Understanding the Behavioural Determinants of Nutrition Practices in Ultra-Endurance Runners in Ireland.

Ultra-endurance running presents significant physiological demands, with adequate nutritional intake being critical for optimal preparation, performance, and recovery. However, athletes of this sport often consume an insufficient diet. Semi-structured interviews were conducted virtually with ten recreational ultra-endurance runners (age mean ± standard deviation 46 ± 12 years; eight male, two female), all with experience competing in ultra-endurance events, to explore their ultra-endurance experience, dietary intake, nutrition knowledge, and gastrointestinal symptom management. Interviews were transcribed verbatim and thematically analysed in accordance with the COM-B model and the Theoretical Domains Framework. Themes were defined under each of the three COM-B model components: Capability: (1) knowledge and skills, (2) the intention-behaviour challenge; Opportunity: (1) facilitators and barriers to optimal nutrition, (2) information sourcing and learning; and Motivation: (1) drivers of nutrition behaviours, (2) risk perceptions' influence on fuelling strategies. Participants demonstrated strong psychological capability, that is, awareness of nutrition's importance, yet limited behavioural regulation to translate this knowledge into practice. Nutrition strategies were largely self-directed, relying on peers and social media over professional support. Fear of gastrointestinal symptoms and time pressures further shaped dietary decisions. These findings emphasise the importance of evidence-based nutrition guidance to support performance and recovery while minimising gastrointestinal symptom risk. For practitioners and self-coached athletes, recommendations should reflect the practical realities and competing demands of ultra-endurance runners' daily lives and be feasible within real-world settings.