People may be familiar with products that claim to impact gut health via probiotics and prebiotics, like kombucha and supplements. Prebiotics are plant fibers that encourage growth of beneficial gut bacteria, while probiotics are usually live bacteria that are associated with good gut health (Mayo Clinic Staff, 2021). Diversity comes up often in discussions about gut microbes. Diversity refers to the amount of different species present in a sample, and more diverse samples contain a wider range of microbes. We know that a less diverse community of gut microbes is associated with inflammatory bowel diseases (Cénit et al., 2014) and obesity (Turnbaugh et al., 2008). However, relatively little is known about how the performance of endurance exercise might impact the microbes living within the gut.
Many of us have probably been told at some point that exercise is good for us. In fact, exercise can even prevent the development of more than 40 chronic diseases (Ruegsegger and Booth, 2018). Athletes are prime examples of physically active individuals who train extensively to perform well in their respective sports. Training is known to have clear impacts on muscle tone (Higbie et al., 1996), cardiovascular fitness (Schjerve et al., 2008), and the amount that an individual needs to eat to fuel their body. It has been observed that there are differences in gut microbiota between athletes and people living a less active lifestyle (Barton et al., 2018) Thus, we know people who are in-shape seem to have a distinct microbial community when compared to the general population.
One particular subset of athletes that push their bodies to the limit are ultra-endurance athletes. These are the people many of us would consider crazy, who do things like run hundreds of kilometers over multiple days or rowing a boat across an ocean. Similar to patterns observed in other athletes (Barton et al., 2018),we expect to see a distinct set of microbiota associated with increased physical activity. One question not investigated in previous studies is how an athlete’s gut microbiota may be impacted by participation in an ultra endurance event. The researchers wanted to understand how the gut microbiota might change in response to sustained intense exercise. The research that will be shared in this blog post comes from a research article titled “Four men in a boat: Ultra-endurance exercise alters the gut microbiome” (Keohane et al., 2019). This particular study looks at an extreme example of an ultra-endurance event to see what patterns may emerge.
How might an individual’s gut microbes be impacted by extreme amounts of ultra-endurance exercise?
The subjects of the study are four male athletes who are already fit going into a 33 day, 5000km transoceanic rowing race. The researchers used poop samples from each of the men taken before, during, and after the race to observe how their gut microbes responded to the event.
The researchers analyzed the poop samples collected from the athletes to determine if there were any notable changes in diversity observed. Illumina sequencing, which is one of the most popular modern methods for analyzing DNA present in a sample, was used to process the samples. Next, the researchers used computer analysis to interpret their raw data. The researchers found that there was an increase in microbe diversity in the guts of three out of four athletes in the study (Fig 1). The increase in microbe diversity within each individual, or alpha diversity, was noticeable around day 17 of the study, and continued to increase throughout the rest of the endurance event. Athlete 3 did not show an increase in microbe diversity because he had to take antibiotics early on in the study which may have impacted the results. Antibiotics can have widespread effects on microbes throughout our bodies. The antibiotics were used to treat cellulitis, a bacterial skin infection (CDC, 2020) during the race. The researchers stated that the decline in diversity of the afflicted athlete was what you would expect to see from taking oral antibiotics. Athlete 4 did not experience changes that were as dramatic as athlete 1 or athlete 2, but this may have been because his baseline diversity was higher to begin with and more similar to the end result of other athletes.
The researchers found that there were 3 common bacterial species that increased in all 4 athletes over the course of the race: Dorea longicatena, Roseburia hominis, and unclassified members of genus Subdoligranulum. Bacteroides finegoldii decreased in all four athletes. Specifically, Roseburia hominis and members of the Subdoligranulum genus produce butyrate. Butyrate helps to regulate gut health, reduce inflammation, and it may possibly reduce the risk for certain cancers (Canani, 2011). Dorea longicatena has been shown in some cases to improve metabolism and increase insulin sensitivity (Brahe et al., 2015). Many people may have heard the term “insulin” tossed around here and there when someone is referring to blood sugar or diabetes. Insulin is a hormone involved in controlling the amount of sugar in the blood (Wilcox, 2005). Insulin plays a key role in allowing the body to absorb sugar from the bloodstream and use it for energy or other purposes (Wilcox, 2005). If someone is more sensitive to insulin, it allows them to more efficiently absorb and use sugars from the bloodstream (Wilcox, 2005). On the other hand, individuals with diabetes are either insensitive to insulin or do not produce insulin at all.
Metabolic gene pathways are essentially sets of closely linked different genes often expressed together, that function together in the same series of biological steps. Metabolism refers to a series of chemical reactions that either breakdown or build up molecules in the body that are important for maintaining health and internal balance in the body. The researchers used computer software to explore which metabolic gene pathways were expressed at a higher level as the study went on. There was a pattern of increases in microbes that are known to be involved in the production of some essential amino acids, specific medium and long chain fatty acids, SAMe biosynthesis, and fatty acid elongation as well as glycolysis (Keohane et al., 2019). The increased production of amino acids L-isoleucine and L-lysine is important because these amino acids are involved in improving muscular performance, specifically, muscular fatigue and slowing the breakdown of muscles. This would be very helpful for endurance athletes because to do well in their respective sports, they need to maintain some level of performance for an extended period of time without too much fatigue. It is possible that these amino acids may also help with cardiovascular fitness by allowing oxygen to be more efficiently carried throughout the body. The researchers tell us that SAMe plays several important roles in the body, the most relevant is that it acts as a precursor of a compound called glutathione (Leber and Packer, 2002). Glutathione is relevant here because it is an antioxidant, which can help counteract the negative effects of oxidative stress that is associated with endurance exercise. The researchers speculate that there may be benefits associated with the increase in long chain fatty acid production, such as reduced inflammation, but more research needs to be done to better understand possible benefits.
Ultimately, the results of the study suggest that endurance exercise has distinct effects on gut microbiota. Specifically, the changes observed by the researchers were associated with beneficial adaptations to help make our bodies more efficient with energy and help reduce inflammation associated with endurance exercise.
Researchers did consider the impacts of diet in this study, and they did record that subjects were getting roughly the same amount of nutrients each day. However, diet could still be a confounding variable in this study since the subjects did not eat exactly the same foods during the study. Controlling the diets of subjects would confirm that changes in exercise are what is directly responsible for any changes in diversity of gut microbes. This is a common challenge when researching factors that may impact gut microbiomes (Maneesh et al., 2012). Unless researchers are able to find people willing to completely alter their diet so they eat exactly the same amounts and types of food, this will likely continue to be an issue in future studies as well. Additionally, this study only included four people, so it would be important to do another study involving more people to see if the results remain the same. An interesting group to conduct a similar study on a larger scale would be athletes competing in the olympics, since in theory they represent the best of the best within their respective sports. Another consideration is that the exercise that the athletes in this study underwent was very extreme. More research would need to be conducted to see if similar effects can be observed across other endurance athletes who participate in less extreme events (short distance running, cycling, or lap swimming). It would also be really fascinating to see more research done comparing the gut microbiomes of different types of athletes (for example, comparing a sprinter to an ultra-marathon runner).
Further Reading and Information
This website provides broad information about the interactions between microbes and the human body in a way that is easy to understand. It is suitable for young readers as well as adults:
The following resources provide more information about physical activity and gut microbes:
This resource specifically discusses the gut in general, and includes discussion of how microbes fit into our gut health:
For people interested in learning more about insulin sensitivity in athletes, the following journal article discusses the topic in depth. The study looks specifically at the mechanisms involved in insulin sensitivity and the effects it has on the body:
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Ruegsegger, Gregory N, and Frank W Booth. “Health Benefits of Exercise.” Cold Spring Harbor perspectives in medicine vol. 8,7 a029694. 2 Jul. 2018, doi: 10.1101/cshperspect.a029694
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CDC. “Cellulitis.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 29 May 2020, https://www.cdc.gov/groupastrep/diseases-public/Cellulitis.html#:~:text=Cellulitis%20is%20a%20common%20bacterial,Many%20Bacteria%20Can%20Cause%20Cellulitis.
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