The role of the gut in athletic performance
The role of the gut in athletic performance is more extensive than one might believe. There is the obvious fact that athletes absorb the nutrients they need to recover from athletic activity through the GI tract. In order to do this properly, athletes must break down their food by producing adequate digestive enzymes as well as providing a healthy GI barrier for the proper absorption of nutrients including macronutrients as well as vitamins and minerals.In addition, the gut also plays several other roles that may impact athletic performance. More than 70% of the cells of the immune system are located in the gut, and the resident bacteria found in the gut are known to exert many of their functions via the immune system by regulating inflammation as well as the adaptive immune system responsible for fighting off infections(1). Since inflammation is something an athlete encounters as a byproduct of their sport and is essential to the healing process, and proper immune function is critical to avoiding infection when stress is high, a fine tuned immune system plays an important role in sport performance.
Gut health also plays a critical role in management of the stress response. Multiple studies have shown that the bacteria in your gut can have dramatic effects on the way the brain functions via two-way communication(2, 3, 4, 5). This means that the bacteria in your gut can affect the way you respond to stress and that stress can change the bacteria found in your gut. The conduit through which this 2 way communication works is called the vagus nerve.
To further illustrate the importance of gut health, it's interesting to note that severing the vagus nerve does not cause the GI tract to stop working. This means that the GI tract is actually a separate component of the autonomic nervous system called the enteric nervous system. Since the autonomic nervous system is the regulator of physiological processes that are out of our control such as heart rate and blood pressure, there is little question that this part of the ANS likely has huge implications to athletic performance.
The bacteria in the gut also help with nutrient and mineral absorption, generate nutrients that we cannot get from food alone including vitamin K2, and help regulate blood sugar and insulin sensitivity through the generation of short chain fatty acids as a byproduct of fermentation(6, 7). These short chain fatty acids are preferential fuel for the the cells of the intestine and thus have the ability to maintain the health of the colon by promoting peristalsis, reducing inflammation, and regulating the permeability of the intestine to the contents within it. This is important because increased intestinal permeability can wreak havoc on recovery and blood glucose regulation by triggering the immune system and tying up resources better used for recovery.
Impaired gut function and athletic performance
In athletics, every edge counts no matter how small. Over the course of the 10,000+ hours it requires to become elite at something, small changes add up to big advantages. Getting sick or entering a period of overtraining due to insufficient recovery can pull an athlete out of their groove and take away precious practice time. In addition, achieving optimal recovery from workouts, practices, and competitions allows an athlete to become bigger, stronger, and faster. There should be no doubt that being able to get the most out of your food is in your best interest, but how can poor gut function directly impact performance in athletes?Increased intestinal permeability can cause a multitude of issues that an athlete should try to avoid. Increased intestinal permeability occurs when areas between the cells of the intestinal tract called tight junctions dissolve. This allows undigested food within the digestive tract to interact with the immune system and enter the blood circulation. Under several circumstances including high levels of stress, inflammation, and certain dietary influences, intestinal permeability is increased and can negatively impact performance. In addition to poor nutrient absorption, it can increase insulin resistance and delay recovery.
When bacteria from the gut are able to translocate in to the bloodstream from a leaky gut, it sends the immune system in to a frenzy. Inflammation is created and signaling molecules induce insulin resistance in muscle and fat tissue to conserve glucose for the immune system as glucose is the fuel of choice during an infection. This reduces an athlete's ability to store glucose as this is the primary function of insulin. Glucose can be used as fuel, but an athlete would need to clear the "infection" before being able to utilize insulin properly and store glycogen for later use. I've broken this down thoroughly in two other blogs found here and here.
One of the primary proteins involved in this process is called zonulin. When zonulin is secreted, it dissolves the tight junctions that secure the intestinal wall and typically prevent unwanted particles from entering circulation. Zonulin is hypersecreted in people with Celiac disease and gluten sensitive enteropathy and people with these conditions have a difficult time re-sealing the tight junctions when compared to a healthy individual. However, tissues from healthy people also react to gluten by secreting zonulin, but the intestinal permeability seen in these people is much more transient in nature than that of people sensitive to gluten(8). This effect is likely due to changes in gut bacteria and a lack of the fermentation byproduct butyrate, which helps reseal tight junctions exposed to zonulin. In addition to regulating the tight junctions between enterocytes of the intestinal wall, zonulin also dissolves the tight junctions in the blood brain barrier as well as the epithelial cells in the lungs(9, 10), allowing unwanted substances to interact with the brain or enter the body during respiration.
For the most part, one of the primary goals for athletes should be to promote good gut health by maintaining a healthy microbiota, the collection of bacteria that are inhabitants of our GI tract. There are 10x as many bacteria in your gut as there are cells in your body, and the genetic material contained in these bacteria outnumber your own genes 150 to 1. This means they perform many biological functions that are necessary for optimal health that you cannot.
There are 2 steps that can help in this situation. First, probiotics and probiotic/fermented foods can be consumed to populate the gut with healthy bacteria. The interesting part here is that these bacteria don't even need to populate the gut to exert their effects, they also interact with the immune system and promote health through that route. The second thing that needs to be taken in to consideration is prebiotic foods. Prebiotics are essentially food for your gut bacteria and include fiber, resistant starch, polyphenols, and more. Since we don't break these nutrients down, they are free to feed the many bacteria that line our GI tract. This is one of the problematic things about gluten, no human can digest it. To date there is no evidence that any human can digest gluten and humans lack the adequate amount of the enzyme necessary to do so, therefore it is free to interact with the microbiome.
Most people focus on probiotics but neglect prebiotics, a major mistake. Prebiotics feed the good bacteria and keep them as permanent residents of your GI tract, taking probiotics is worthless if you aren't providing an environment that they can proliferate in. This is why probiotic foods such as kimchi, sauerkraut, kombucha, and kefir are likely your best bets, they contain both the bacteria as well as the foods they consume in one package. For the most part, prebiotics haven't been studied at length in athletes, but probiotics have. Let's take a look at some of the studies.
Probiotics in sport
A recent study in endurance athletes found that intense endurance exercise performance in trained men led to an increase in zonulin that was attenuated by use of a multi-species probiotic strain(11).
Supplementation with the probiotic also decreased inflammation and reduced exercise induced protein oxidation. Another study using two Lactobacillus strains of probiotic bacteria found that supplementation with these strains reduced exercise induced oxidative stress through increased endogenous antioxidant activity(12).
Probiotics may also have an effect on overtrained or fatigued athletes. A study looking at fatigued, well trained recreational athletes found that supplementation with a lactobacillus strain reversed fatigue and led to improvements in interferon-gamma secretion(13). Interferon-gamma plays a role in preventing the reactivation of viruses including the Epstein-Barr virus(EBV). The fatigued athletes in this study showed evidence of EBV reactivation that was attenuated with 4 weeks of probiotic supplementation.
These studies are fairly small and further testing needs to be done to determine if probiotic supplementation can provide an ergogenic effect to athletes. However, it's important to re-emphasize that gut health is not dependent on probiotic supplementation. Other factors such as diet and stress likely play a much larger role than probiotic supplementation ever will. If probiotics have a role in athletic performance, it is likely as a therapeutic tool to replace bacteria that may be lost due to poor diet, the use of antibiotics that wipe out the gut flora, or other factors. Even if an athlete gets the desired effect from a probiotic, they still need to support colonization of the probiotic by providing a healthy GI tract that the bacteria can live in. This is only possible through diet.
Special considerations
Activation of the stress response causes the sympathetic nervous system to divert blood towards muscles and away from the organs of digestion. When the stressor ends, blood is diverted away from the muscles and back towards the organs of digestion. Intense endurance exercise can cause decreased motility, maldigestion, and poor nutrient absorption in many who participate in it(14) through this mechanism. This means that it's crucial to make sure that the athlete recovers appropriately from training and competition. Optimizing gut health is a critical step in preventing these symptoms from becoming a nuisance, even transiently. Since the enteric nervous system can function autonomously from the autonomic nervous system, and many of the functions within the enteric nervous system are performed by the bacteria there, maintaining a healthy gut and a diverse population of bacteria within it should be a consideration for optimal athletic performance.
Another issue of relevance to athletes is the use of non-steroidal anti-inflammatory drugs(NSAIDs) such as ibuprofen. Athletes commonly use NSAIDs to help recover from exercise induced inflammation. Exercise, in and of itself, causes injury to the small intestine and increases intestinal permeability in otherwise healthy individuals. The use of NSAIDs aggravates this response and leads to greater levels of intestinal permeability than in those not using NSAIDs(15). Long term use of NSAIDs has also been shown to lead to chronic levels of gut inflammation as well as blood and protein loss(16). Therefore the use of NSAIDs, while they help reduce inflammation in the local musculature, may delay recovery in the long run. In the event an athlete needs to use these drugs, special care should be taken to support overall gut health in the process.
