Introduction <p>Ultra-endurance sports are characterized by prolonged durations of activity, high energy expenditure, and unique physiological and psychological demands. These factors could exacerbate risks for Relative Energy Deficiency in Sport (REDs) and the related conditions of low energy availability (LEA), eating disorders (ED), and disordered eating (DE), amongst others. However, presently there is a lack of information about these topics in ultra-endurance sports.</p> Methods <p>To explore risk factors, lived experiences, and research gaps, we conducted a narrative review of studies exploring REDs in ultra-endurance sports. A keyword search was performed in Scopus and PubMed, with supplemental searches of reference lists and Google Scholar. Both quantitative and qualitative studies were included to capture the scope of available evidence. Following selection and extraction, key quantitative findings were summarized and qualitative research was thematically analyzed.</p> Results <p>Sixteen studies (<i>n</i> = 10 quantitative; <i>n</i> = 6 qualitative/case reports) were included in the results. Quantitative studies highlighted the presence of REDs, LEA, and DE/ED among ultra endurance athletes, with one studying showing that up to 65% of athletes may be at-risk. Qualitative studies provided insights into psychological distress, body image concerns, intentional caloric restriction, and the impact of sociocultural pressures on fueling behaviors in athletes.</p> Conclusions <p>REDs and related conditions pose risks for ultra-endurance athletes, with the combination of high-volume training and psychological drivers likely increasing susceptibility to energy imbalances. Nevertheless, the REDs model may inadequately address underlying psychological factors contributing to LEA and could require integration with ED-based protocols for improved treatment outcomes in ultra-endurance sports and elsewhere.</p>