Abstract
ound:Gastrointestinal (GI) symptoms are common in endurance and ultra- endurance sports and may impair performance or lead to race withdrawal. While nu- tritional strategies are frequently emphasized, the respective roles of baseline digestive susceptibility and cumulative exercise exposure remain insufficiently characterized.Meth- ods:Two complementary cross-sectional questionnaire-based studies were conducted in endurance athletes. Study 1 included 230 ultra-trail runners and examined determinants of systematic GI symptoms during competition using a composite digestive vulnerability (DV) score reflecting susceptibility indicators. Study 2 included 497 endurance and ultra- endurance athletes from multiple disciplines and investigated multivariable correlates of GI symptoms and GI-related race withdrawal, integrating training-related GI symptoms (proxy of digestive vulnerability), habitual competition duration (≥6 h), sport category and specific digestive symptoms. Logistic regression models were adjusted for age and sex.Results:In Study 1, the DV score was independently associated with systematic GI symptoms during competition (adjusted OR per point = 1.93, 95% CI 1.33–2.80). In Study 2, athletes reporting GI symptoms during training had markedly higher odds of experiencing GI symptoms during competition (adjusted OR = 3.96, 95% CI
specific digestive symptoms. Logistic regression models were adjusted for age and sex.Results:In Study 1, the DV score was independently associated with systematic GI symptoms during competition (adjusted OR per point = 1.93, 95% CI 1.33–2.80). In Study 2, athletes reporting GI symptoms during training had markedly higher odds of experiencing GI symptoms during competition (adjusted OR = 3.96, 95% CI 2.67–5.87). Habitual exposure to events lasting≥6 h was independently associated with increased odds of GI-related race withdrawal (adjusted OR = 2.25, 95% CI 1.35–3.78). GI symptoms during competition represented the strongest proximal correlate of withdrawal (adjusted OR = 7.04, 95% CI 4.00–12.30), indicating a sequential relationship between baseline diges- tive vulnerability, symptom expression during competition and race termination. After adjustment for digestive vulnerability and exercise exposure, no individual nutritional cate- gory remained independently associated with GI outcomes.Conclusions:Gastrointestinal symptoms and race withdrawal in endurance athletes were more consistently associated with digestive vulnerability expressed during training and cumulative exercise exposure than with isolated nutritional items. These findings support a vulnerability–exposure framework in which individual digestive susceptibility interacts with prolonged phys- iological stress during endurance exercise. Identifying athletes with elevated digestive vulnerability during training may represent a practical strategy to improve individualized nutritional preparation and reduce GI-related race interruption. Nutrients2026,18, 1033 https://doi.org/10.3390/nu18071033
Nutrients2026,18, 1033 2 of 21 Keywords:exercise-induced gastrointestinal syndrome; digestive vulnerability; ultra- endurance; race withdrawal; endurance nutrition 1. Introduction Trail running and ultra-endurance events have expanded markedly over the past two decades. These competitions are characterized by prolonged exercise durations—frequently exceeding six hours—substantial elevation gain, and sustained metabolic and thermoreg- ulatory strain [1,2]. Despite their growing popularity, dropout rates remain high, and gastrointestinal (GI) symptoms are frequently reported as important contributors to perfor- mance impairment and race withdrawal in long-distance events [3–5]. Despite increasing scientific attention, the determinants of GI symptoms during prolonged endurance exercise remain incompletely understood, particularly regarding the relative contributions of cumu- lative physiological exposure, individual digestive susceptibility, andnutritional practices. Exercise-associated GI symptoms encompass a spectrum of upper and lower gastroin- testinal symptoms, including nausea, vomiting, gastroesophageal reflux, abdominal pain, bloating, diarrhoea, and urgency to defecate [6]. Reported prevalence in ultra-endurance settings is high, with substantial proportions of athletes experiencing GI distress during competition and a notable fraction of non-finishers attributing withdrawal to digestive symptoms [5]. Nevertheless, prevalence estimates vary widely (approximately 11–80%) depending on symptom definitions, severity thresholds, timing of assessment, and method- ological approaches [7]. While most studies have described the occurrence of GI symptoms, fewer investigations have examined the hierarchical determinants that may contribute to symptom development and race withdrawal under real-world exposure conditions. From a mechanistic perspective, gastrointestinal disturbances during prolonged exer- cise are commonly interpreted within the framework of Exercise-Induced Gastrointestinal Syndrome (EIGS) [8]. Redistribution of blood flow toward active skeletal muscle and skin results in marked splanchnic hypoperfusion, with reductions in mesenteric blood flow of up to 70–80% [9,10]. Sustained hypoperfusion may induce epithelial injury, increased intestinal permeability, and inflammatory responses. Beyond circulatory redistribution alone, system- atic evidence indicates that exercise per se increases biomarkers of intestinal epithelial injury and permeability, with effects amplified under heat stress conditions, further reinforcing the pathophysiological basis of EIGS [11]. Recent integrative analyses further emphasize the central role of intestinal barrier disruption, endotoxemia, and inflammatory signaling in exercise-induced gastrointestinal stress, while highlighting substantial inter-individual variability in biomarker responses [12]. Reperfusion phenomena, oxidative stress, hyper- thermia, environmental
se increases biomarkers of intestinal epithelial injury and permeability, with effects amplified under heat stress conditions, further reinforcing the pathophysiological basis of EIGS [11]. Recent integrative analyses further emphasize the central role of intestinal barrier disruption, endotoxemia, and inflammatory signaling in exercise-induced gastrointestinal stress, while highlighting substantial inter-individual variability in biomarker responses [12]. Reperfusion phenomena, oxidative stress, hyper- thermia, environmental heat exposure, and repetitive mechanical impacts—particularly in running-based disciplines—may further exacerbate gastrointestinal strain. Under these conditions, nutritional factors such as high carbohydrate loads, hyper- osmolar solutions, fat- or protein-rich foods, inadequate hydration and ingestion timing may interact with reduced intestinal perfusion. Importantly, these nutritional factors likely interact with an already physiologically stressed gastrointestinal system rather than independently determining symptom occurrence. In applied settings, GI symptoms are frequently attributed to specific foods or car- bohydrate intake rates during competition. While inappropriate nutritional practices can provoke symptoms [6], contemporary reviews of carbohydrate supplementation strate- gies emphasize that optimal performance nutrition requires individualized approaches that account for gastrointestinal tolerance, exercise intensity, duration and environmen- tal constraints [13]. These observations suggest that fueling strategies operate within a https://doi.org/10.3390/nu18071033
Nutrients2026,18, 1033 3 of 21 broader physiological context rather than acting as isolated determinants of GI symptoms. Accordingly, evidence supporting a consistent and independent causal role of specific food categories under typical ultra-endurance conditions remains heterogeneous. This discrepancy raises the possibility that nutritional factors may act more as modulators of symptoms in individuals with pre-existing digestive susceptibility rather than as primary causal triggers. Ultra-endurance exposure is often operationalised as continuous exercise lasting ≥6 h, reflecting cumulative metabolic, thermal, mechanical and circulatory stress rather than an abrupt physiological threshold. In endurance sport research, this duration is fre- quently used as a pragmatic operational boundary because events exceeding approximately six hourstypically involve sustained splanchnic hypoperfusion, progressive metabolic strain and increasing risk of gastrointestinal perturbation. This variability suggests the existence of an individual digestive vulnerability profile. Digestive vulnerability may be reflected in recurrent GI symptoms during training, height- ened digestive sensitivity, or increased symptom reactivity under physiological stress. No single validated instrument formally termed a “Digestive Vulnerability Scale” currently exists. Instead, the construct overlaps with domains assessed in validated questionnaires such as the Gastrointestinal Symptom Rating Scale (GSRS), the Gastrointestinal Symptom Questionnaire (GSQ), and the Visceral Sensitivity Index (VSI). However, these instruments were not specifically designed to characterize digestive susceptibility in ultra-endurance contexts or to predict race withdrawal under prolonged physiological exposure. Establishing whether recurrent GI distress during training represents a stable marker of baseline digestive vulnerability that predicts competition symptoms and withdrawal is therefore critical. Moving beyond the question of “what was eaten,” understanding how baseline digestive susceptibility interacts with cumulative exposure may provide a more coherent explanatory framework for GI symptoms in endurance and ultra-endurance sport. The present study does not aim to validate a standardized digestive vulnerability scale. Rather, it tests whether training-expressed gastrointestinal disturbances function as a pragmatic marker of susceptibility under prolonged physiological stress within en- durance settings. The aim of the present study was to examine whether digestive vulner- ability expressed during training and cumulative exercise exposure are associated with gastrointestinal symptoms during competition and race withdrawal in endurance and ultra-endurance athletes. Study Objectives Building on the
tests whether training-expressed gastrointestinal disturbances function as a pragmatic marker of susceptibility under prolonged physiological stress within en- durance settings. The aim of the present study was to examine whether digestive vulner- ability expressed during training and cumulative exercise exposure are associated with gastrointestinal symptoms during competition and race withdrawal in endurance and ultra-endurance athletes. Study Objectives Building on the vulnerability–exposure framework outlined above, the present in- vestigation aimed to examine the relative contributions of nutritional practices, baseline digestive vulnerability, and cumulative exercise exposure to the occurrence of gastrointesti- nal symptoms during endurance and ultra-endurance competition. Specifically, Study 1 investigated determinants of systematic gastrointestinal symp- toms during competition in ultra-endurance athletes, with particular attention to the respective roles of fueling practices and baseline digestive vulnerability expressed through recurrent gastrointestinal symptoms during training and symptom reactivity to exercise duration or intensity. Study 2 extended this framework by examining multivariable corre- lates of gastrointestinal-related race withdrawal across endurance disciplines, integrating digestive vulnerability, habitual exercise exposure (≥6 h), sport category, and specific digestive symptoms. Digestive vulnerability was conceptualised as a functional susceptibility profile re- flected by recurrent or stress-reactive gastrointestinal symptoms rather than as a clinical diagnosis derived from a validated medical scale. https://doi.org/10.3390/nu18071033
Nutrients2026,18, 1033 4 of 21 Based on this framework, we hypothesised that gastrointestinal symptoms during competition would be more strongly associated with baseline digestive vulnerability and cumulative exercise exposure than with isolated nutritional items, and that gastrointestinal symptoms occurring during competition would represent the strongest proximal correlate of race withdrawal. 2. Materials and Methods 2.1. Study Design and Ethical Approval The present investigation comprised two complementary cross-sectional observational studies conducted using anonymous, non-interventional, structured self-administered online questionnaires targeting endurance and ultra-endurance athletes. No intervention or experimental manipulation was performed in either study. Both studies were designed and reported in accordance with the STROBE statement for observational research. The study was conducted in accordance with the Declaration of Helsinki and current European regulations governing personal data protection. The overall research protocol was approved by an independent ethics committee for research (CPP Comité de Protection des Personnes Ouest III, 21-0166, 21.09.61/SIRIPH 2 G 21.01586.000009), and the informed consent of each major participant was obtained. The protocol was conducted according to the Declaration of Helsinki and was regis-tered in Clinical Trials (NCT06297317). Participation was voluntary and anonymous. Prior to accessing the online ques- tionnaire, participants were presented with a mandatory information page detailing the objectives of the study, eligibility criteria (age≥18 years, ability to read and understand French, and practice of endurance or ultra-endurance sports), the estimated completion time, and information regarding data protection and voluntary participation. Access to the questionnaire required explicit electronic consent through mandatory confirmation checkboxes indicating that participants: •Were at least 18 years of age; •Had read and understood the study information; •Agreed to participate voluntarily. Informed consent was therefore obtained electronically before participation. Data were collected using the Framaforms online platform (Framasoft, France), which complies with the General Data Protection Regulation (GDPR) and ensures secure data hosting within the European Union. No directly identifiable personal data (e.g., name, contact information, or IP address) were collected. 2.2. Data Collection Timeframe and Statistical Environment Participants were recruited using a convenience sampling approach through en- durance sport networks, mailing lists, and social media platforms targeting endurance and ultra-endurance athletes. Recruitment messages
Data Protection Regulation (GDPR) and ensures secure data hosting within the European Union. No directly identifiable personal data (e.g., name, contact information, or IP address) were collected. 2.2. Data Collection Timeframe and Statistical Environment Participants were recruited using a convenience sampling approach through en- durance sport networks, mailing lists, and social media platforms targeting endurance and ultra-endurance athletes. Recruitment messages invited athletes currently engaged in endurance disciplines to participate voluntarily in an anonymous online questionnaire focusing on gastrointestinal symptoms and nutritional practices in endurance contexts. Data collection took place between 5 January 2023 and 30 April 2023. The questionnaires were administered using the Framaforms online platform (Frama- soft, Lyon, France), which complies with the General Data Protection Regulation (GDPR) and ensures secure data hosting within the European Union. The questionnaires required approximately 10–12 min to complete for the ultra-trail cohort (Study 1) and 15–20 min for the multisport endurance cohort (Study 2). Analyses were performed on complete-case datasets, and no imputation of missing data was conducted. https://doi.org/10.3390/nu18071033
Nutrients2026,18, 1033 5 of 21 The anonymized datasets from Study 1 and Study 2, including the full question- naires and accompanying methodological documentation, are publicly available in the Zenodo repository (https://doi.org/10.5281/zenodo.18823509) to promote transparency and reproducibility. 2.3. Questionnaire Structure and Variables The questionnaires used in both studies were developed specifically for the present investigation based on existing literature on exercise-induced gastrointestinal syndrome and sport-related symptom assessment. The instruments were designed to capture gas- trointestinal symptoms occurring during training and competition, nutritional practices, and sport exposure characteristics in endurance and ultra-endurance contexts. The questionnaires do not correspond to previously validated clinical diagnostic scales. Instead, they were constructed as structured epidemiological instruments intended to capture sport-relevant gastrointestinal outcomes under ecological conditions. Across both studies, the questionnaires collected information in four main domains: 1. Individual characteristics—Participants reported demographic information including age and sex. 2. Sport practice—Participants provided information regarding their primary disci- pline, training background, and typical competition formats. In the multisport co- hort (Study 2), disciplines were further classified into biomechanical sport groups to explore potential differences in gastrointestinal symptom expression according to movement constraints. 3. Gastrointestinal symptoms—Participants reported the occurrence of gastrointestinal symptoms during training and during competition. The questionnaires assessed common upper and lower gastrointestinal manifestations including nausea, vomiting, gastro-oesophageal reflux, abdominal pain, bloating and diarrhoea. Participants were also asked whether gastrointestinal symptoms had previously forced them to abandon a race. 4. Nutritional practices—Participants reported food categories and fueling practices used during training or competition, including carbohydrate sources, solid foods and other nutritional strategies typically employed in endurance events. In addition, participants were asked about the following: •Self-reported digestive sensitivity in daily life; •Previous gastrointestinal symptoms during exercise; • The use of medications taken preventively or during competition to manage digestive symptoms. The full list of questionnaire items for both studies is available in the publicly acces- sible dataset deposited in the Zenodo repository, ensuring methodological transparency and reproducibility. 2.4. Outcomes and Definitions The primary gastrointestinal outcomes assessed in the present investigation were the occurrence of gastrointestinal (GI) symptoms during competition and GI-related race withdrawal. GI symptoms during competition were
digestive symptoms. The full list of questionnaire items for both studies is available in the publicly acces- sible dataset deposited in the Zenodo repository, ensuring methodological transparency and reproducibility. 2.4. Outcomes and Definitions The primary gastrointestinal outcomes assessed in the present investigation were the occurrence of gastrointestinal (GI) symptoms during competition and GI-related race withdrawal. GI symptoms during competition were defined as the self-reported occurrence of digestive symptoms experienced during an endurance or ultra-endurance event. Participants were asked whether they had experienced gastrointestinal disturbances during competition. Reported symptoms included common upper and lower gastrointesti- nal manifestations frequently described in endurance exercise contexts. Upper gastrointesti- nal symptoms included nausea, vomiting, and gastro-oesophageal reflux, whereas lower gastrointestinal symptoms included diarrhoea, abdominal pain, and digestive discomfort https://doi.org/10.3390/nu18071033
Nutrients2026,18, 1033 6 of 21 in the lower abdomen. This classification reflects commonly used distinctions in exercise gastroenterology separating upper and lower GI symptom domains. The questionnaires did not employ a numerical symptom severity scale. Instead, symptom severity was operationalised pragmatically through the functional impact of symptoms on race continuation. Participants were asked whether gastrointestinal symp- toms had previously forced them to abandon a race. Accordingly, gastrointestinal outcomes were interpreted according to two functional levels: the occurrence of GI symptoms during competition and GI-related race withdrawal, the latter reflecting disturbances sufficiently severe to prevent race continuation. Digestive vulnerability was conceptualised as an individual susceptibility to develop gastrointestinal disturbances under exercise stress. Rather than relying on a validated clinical scale, digestive vulnerability was operationalised through self-reported indicators reflecting digestive sensitivity and recurrent symptom expression in training contexts. The specific operationalisation of this construct differed between Study 1 and Study 2 and is described in the respective study sections below. 2.5. Study 1—Ultra-Trail Cohort 2.5.1. Participants Study 1 targeted ultra-trail runners. Eligible participants were adult athletes (≥18 years) currently practicing ultra-trail running. No upper age limit was applied. As participation in ultra-endurance competitions typically requires a valid medical certificate authorizing sports participation, athletes were assumed to meet standard health eligibility requirements for competitive endurance events. Participants were recruited through ultra-trail community mailing lists, endurance sport social media groups, and direct email contact with athletes previously involved in ultra-endurance research initiatives. A total of 230 ultra-trail runners completed the questionnaire during the recruitment period. Analyses were conducted on complete cases. No additional exclusion criteria were applied beyond incomplete questionnaire responses. Given the exploratory nature of the study, no formal a priori sample size calculation was performed. 2.5.2. Digestive Vulnerability Score Digestive vulnerability (DV) was operationalised as a composite indicator reflecting self-reported susceptibility to gastrointestinal disturbances during exercise. The DV score was constructed a priori using four binary indicators derived from the questionnaire: •Gastrointestinal symptoms during training; •Symptom association with exercise duration; •Symptom association with exercise intensity; •Self-reported digestive sensitivity in daily life. Each item was coded as 0 (absence) or 1 (presence), producing
was operationalised as a composite indicator reflecting self-reported susceptibility to gastrointestinal disturbances during exercise. The DV score was constructed a priori using four binary indicators derived from the questionnaire: •Gastrointestinal symptoms during training; •Symptom association with exercise duration; •Symptom association with exercise intensity; •Self-reported digestive sensitivity in daily life. Each item was coded as 0 (absence) or 1 (presence), producing a cumulative score ranging from 0 to 4, with higher values reflecting greater digestive vulnerability. Internal consistency of the four-item composite indicator was assessed using Cronbach’s alpha. The DV score showed a Cronbach’s alpha of 0.66, indicating acceptable internal consistency for an exploratory composite index of digestive susceptibility. Importantly, systematic GI symptoms during competition were deliberately excluded from the score in order to avoid circularity with the primary outcome. The DV score does not correspond to a previously validated clinical scale but was designed as a pragmatic composite indicator capturing functional digestive susceptibility in ultra-endurance contexts. The indicators were selected a priori based on their conceptual https://doi.org/10.3390/nu18071033
Description
This study examines the relationship between digestive vulnerability and gastrointestinal symptoms in endurance athletes.