Microbiology of Endurance: The Microbiome’s Role in Fitness

March 4, 2022

Any aspiring runner can open up Pandora's Box simply by googling "how to train for a race." The internet is awash with snazzy life hacks, tips and tricks for fitness and all the inspiration one could want to don spandex and step out into the cold for a jog, complete with videos and hip soundtracks. New athletes are beseeched with every schema under the sun to increase endurance, avoid injury and grow stronger. And while much of this buffet of advice involves using nutrition to fuel athletic gains, surprisingly little has been discussed about a related aspect of our health and wellbeing: the microbiome.

Nutrition and the Microbiome

The disconnect between microbiome and nutrition in the sports world is stark, given that these microbes are the symbionts that rest in our bowels and metabolize the parts of our dinner (and pre-run snack) that we cannot. They are the nutritional tradesmen of our colon, exchanging what we can't use for metabolites our bodies crave and require. The dissonance is perhaps an artifact of a bygone era where we believed in the strict stoichiometry of calories in, calories out. We know now that it's not quite so simple.

The microbiome is central to a healthy metabolism, endocrine and immune system, and it is deeply influenced by our nutrition. But what about its role in our fitness? Our stamina and perseverance? Our endurance? Surely, these are evolutionary paradigms that demand its participation.

The silence on this subject in the sports world may be because the data in this realm are still in their infancy. While we know much about how our intestinal symbionts play an important role in other areas of our metabolism, their role in our fitness is murky and, at times, inconclusive. Moreover, even as we unearth new findings, such information has to be carefully conveyed to avoid a bonanza of "scientific-ish" claims about how to improve people's relationships with their bodies.

Connections Between Exercise and the Microbiome

We know that exercise is affiliated with greater microbial diversity in the intestine, which is generally regarded as a marker of health. Humans with diverse microbiomes seem to do a better job fighting off illness and staving off chronic disease, which is also independently true for people who exercise consistently. Furthermore, the microbiome of elite athletes is better able to mitigate oxidative stress than average, non-elite athletes and shows an increase in microbial pathways that involve ATP metabolism, sugar transport systems and carbohydrate metabolism.

Recent research has shown that, metabolically speaking, the microbiota is an important component of whether or not our bodies even respond to exercise. "Exercise resistance" refers to the phenomenon in which humans can lack or even have a negative response to exercise and is associated with a diabetic disease course. A recent study found that while some prediabetic people (specifically men) showed limited response to physical activity, others, whose microbiomes were better able to synthesize short-chain fatty acids (SCFA), saw an improvement in their insulin sensitivity after a regimen of physical activity. In other words, the microbiome was an important determinant of whether a person was metabolically resistant to the benefits of exercise, and this was inversely correlated to the production of SCFA.

Intense exercise is associated with microbial production of short-chain fatty acids (SCFA) and bacterial diversity.
Intense exercise is associated with microbial production of short-chain fatty acids (SCFA) and bacterial diversity.

SCFA are produced solely by the microbiota, most often in response to dietary fiber. Roughage in our diet makes it through our small intestine and all the way to the colon, where certain bacteria feast on it and produce SCFA in return. These SCFA, particularly butyrate, can also increase mitochondrial activity and energy expenditure in mice, supporting the idea that the various species of bacteria that produce them may be important for energy homeostasis and, potentially, physical activity. Indeed, the ratio of 2 predominant phyla in the gut, Bacillota (formerly Firmicutes) and Bacteroidetes, predicts both the fecal SCFA content and a higher VO2 max, or maximal oxygen uptake during exercise.

These studies indicate that not only does the microbiome interface with exercise physiology, but also that it does so with crucial inputs from the diet. A pervasive, though outdated, belief in the exercise world is that you can eat what you want if you exercise enough. And yet, research doesn't support such a polarized dichotomy, at least not from a microbial standpoint. We now appreciate that diet is a cornerstone for our microbiome, particularly where it concerns the production of SCFA. Dietary fiber is a driving source for the cultivation of SCFA-producing species and, in turn, SCFA themselves, which support a healthy metabolism and activity threshold.

Recent evidence has uncovered an even more direct relationship between the microbiota and athletic performance. A study published in Nature Medicine identified a single microbe, Veillonella atypica, that was not only associated with the performance of elite athletes, but, when introduced to rodents, could also increase time to exhaustion by 13%. V. atypica is evolved to specifically break down exercise-induced lactic acid into the SCFA propionate. Lactic acid is produced in abundance during intense bouts of exercise and is a primary aspect of fatigue in endurance sports. Not only that, but propionate, the end product of lactic acid metabolism, can increase energy expenditure in both mice and humans, as well as lipid oxidation in humans. Although many species have the lactate dehydrogenase enzyme to break down lactic acid, only V. atypica can carry the pathway all the way through to produce propionate.

An Evolutionary Role for the Microbiome and Diet in Athletic Performance

We ought to resist the temptation to turn V. atypica, or other fitness-associated microbes, into a performance wonder-boost. Instead, we must emphasize the ecological relationships. The symbiotic relationship between V. atypica and exercise suggests an ancestral relationship in which a member of our microbiota adapted to benefit from our physical activity, while also giving us a literal fitness advantage. It also uncovers the question—can V. atypica survive and compete in the inactive gut? In a time when being sedentary is more prevalent than ever before, what happens to V. atypica upon a starvation of activity, and thus, lactic acid?

We know that V. atypica can metabolize multiple organic acids besides lactate, and that it seems to function in partnership with other microbes. It can make its home in the oral cavity or the gut. A separate study even found that it was associated with obesity in an Italian cohort. It may be that V. atypica has a manageable niche even without regular exercise to give it a competitive boost, or that strain differences can impact its function.

On the other hand, perhaps Veillonella is the secret behind the magic of consistency in training; the microbial answer to "practice makes perfect." It's not just that we are building strength in our muscles and capacity in our lungs, but also that we are undergirding our physical strength by creating an exercise-friendly microbial ecosystem. We feed it; it fuels us.

Yet, we must be careful extracting microbes out of their complex contexts. Our microbiome exists in the environment we create for it, and that very much includes diet and other microbes in the milieu. To date, much of the exercise-microbiome research supports an important, if not crucial, role of our food in supporting microbe-exercise dynamics. It's worth noting that propionate, the star of the lactic acid story, is a SCFA that can also be produced by microbes that break down fiber. Not to mention that lactic acid-producing bacteria in the gut (such as Lactobacillus), an alternative source of lactic acid for V. atypica, are strongly reinforced by the regular consumption of fermented foods. Other research has shown that exercise could not rescue mice from the anxiety-like behaviors brought on by a high-fat diet, despite inducing weight loss. It turns out we are a whole ecosystem, and we can't simply outrun what we eat.

Rather, the more we learn about the microbiome and its complex and even evolutionary role in our physical fitness, perhaps it may foster an ecosystem of activity and mindful consumption in our lives (and our guts).

Author: Christy Clutter, Ph.D.

Christy Clutter, Ph.D.
Christy Clutter, Ph.D., is a scientist, microbiome aficionado and writer whose research work focused on gut immunity and the microbiota, along with public health disparities in nutrition.