Few topics in sports nutrition are in vogue today as the gut microbiota, with numerous companies now selling tests that come straight to your front door. Supposedly all you need to do is drop some of your poo in the mail, and presto, you’re on your way to your best performances ever. Indeed, some of these companies outright claim these tests can help improve performance, enhance sleep, hasten recovery, and prevent illnesses.
Supplement companies have also jumped on the microbiota bandwagon, creating a seemingly ever-expanding marketplace offering everything from probiotics to prebiotics to synbiotics. In fact, the global probiotics market was estimated to be worth over $48 billion in 2018. Much like with the poo-testing companies, many of these supplement producers make bold claims. After one minute of searching online, I found a company that claimed their probiotic could “support energy, stamina, and endurance at the microbial level,” whatever the heck that means.
To be sure, there is reason to feel optimistic that the gut microbiome may have the power to impact athlete performance and health. But, as with many novel and exciting topics in the field of sport science, the hype seems to be outpacing real scientific inquiry. I loathe to be a hater, but this is one topic that needs to be brought back down to earth.
Defining and Assessing the Gut Microbiota
Before we look at microbiota research that is relevant to athletes, we first need to define what we mean by the phrase gut microbiota. Like any of your body’s external surfaces, your gut is populated with numerous micro-organisms: bacteria, viruses, fungi, and the like. We’ve known the gut serves as a home to bacteria since at least the 1840s, when a surgeon named John Goodsir discovered a bacterium in the ejected vomit of a 19-year-old patient. These days it’s recognized that bacteria residing in the gut do more than just hang out. These bacteria—known collectively as the gut microbiota—play a role in regulating your immune function, metabolism, mood, and disease risk. A slightly different phrase, the gut microbiome, describes not only the microbes themselves but also their genes and the entire habitat in which they reside.
Of any place in the gut, the colon is undoubtedly the most abundantly populated with bacteria. The exact count varies some from person to person, but the average man or woman might house roughly 40 trillion bacterial cells, with over 95 percent in the colon. You may find it hard to believe, but you have more than one bacteria cell in and on your body for every one of your own cells.
When it comes to characterizing a person’s gut microbiota, the most common approach is taking a sample of caca. While this is relatively easy (and somewhat gross), fecal samples are just a proxy measure for the gut microbiota. Indeed, microbes predominating in stool and those that tend to live in close proximity to the mucosa (the lining of the gut) are not entirely the same. This issue—along with other methodological difficulties—has made it challenging to figure out which microbes populate the gut and how these populations change in response to various lifestyle and environmental factors.
Even so, there are some points of agreement among scientists in this field. First, the gut microbiota tend to develop in a somewhat predictable manner early in life. For example, in the first days after birth, an infant’s gut microbiota shifts to become richer in Bifidobacteria, and this persists until they transition to eating solid foods. Second, it also seems true that the gut microbiota is relatively stable throughout much of adulthood, though it does shift somewhat in response to diet changes, weight loss, and antibiotic use. Simply put, the gut microbiota seem to have both stability and responsiveness characteristics.
The Athlete’s Gut Microbiome Is Different. But Why?
One of the first studies to look at the microorganisms residing in athletes’ guts used male (primarily Irish) rugby players. In comparison to non-athletes, the rugby players had more diverse microbiotas based on several measures. Notably, they had greater proportions of 48 bacteria phyla than non-athletes of similar body sizes, and just one phylum type, Bacteroidetes, was less plentiful in the athletes. Somewhat similar results were found in a later study of elite Polish marathoners, in that they had a lower abundance of Bacteroidetes than couch potatoes. In contrast, runners had greater abundance of Prevotella than non-athletes. The findings of these studies also reflect the overall body of research, as a 2020 review from the International Society of Sports Nutrition concluded that, “active individual’s microbiota display a higher abundance of health-promoting bacterial species. . .and increased diversity.”
One issue that scientists are grappling with is exactly where this extra microbiota diversity comes from. Is it the athletes’ training, diet, or something else? As you can imagine, it is exceedingly difficult to tease these factors apart and make broad-sweeping conclusions. Still, some findings are fairly consistent between studies. The first is that higher intakes of fiber and other complex carbohydrates are associated with greater abundances of certain microorganisms, some of which are linked with good health. Further, experiments confirm that supplementing dietary fiber enhances the abundance of Bifidobacterium and Lactobacillus species. When people exercise a lot, they often up their carbohydrate consumption, which may partly explain the greater microbiota diversity observed in athletes.
Beyond fiber and complex carbohydrates, protein intake is often higher in athletes. Interestingly, that study of Irish rugby players found that protein consumption positively correlated with microbiota diversity. Amino acids (the products of protein digestion) can be broken down by certain bacteria in the colon if they are left unabsorbed (which is more likely with high protein intakes). Although this may seem like a positive, we need to interpret this finding with a good dose of caution. Some of the byproducts of this colonic amino acid metabolism may negatively affect epithelial cells residing there.
Additionally, adding extra protein to the diet may directly impact the types of bacteria that thrive in the gut, sometimes not in a good way. One study of endurance athletes, for instance, found that supplementing with protein (a blend of whey isolate and beef hydrolysate) reduced potentially health-promoting bacteria such as Roseburia and Bifidobacterium. Ultimately, the effects of protein on the microbiota will undoubtedly vary based on dietary source (plant-based, meat, fish, dairy), and we’re still trying to flesh out what each of these different sources does to the gut microbiome.
Does the Gut Microbiota Matter for Athletic Success?
It’s well and good to know that some of the microbes living in athletes’ guts differ from people who are sedentary, but does this matter on a practical level? There are various ways to answer this question, but one simple approach is to see if gut microbiota markers correlate with aspects of athletic prowess. A 2016 investigation did exactly that, finding that performance on a VO2max test was moderately correlated with gut microbiota species richness among young adults. Other studies have also found that certain gut microbiota profiles correlate with aerobic fitness levels in healthy non-athletes (see here and here).
Yet, these types of studies cannot prove the gut microbiota directly modifies aerobic fitness. It’s equally, if not more plausible, that the fitter people ended up with different gut microbiotas because of their diet and training. In other words, their gut microbiota could simply be a byproduct of their lifestyle. To really figure out whether the microbiota is vital to athletic success, the ideal approach would be to experimentally manipulate the microbes living in athletes’ guts.
Supplementing with probiotics is one such option, but disappointingly, experiments that have administered them to athletes and tested performance have, by and large, rarely found improvements (though respiratory immune benefits are not uncommon). It’s conceivable that these studies didn’t use the right type or dose of probiotic, but it could also be because bacterial metabolism in your gut just simply isn’t powerful enough to affect your muscles or heart during vigorous exercise. Regardless, probiotics are pretty low on the list of evidence-based strategies an athlete should try to directly improve their performance.
A Performance Probiotic on the Horizon?
One criticism of the abovementioned research is that the probiotics weren’t chosen with the express purpose of improving performance. Instead, the specific species were typically selected because of their supposed health properties or simply because they were used in earlier investigations.
In 2019, a group of scientists published a paper in Nature Medicine that took a more methodical approach to identifying gut microbes that may alter athletic performance. In a first step, the researchers evaluated the microbiotas of 15 runners from the 2015 Boston Marathon, which involved collecting stool samples over several days preceding and following the race. Overall, the runners showed increases in Veillonella microbes after the marathon relative to pre-marathon. The scientists replicated these results in ultramarathoners and elite rowers, and they went on to speculate that Veillonella species could directly modify running performance because they are known to metabolize lactate into short-chain fatty acids. In theory, this could lower lactate levels in the body and provide another source of fuel in the form of short-chain fatty acids.
In a final part of their investigations, the scientists tried proving that Veillonella bacteria can directly enhance performance. They did this by inserting Veillonella atypica into the GI tracts of mice and making them run on a treadmill several hours later. The speed of the treadmill began at 5 meters per minute and was accelerated by 1 meter per minute every minute thereafter. In comparison to when they were treated with a control microbe (Lactobacillus bulgaricus), the mice ran 13 percent longer when they got the Veillonella treatment.
I was impressed with this research for several reasons. It combined observations from human athletes and showed that, in principle, resistance to fatigue can be improved through manipulating gut microbes. It’s worth mentioning, though, that some of the authors work for or hold equity in a company called FitBiomics, which is working to commercialize a product off of this research. There is nothing inherently wrong with that, but it does raise my level of skepticism a notch, and I’d like to see the results replicated by other groups.
More importantly, though, “performance improvements” in mice do not mean the same thing will happen in humans. As has been pointed out by others, experiments with mice often fail to be duplicated with humans. Also, given that endurance athletes already see substantial jumps in Veillonella with exercise, it’s debatable whether they would benefit from a supplement that attempts to add more Veillonella. In the next few years, I’m guessing we will have more concrete data on this potential performance-boosting micro-bug as more human experiments are carried out.