and so we should try and do more physical activity. This does not have to be in the form of structured exercise or sports, but could instead include walking more, doing housework or gardening or even just choosing to take the stairs rather than the lift!
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