Most of the microorganisms that live on and inside us are contained in our digestive tract with increasing concentrations generally found the further down you go (💩). While research investigating these gut-specific microbes is still emerging, what is clear thus far, is this community is essential to our health. In relation, a recent and intriguing research area is aimed at assessing the potential linkage of these gut microbes with features of athleticism.
What is the gut microbiota?
The gut microbiota is defined as diverse ecosystem consisting of (primarily) bacteria, archaea, viruses, protists, and even fungal communities all residing in the gastrointestinal tract (1). What we are trying to establish in this blog is how important factors like exercise and diet influence this community. And, for our own selfish desires, how does this community might benefit us. A sort of “two-way street” if you will.
Those that engage in regular exercise and have a specific diet regimen appear to have a community distinct from those more sedentary (2, 3). The structure of the athlete gut microbiota is likely, in part, the result of adaptations to these long‐term lifestyle factors (4). Indeed, athletes adhering to fairly spartan schedules for years and even decades are not uncommon.
What makes an athlete's gut microbiota "distinct"?
A key feature to highlight, in the context of athletes, is the role the gut has in producing short-chain fatty acids (SCFAs). These molecules can be used as a fuel substrate by the body and even act as signaling intermediates involved in the regulation of metabolism and inflammation (5). SCFAs are produced by the fermentation of non-digestible food components such as dietary fiber and other components, including those derived from our own bodies (6).
In comparison to sedentary individuals, athletes have increased fecal metabolites and improved overall health (unless over-trained or in energy deficiency) (7). While speculative, athletes may also possess gut microbiota “resilience”. What this refers to is the ability of the gut flora to return to “baseline” following stressful situations like extreme dietary or exercise pressures. This is recognized as an important feature of a health-associated gut community (8).
"In comparison to sedentary individuals, athletes have increased fecal metabolites and improved overall health (unless over-trained or in energy deficiency)".
What effect does exercise have on the gut microbiota?
In the last few years several research groups have been able to capture the effects of extreme exercise on the gut microbiota. For example, examining Boston marathon participant’s stool samples, Scheiman and colleagues noted an increase abundance of a microbe called Veillonella after the race (9). Through a series of experiments involving isolating a specific strain of Veillonella from participant’s stool, mice inoculated with this bacterium dramatically increased exercise performance. What appeared to be occurring was this microbe metabolized lactate (a metabolite of muscle metabolism) into SCFAs (a fuel source). Scheiman and colleagues theorized that higher levels of lactate in the gut of athletes might favor the growth of these bacteria which in turn could help aid performance. But exactly how this these bacteria are linked to improved performance is currently unknown.
SCFA-producing microbes have shown up in other investigations of endurance-based exercise events, including the grueling Western States Endurance Run, a 163 km mountain footrace. Here, changes in the gut microbiota of a world-class ultramarathon runner before and after competing were reported, showing a massive increase in Veillonella two hours post-race (10). In another extreme case, four well-trained male athletes had their gut microbiotas traced before, during, and after a continuous, unsupported 33-day, 5000 km transoceanic rowing race averaging close to 400 hours of rowing for each athlete (11). Increased abundance of SCFA (i.e., butyrate) producing species and species associated with improved metabolic health were noted.
More recently, two unfit males had their gut flora tracked for 6 months as they undertook progressive exercise training with one training for a marathon and the other an Olympic-distance triathlon (12). There were increases in health-associated metrics like community diversity and abundance of microbial species that have been shown to influence SCFA production. Importantly, these two participants had differential changes in specific health-associated microbes highlighting a very important feature of the human gut microbiota. It is individualized.
As with training adaptations, the response of the gut flora to exercise is likely quite variable and, as noted above, individual. Moreover, it is extremely hard to separate out factors like diet, especially since many athletes are on a very specific regimen (13). Finally, not all exercise stress is necessarily good for the gut. For example, athletes training at high intensities for long periods without adequate fueling are at risk for disturbances in gut integrity and function and gastrointestinal symptoms (14). These issues do raise some caution when looking at the athletic gut microbiota and generalizing findings.
"As with training adaptations, the response of the gut flora to exercise is likely quite variable and... individual".
So, does an athlete’s gut microbes help aid performance?
From the limited evidence, athletes as a group appear to harbor an increased abundance of functional pathways within the microbiota that could support exercise metabolism and athlete health (7). Like the SCFA-producing microbes and SCFA production noted. In some sense, the gut microbiota may be viewed as an energy harvester for athletes. Indeed, the digestive tract offers an incredibly large exchange surface area (80 m2!) for gut-derived metabolites (15).
In instances like endurance-based exercise, which can be extremely metabolically demanding, SCFA may be an important consideration for performance as they are readably absorbed into systemic circulation (16). These SCFAs can then be directly used in muscle and other tissues (17). In skeletal muscle, SCFAs can support energy metabolism during exercise (18). SCFAs also contribute to increased blood flow, insulin sensitivity, skeletal muscle mass preservation, and an oxidative phenotype (17).
The main takeaways...
The gut microbiota of athletes appears to have increased fecal metabolites like SCFAs, which may play a role in exercise performance and overall health compared to less active individuals (7). These differences are likely driven by the effects of exercise training and/or dietary intake. They may also have a greater ability to harness energy from the diet and products of exercise metabolism.
Overall, the mechanisms by which exercise may promote a rich bacterial community, increased functional pathways, and exercise-enhancing metabolites are not fully understood, but likely involve a multitude of factors beyond training and diet. Finally, most of the studies are correlative (we all know the adage; “correlation does not imply causation”). However, there is a growing interest in researching how the gut is modified by longitudinal designs and if the microbiota can be “trained”. A topic we will cover in Part II (to be published soon).
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