*3.5. Low-FODMAP Diet*

Exercise-related GI problems affect performance and health conditions in approximately 70% of endurance athletes [179]. Several foods are believed to trigger these GI symptoms, including foods high in fructose, lactose, digestible fibers, and undigested fermentable carbohydrates such as inulin and oligofructose, named "prebiotics" [180]. These fermentable short-chain carbohydrates are classified as FODMAP, including animal milk (lactose), legumes (galactooligosaccharides; GOS), wheat (fructans), fruits (high in fructose), and prebiotic foods (high in inulin, fructooligosaccharides (FOS) and oligofructose) [180,181]. Prebiotics are known for their beneficial effects on health, including reducing disease risks by increasing the microbial abundance of beneficial bacteria such as Bifidobacterium and butyrate producers [182]. However, they reach the colon and are fermented by colonic bacteria [183]. Thus, they can cause GI symptoms such as abdominal distress, bloating and gas, resulting in gas production, including hydrogen and methane and osmotic water translocation [184]. As a result, luminal distention and GI symptoms such as bloating, and cramps, can increase, impairing well-being and athletic performance [185]. Therefore, endurance athletes tend to remove high-FODMAP foods from their diets to eliminate their undesirable effects on the GI system [67]. In endurance

athletes with exercise-induced GI symptoms, low-FODMAP diets could apply in two different processes, including the long or short term (both described in detail in Table 2) [8].

#### 3.5.1. Several Points Indicating That a Low-FODMAP Diet Is Advantageous

Endurance athletes' expectations of a low-FODMAP diet are the same as those they have of GFD, including reduced GI symptoms, and thereby increased performance [8]. It is estimated that approximately 22% of endurance athletes have IBS [186]. Exerciseinduced oxidative stress and physiological changes in the body can lead to impaired GI motility and intestinal permeability, which also occur as a result of IBS [147]. Foods rich in FODMAPs can further trigger GI symptoms in athletes with impaired GI function or in IBS patients [187]. In addition, foods high in FODMAPs can also cause upper-GI symptoms, such as stomach swelling due to the high consumption of fructose and glucose [184]. For example, upper-GI distress syndromes such as bloating, nausea, and stomach pain/cramps are common in cyclists, which can impair performance and well-being during exercise and daily life [188]. The potential efficiency of a low-FODMAP diet on exercise-induced GI symptoms has been studied in four studies, two randomized controlled crossover studies [67,70], and two case reports [68,69]. All studies suggested the low-FODMAP diet as an efficient treatment for reducing exercise-associated GI symptoms. A case study investigating a multisport athlete with exercise-induced GI symptoms showed that a short-term (6 day) restriction of foods high in FODMAPs (from 81.0 ± 5.0 g to 7.2 ± 5.7 <sup>g</sup>·day−1) resulted in a decrease in GI symptoms both during exercise and daily life of the athlete [69]. Another case report evaluated a long-term (4 week restriction of foods high in FODMAPs followed by reintroduction of foods high in FODMAPs for 6 weeks) low-FODMAP application before an aggressive multistage ultra-marathon race [68]. Apart from severe nausea, minimal GI symptoms including bloating and flatulence were observed throughout the race. Examining the influence of a 6-day low-FODMAP diet on recreationally competitive athletes with non-clinical GI symptoms in a single-blind, crossover design, Lis et al. [67] reported a significant decrease in exercise-induced GI symptoms, particularly in flatulence, urge to defecate, loose stool, and diarrhea, in nine of 11 athletes after the low-FODMAP trial. Another well-designed crossover study also applied 1 day low-FODMAP or high-FODMAP diet before exertional-heat stress to evaluate its impact on GI integrity, functions, and discomfort [70]. An exercise protocol that includes 2 h of work at 65% VO2max at 35 ◦C ambient temperature was applied after the diet applications. The study findings indicated that lower exercise-induced GI symptoms and I-FABP concentrations were observed after 1 day low-FODMAP diet, suggesting that 1 day low-FODMAP diet provided a crucial advantage by decreasing exercise-associated disruption of GI integrity, and attenuating GI symptoms [70]. Therefore, studies evaluating exertional-heat stress during long-term exercise have administered a 24 h low-FODMAP diet as a control diet to eliminate GI symptoms associated with food and fluid intake [189–191].

It should be noted that endurance athletes typically eat foods high in FODMAPs [8]. A study investigating the content of FODMAPs in various sports foods has shown that FODMAPs are often included in sports foods, such as dry dates (fructans), fructose, inulin (fructans), honey (fructose), and chicory root (oligosaccharides) [8]. Therefore, sports food alternatives low in FODMAPs could be a better choice for endurance athletes, in particular, those who have previously experienced GI symptoms.

A meta-analysis of nine randomized trials reported the administration of a low-FODMAP diet for short-term attenuated GI symptoms, abdominal pain, and quality of life in patients with IBS [192]. However, 25% of patients did not respond to the diet, and responders experienced the diarrhea-predominant type of IBS. These findings sugges<sup>t</sup> that the higher response rate of diarrhea-type IBS may be due to osmotic changes in the gu<sup>t</sup> following a low-FODMAP diet [192]. Note that runner's diarrhea is known to be one of the most common exercise-related GI problems [168]; the chances of responding positively to a low-FODMAP diet are high in endurance athletes, especially athletes with runner's diarrhea.

Exercise-induced GI symptoms may become detectable after intense exercise, affecting recovery and refueling periods [188]. The managemen<sup>t</sup> of this process becomes crucial in multistage events that last multiple stages in a day or over multiple days [112,193]. In endurance athletes with exercise-induced GI symptoms, the FODMAPs restriction may also be needed for the post-exercise period [129], which is crucial to provide optimal nutrient delivery to the body after exercise, particularly intense training periods.

Taken together, as all studies on a low-FODMAP diet and exercise-associated GI symptoms confirmed the efficiency of the diet, we can consider a low-FODMAP diet as an efficient therapy to attenuate exercise-associated GI symptoms. However, the response rate to the low-FODMAP diet should also be determined before planning any long-term low-FODMAP diet application for endurance athletes.

#### 3.5.2. Potential Risks to Consider When Applying a Low-FODMAP Diet

A low-FODMAP diet may result in decreased consumption of prebiotics, which is highly recommended for maintaining a healthy gu<sup>t</sup> microbiome [178]. Additionally, adherence to the diet may be problematic for athletes due to difficulties during the application process [3].

By assessing the low-FODMAP diet based on nutrients instead of general composition, we can realize that complex polysaccharides, the most significant prebiotic metabolites, are restricted with the diet, thus negatively affecting microbiome composition [194]. Although highlighted in several studies on humans with IBS [195–197], randomized controlled studies are needed to investigate the gu<sup>t</sup> microbiome and low-FODMAP diet to evaluate the potential effects of a low-FODMAP diet in endurance athletes.

A low-FODMAP diet should not only attenuate GI problems but provide sportsspecific nutrients and energy intake efficiently as well [171]. Subjects that fail to show any improvement during the first phase of long-term FODMAP application should not continue the diet [3]. Additionally, the reintroduction phase should be carefully applied to subjects by trained dietitians and professionals to identify which foods high in FODMAPs cause these symptoms, and personalize the diet to attenuate IBS symptoms, and thereby maintain healthy gu<sup>t</sup> functions [8].

A general recommendation to reduce the FODMAP content of the diet consists of reducing FODMAP intake from 15–30 g FODMAP·day−<sup>1</sup> to 5–18 g FODMAP·day−<sup>1</sup> [198]. It is recommended for patients with IBS that less than 0.5 g FODMAP per meal or less than 3 g per day be consumed [199]. However, endurance athletes with exercise-induced GI symptoms consume 2-fold higher FODMAPs than the diet classified as high in FODMAPs in clinical research (up to 43 <sup>g</sup>·day−1) [67]. Therefore, foods high in FODMAPs could be a contributing factor for exercise-induced GI symptoms. A recent study on athletes reported that 55% (*n* = 910) of athletes removed at least one high FODMAP from their diet to attenuate exercise-induced GI symptoms, and approximately 85% reduced GI symptoms by removing food from their diet [171]. Lactose is often reported as the most problematic nutrient high in FODMAPs [163]. The most frequently eliminated foods are reported as lactose (86%), GOS (23.9%), fructose (23.0%), fructans (6.2%), and polyols (5.4%). Therefore, before strict FODMAP restriction, it should be considered that lactose and fructose are the most common inductors for GI distress [200]. Lactose consumption of athletes may be greater than that in the general population due to high protein ingredients, good sources of calcium, and rehydration [69]. Furthermore, higher fructose consumption may be greater in endurance athletes, especially during exercise due to sufficient energy supply during longduration (> 90 min.) events or training [201]. Higher fructose intake may be more likely to trigger exercise-induced GI symptoms [202]. Therefore, just reducing or eliminating lactose and fructose instead of all high FODMAPs may inhibit the detrimental gu<sup>t</sup> alterations and may solve the GI problems in endurance athletes.
