*3.4. Gluten-Free Diet*

Exercise-induced GI symptoms in endurance athletes share common characteristics with Irritable Bowel Syndrome (IBS), including altered bowel functions (e.g., diarrhea, constipation), bloating, intestinal cramps, urge to defecate, and flatulence without any known organic disease [168]. These symptoms strongly affect the quality of life, psychological well-being, and also have quite a detrimental influence on exercise performance [1,168]. Therefore, several therapies have been developed for manipulating and attenuating these GI symptoms [169]. While drug-based treatments can be of benefit, certain foods are thought to trigger GI symptoms. In a research study, 63% of patients with IBS reported that some foods trigger their IBS symptoms [170]. Therefore, diet therapies gain more interest than other therapy options in patients with IBS and endurance athletes with GI symptoms. For example, a gluten-free diet (GFD) [128] and a low Fermentable Oligo-, Di-, Mono-saccharides, and Polyols (FODMAP) diet [171] are classified as elimination diets that both exclude or limit certain foods or nutrients that may cause undesirable GI problems such as abdominal bloating, cramps, flatulence or urge to defecate.

#### 3.4.1. Why Do Endurance Athletes Consider a Gluten-Free Diet to Be Beneficial?

A gluten-free diet is a strict elimination diet that requires the complete exclusion of gluten, a storage protein found in wheat, rye, barley seeds, and includes gluten-free foods and food products that do not contain gluten or have a gluten content of less than 20 ppm, as per European legislation [172]. It has been used for decades as a treatment for celiac disease (CD) or to treat other gluten-related disorders that require strict gluten elimination from the diet [173]. However, recently, gluten has been considered to be an inducer that triggers the pathophysiology of various conditions. Based on this theory, endurance athletes have widely practiced GFD even if CD or non-celiac gluten sensitivity (NCGS) has not been diagnosed [7]. Although they applied GFD as a possible dietary therapy because of their belief in a diet that could improve metabolic health and performance or alleviate exercise-induced GI symptoms, the results show no significant improvement in performance with GFD in non-celiac athletes [129].

A study of 910 athletes (male = 377, female = 528, no gender selected = 5) found that 41% of the athletes reduced their gluten consumption by approximately 50% to 100% due to their belief that gluten causes GI symptoms, inflammation, and decreased performance [7]. Endurance athletes in particular (70%) tend to exclude gluten from their diet. Almost half of the athletes who consumed GFD reported that at least one of their GI symptoms was attenuated with ongoing GFD [7]. Inconsistent with the study, a randomized controlled, double-blind, crossover study of 13 endurance cyclists with no known gluten-related disease who followed GFD or gluten-containing diet for a short period (7 days) showed that gluten elimination did not alleviate GI symptoms [66]. Additionally, neither plasma intestinal fatty acid binding protein (I-FABP), a marker of intestinal damage, nor TT performance differed between the groups. This is the only randomized-controlled study investigating the influence of GFD vs. gluten-containing diet on endurance performance and intestinal injury, and perceived well-being in endurance athletes [66]. Further research is required to elucidate the GFD, endurance performance and GI symptoms.

The best technique for identifying gluten-related issues is to remove gluten from the diet and check it for health effects in clinical practice [174]. With this gluten-related practice, athletes often self-diagnose that they have gluten-related disorders, resulting in gluten being excluded from the diet [129]. Assessing the presence of celiac symptoms, prevalence, and comorbidities in 141 collegiate athletes, Leone et al. [175] found that athletes reported being 3.85 times more likely to be diagnosed with CD and 18.36 times more likely to be associated with CD than the general population. This close association negatively alters the

athlete' health, leading to several detrimental consequences, including higher depression and perceived stress levels [175]. A possible explanation is that CD can be diagnosed faster as athletes monitor their health on a regular basis and work with an interdisciplinary team. The rapid detection of CD can provide an advantage to begin treatment as soon as possible, thereby reducing other harmful consequences associated with celiac disease.

A study on endurance athletes showed that they generally believed in GFD and its benefits to GI stress and exercise performance [176]. It is well known that the "belief effect" in athletes is an influential factor that can increase sports performance by 1 to 3% [177]. Whether gluten triggers exercise-related GI symptoms or whether endurance athletes with GI issues have a higher rate of NCGS remains unclear [66]. Additionally, switching to GFD can cause some healthy dietary changes in athletes, such as increased consumption of fruits, vegetables, legumes, and whole grains, and these changes may have more significant benefits on the GFD than gluten elimination [96]. Therefore, the gluten-free diet should not be recommended to non-celiac athletes (NCAs), as there is no evidence in the literature about its benefits to GI stress, immune response, and athletic performance [8,66].

#### 3.4.2. Possible Risks of a Gluten-Free Diet

The main concerns of GFD for endurance athletes can be classified as low energy availability [96] and the potential to create an energy deficit, micronutrients and fiber, leading to the RED-S [3]. Although GFD limits the consumption of certain gluten-containing foods rich in CHO that could lead to an energy deficiency [173], there is insufficient data to investigate the effect of GFD on energy deficiency in endurance athletes. We recommend that more studies are required on this topic, especially with a well-planned GFD for endurance athletes.

In addition, athletes consuming GFD need to greatly consider their diet as they need to control all foods for gluten content, which can negatively affect psychology [128]. For athletes with CD or other gluten-related clinical conditions, removing gluten from the diet is the only effective treatment [173]. In endurance athletes with CD, an increase in exercise performance and a decrease in GI problems were found after a gluten-free diet was adopted [178]. However, it is worth noting that endurance athletes need more energy to perform better in prolonged training and races, and gluten is present in carbohydrate-rich foods, which are the primary common source to meet their energy needs [112]. Gluten-free products are also known for their high cost and can sometimes be difficult to find [128]. Therefore, dietary gluten elimination may be an effective strategy for athletes with CD [173]. However, when applied to non-celiac athletes, it can create a large energy deficit and low energy availability, impairing both metabolic health and performance.
