Our gut and its microbiome can have a major impact on the athletic performance. It plays a vital role in producing, storing, and supplying the calories needed by an athlete or any individual practicing endurance exercises, such as running, cycling, aerobic exercise or swimming.
A Healthy Gut can adapt itself to sudden bodily changes during High-intensity workouts
An athletes’ body, including the gut, tends to encounter several physiological adaptations. One such adaptation is “the gut leak.” In particular, athletes who perform endurance exercises have a high incidence of GI issues such as bloating, cramping, and diarrhea due to increased gut leakage. An unhealthy gut is at greater risk of leakage, allowing unfriendly bugs to migrate into the bloodstream, leading to several chronic health problems. For instance, the upper airways of athletes are almost all the time exposed to allergy-inducing agents, which can give rise to respiratory tract infection. A study published in a scientific journal showed that supplementing probiotics can reduce the odds and severity of airway infections in some athletes.  This further draws attention to the role of gut bacteria in boosting the athletic functioning.
A study conducted by several renowned experts revealed that probiotics when consumed prior to a prolonged heavy workout can speed up the recovery of naturally-occurring friendly gut bacteria, which are vital to boosting performance in a variety of ways. 
A Healthy Gut Microbiome partakes in Bone Health
An optimal bone health is critical to cutting the risk of injury and speeding recovery in athletes. Just like the brain communicates with the gut microbiome and vice versa through the gut-brain axis, research shows that our gut can also interact with our bones through the gut-bone axis.  A healthy gut and its microbiome can modify the bone mass by shaping the immune system and altering the bone metabolism “in a good way.”
Some authors report that prebiotic supplementation can accelerate the absorption of the vital bone mineral, calcium.  The resultant healthy, strong bones are a key to optimal athletic performance.
The makeup of the Gut Microbiome and its effect on Athletes’ health
The composition of gut bacteria in athletes is quite different from people who lead a sedentary lifestyle.  The highly active individuals have a predominance of beneficial bugs. Moreover, their stools reveal high levels of butyrate, a fatty acid that not only protects against colon cancer and inflammatory bowel disease but also acts a fuel when other energy supplies get exhausted.  – see below
A healthy digestive system and its beneficial bugs can boost the immunity, alleviate fatigue, and help release “feel-good chemicals,” thus, driving the required energy levels of a sportsman.
A Robust Digestive System can be “Trained” in Athletes
Athletes can train their gut to optimize the delivery of nutrients during workout while simultaneously minimize some — if not all — of the GI concerns. This training has been referred to as “training the gut,” by a UK expert. 
Athletes need carbohydrate and fluids (even more during intensive training) to help kick off their performance and at the same time keep themselves energized. These fuels reach an athletes’ body, including the working muscles after traveling through the gut. The stomach should be able to empty these nutrients when needed. But an unhealthy stomach cannot function to its max when required. Instead, such athletes may often feel bloated after consuming large volumes of drinks such as during high-intensity workouts. This is because our stomach can only hold — and then empty — certain amounts of nutrients within a certain time window. Overloading our stomach can cause the food to build up, paving the way for GI complaints. Dehydration, on top of this, can further add to the problems in athletes.
In contrast, a healthy stomach can adapt itself to ingesting large amounts of a meal. It can be “trained” to hold larger volumes of food with less discomfort. With regular training, an athlete can consume large volumes of food within a small time frame. This helps ensure the continuous delivery of the essential nutrients to the working muscles, thus, preventing muscle fatigue. 
A healthy Gut can “fuel” the energy center
In addition to the fuel provided by diet, our bodies also require natural fuel, supplied in the form of ATP (Adenosine Triphosphate) from the “powerhouse” of the cell. Our gut microbiome cross-talks with — and regulates the functions of —this powerhouse of the cell — to ensure a constant supply of energy.  This prevents muscle fatigue in athletes, who need even greater amounts of this energy.
It wouldn’t be wrong to say that if the gut along with the bacteria residing in it are not in their optimal shape, it will fall short of providing the necessary fuel to an athletes’ body, thereby compromising performance as well as increasing the risk of GI, bone, and other health concerns.
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