30-70% of athletes experience some sort of gastrointestinal issue during their training cycles, with the most common sports affected being running, cycling, and weightlifting. Gut health is a central component of optimal athletic performance, as the gut plays a role in energy metabolism, nutrient absorption, and immune function. The gut microbiome also has important relationships with stress and hormone production. ‍

Functional medicine is uniquely positioned to help athletes support performance by addressing gut-related issues due to the holistic lens through which functional medicine practitioners view health and the body. Helping athletes uncover underlying causes of gut-related symptoms and adjusting their nutrition, lifestyle, and training accordingly can help athletes not only feel their best but perform their best as well.  

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The Importance of Gut Health in Athletes

Gut health can impact athletic performance in several ways. While gut-related symptoms such as bloating or reflux can directly affect how well an athlete performs in a given training session or competition, the health of the gut also impacts things like nutrient absorption, energy production and metabolism, and immune function. For athletes, having the ability to produce sustained energy is at the core of high performance, and relies on how well an individual can break down food, absorb nutrients from that food, and turn it into usable energy - processes that require the gut to be functioning optimally. Additionally, the gut microbiome can modulate inflammation and the immune response, and studies have found that poor gut microbiome health may be linked to a breakdown of musculoskeletal tissue integrity.  

Gut-related symptoms often increase with difficulty and intensity of training sessions, and fluid restriction or dehydration can worsen these symptoms. Endurance athletes in particular face a higher incidence of gastrointestinal disorders, and it’s been suggested that increased intestinal permeability or “leaky gut” may be associated with a higher rate of sports injuries. It has been found that greater than 60 minutes of vigorous-intensity endurance training (at roughly 70% of an individual’s maximum work capacity) led to changes in the body characteristic of leaky gut syndrome. To the athlete, symptoms may appear such as bloating, diarrhea, reflux or heartburn, or abdominal pain; however, the underlying physiological changes can impact their energy levels, recovery ability, and immune response to common infections in the long run as well. 

Common Gut-Related Issues in Athletes

A few of the most prevalent gut-related issues amongst athletes include irritable bowel syndrome (IBS), leaky gut syndrome, and exercise-induced gastrointestinal distress. Up to 70% of athletes experience some sort of gastrointestinal issue during their training cycles, with the most common sports affected being running, cycling, and weightlifting.  

Endurance athletes in particular tend to experience IBS more frequently, and many more go undiagnosed despite experiencing many of the hallmark symptoms of IBS. Training may be beneficial for athletes with IBS and moderate-intensity training can ameliorate symptom severity, though higher-intensity sessions do increase the likelihood of experiencing symptoms. 

Supporting athletes with IBS may be an important focal point for functional medicine practitioners, as studies have observed that runners with IBS need a better understanding of and guidance around their nutrition - many will avoid self-identified “trigger” foods before a race, but still choose historically aggravating foods concerning IBS regularly, potentially impacting their performance ability.

As mentioned earlier, it has been found that upper thresholds of training (such as vigorous training sessions that last greater than an hour or are equal to about 70% of maximum exercise capacity) can increase intestinal permeability, leading to leaky gut syndrome over time. This effect on the gut is enhanced at higher altitudes, in hot environments, or when athletes are dehydrated, making awareness of race environments and fueling/hydrating strategies particularly important for supporting gut health in athletes. It has been postulated that the exercise-induced heat in the body as well as hypoxia that can accompany prolonged vigorous training sessions are the drivers of leaky gut. However, shorter, moderate-intensity training sessions may help preserve the integrity of the gut mucosa, as well as improve intestinal motility and increase beneficial SCFA-producing bacteria in the gut, so exercise remains an important component of any gut healing plan.  

Strenuous training (without adequate recovery) as well as endurance sports can lead to something called exercise-induced gastrointestinal distress in up to 70% of athletes at some point in their careers. This can manifest as upper GI symptoms, including heartburn, burping, high belly bloating, or reflux, or as lower GI symptoms like gas, abdominal pain, or diarrhea. If severe enough, athletes may be unable to complete or may quit their sport due to interference from their symptoms. Changes to the enteric nervous system may be an underlying cause, as exercise induces the sympathetic nervous system and can lead to alterations in enteric nervous system signaling. Splanchnic hypoperfusion may also be to blame, which occurs when blood is redirected away from the abdominal organs to muscle, heart, and lung tissue during bouts of exercise. While typically transient, it can lead to episodes of abdominal pain and other symptoms with prolonged exercise or particularly vigorous training sessions.  

Functional Medicine Testing for Gut Health Assessment

Functional medicine lab testing can be a helpful tool to provide insight for athletes into the underlying causes of gut-related symptoms. Testing such as comprehensive stool analysis, food sensitivity testing, and microbiome analysis can help athletes adjust their nutritional strategies to optimize athletic performance, while also making adjustments to their lifestyle and supplement use (if needed) based on their unique microbiome makeup. A “health-associated” gut microbiome is a diverse microbiome with a higher abundance of health-promoting bacteria, with healthy barrier function and a functional metabolic capacity - all aspects that can be evaluated using functional medicine lab testing.  

Stool tests such as the GI-MAP by Diagnostic Solutions can detect imbalances in the gut microbiome as well as evaluate digestion, absorption, inflammation, and immune function. Based on the results, a practitioner can help an athlete work towards a healthier microbiome composition and improve digestion and absorption of nutrients by implementing a tailored, personalized plan encompassing nutrition, lifestyle, and targeted supplementation. 

Microbiome analysis, which can be accomplished by tests such as the GI360 Microbiome by Doctor’s Data, can evaluate the composition of microbes in an athlete’s gut environment, bringing to light any imbalances between healthy, commensal bacteria and opportunistic or pathogenic bacteria. Therapeutic foods and specific probiotic supplements can then be tailored to an athlete’s results, ensuring the most streamlined approach to optimizing the gut microbiome.

Lastly, food sensitivity tests like the 96 IgG Food Sensitivity Panel by Alletess Medical Laboratory may be appropriate for athletes who are having gut issues and are having trouble identifying trigger foods that aggravate their symptoms. While food sensitivities are not in and of themselves a root cause for poor gut health, continuing to consume foods that aggravate the gut and result in an immune response and inflammation after eating them can prolong an athlete’s path to recovery and optimal gut health. Such foods may also be culprits leading to symptoms while training, especially if offending foods are consumed immediately pre- or post-workout or during the workout itself. 

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Nutritional Strategies for Optimal Gut Health

Athletes can use personalized nutritional strategies to support optimal gut health and athletic performance, based on health history, training goals, and functional medicine lab testing. A diet that includes fiber, fruit, and fermented foods, such as the Mediterranean diet, will help to promote microbiome diversity and overall function of the gut in a way that can benefit athletic performance.  Adjustments for athletes often include higher protein intake and higher carbohydrate intake than the general population to better support physical strength, muscle growth and maintenance, and athletic performance. However, ketogenic Mediterranean approaches that include healthy fats, fiber, quality protein, and fermented foods have also been found to improve exercise performance and promote gut microbiome health due to increases in butyrate-producing bacteria and other beneficial strains such as Akkermania. Such an approach may be used strategically by an athlete to help improve fat oxidation rates and metabolic flexibility in training sessions.

Athletes can experiment with different strategies to help prevent or manage gut-related symptoms while training and competing. For most athletes, avoiding dietary fat and fiber intake immediately before or during workouts can be helpful to minimize gastric distress, instead emphasizing simple, easy-to-digest carbohydrates and protein sources. Identifying and limiting inflammatory foods, as determined by functional lab testing, can help reduce overall inflammation in the gut and minimize symptoms associated with consumption of those foods, such as bloating, reflux, or abdominal pain. Nutritional “gut training” is another strategy employed by athletes and their coaches to “train” gastric emptying and adaptations to meals to optimize meal timing around training events.  

The Role of Hydration in Gut Health and Performance

Adequate hydration is another factor to consider when addressing gut-related issues in athletes.  Dehydration can contribute to the problem of athletes feeling like fluids they drink “accumulate and sit in their stomach.” This feeling often occurs more frequently during higher-intensity workouts or when working out in hot environments. Gut health is important for athletes to be able to properly absorb fluids and any sodium, electrolytes, or carbohydrates that may be added to fluids to maintain hydration. The more efficient absorption is, the less likely an athlete will experience gastrointestinal discomfort. Even just 2-4% dehydration has been shown to have negative impacts on athletic performance, making it essential for athletes to have a personalized hydration strategy dialed in before competing.

No one recommendation can be applied to all athletes due to differences in training load, sweat rate, body mass, and other factors; however, the following recommendations apply to most athletes and can help determine individual fluid intake needs. First, measuring body weight before and after training can help to estimate the amount of water loss experienced during activity, so an athlete can replace fluids accordingly. Second, athletes should pay attention to urine (it should be a pale color) and thirst sensation (it should be low) to ensure hydration is adequate before training. The American College of Sports Medicine recommends on average a 400-800 mL/hour fluid consumption rate for athletes and for sodium lost through sweat to be replaced via food or electrolyte supplementation. It’s recommended that athletes “practice” their hydration strategies in training sessions before committing to a plan for race day, and athletes may also work to “train” their digestive system to accommodate certain fluid volumes alongside their athletic training. For intense prolonged sessions lasting over an hour, as may be typical of endurance athletes, fluid intake of 600-1200mL/hour of water that also contains 4-8% carbohydrate alongside 0.5-0.7 grams of sodium per liter can help ensure proper hydration during training.

Integrating Lifestyle Modifications

In addition to nutrition, lifestyle factors such as stress management and sleep quality are also important to support gut health and optimal athletic performance. Competitive athletes tend to put more stress on their bodies than the average person, especially during periods of higher intensity or long-duration training.  Stress can affect gut motility and blood flow to the gut, increase intestinal permeability, and affect digestive enzyme production, leading to symptoms of gastrointestinal distress if not addressed. In turn, it’s been found that modulation of the gut microbiome through nutrition, supplementation, and lifestyle changes may improve the stress response in athletes, helping them perform at their peak. Working with a practitioner to identify stressors and create a personalized management plan to help reduce the negative impact of stress on the body can be a beneficial aspect of any holistic training plan for athletes.  

Sleep is essential for optimal athletic performance, as it is when most recovery occurs. Sleep deficits in athletes have been linked to slower reaction time and accuracy, poor endurance, and suboptimal strength. Overtraining or not prioritizing recovery can hurt sleep quality as well, compounding the issue in athletes who are not prioritizing sleep hygiene and are training hard. The diversity of the gut microbiome has been associated with sleep quality, and it’s known that gut microbes can produce neurotransmitters related to sleep and circadian rhythm regulation. By prioritizing sleep hygiene and taking additional measures to optimize gut health, athletes can ensure that they are getting quality sleep and recovery time that will improve their performance in their sport. 

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Gut-Related Issues in Athletes: Key Takeaways

A functional medicine approach to optimizing gut health in athletes can help ensure athletes not only meet their nutrient requirements but also address the underlying causes of any gastrointestinal discomfort they may be experiencing that is negatively impacting athletic performance. Considering digestive capacity, gut microbiome diversity, and personalized nutrition are all central components of a functional medicine approach to gut health, functional medicine lab testing can help guide personalized plans for each athlete. The ability to break down, absorb, and utilize their nutrition is an understated essential for athletes looking to perform at their peak, and the health of the gut is responsible for these abilities.