## 1 Introduction
### Identifying Ultra-Marathon Swimming  
Ultra-marathon swimming presents a unique physiological challenge due to prolonged exposure to water, horizontal body positioning, and thermoregulatory demands. Compared to land-based endurance events, swimmers must manage buoyancy, cold water exposure, and often reduced gastrointestinal tolerance during feeding. Energy expenditure during ultra-swimming is estimated between 600 and 900 kcal/h, depending on intensity and water temperature. Research has shown that even minimal anthropometric differences, such as limb length or torso fat mass, can influence hydrodynamics and fatigue onset. Despite the growth of ultra-distance events globally, there is a lack of standardized data on how body composition impacts performance in ultra-swimmers, especially those performing self-paced efforts in controlled environments.  
However, this varies across studies: [PERSON] et al. [16] identified 29 years as the ideal age, while other work suggested performance declines with age. Still, this was primarily noted in triathlon studies, and when swimming was examined independently, it showed the least age-related performance decline [4]. Another considerable point is the anthropometrics and body composition, whose role is to evaluate and predict performance across various sports [17]. Early studies in ultra-swimming found no significant relationship between performance and body mass, BMI, or limb length [18]. Over time, theories regarding the role of anthropometric features in ultra-swimming performance have evolved, suggesting a more nuanced relationship.  
In their study, [PERSON] et al. [19] examined 18 male and 7 female athletes participating in two ultra-distance open-water swims (26.4 and 88 km). They found a slight but statistically significant correlation between upper limb length and swimming performance, indicating that swimmers with longer arms tended to achieve faster times, potentially due to improved stroke length and propulsion efficiency. Additionally, body fat percentage (BF%) was positively associated with performance, particularly in the longer 88 km swim, suggesting a thermoregulatory or buoyancy advantage in cold-water conditions.  
The authors concluded that although anthropometric characteristics were not the sole predictors of success, they may contribute meaningfully in conjunction with pacing, endurance capacity, and environmental adaptation strategies. Other findings revealed that body fat may decrease after ultra-endurance events [20; 21]. In a study by [PERSON] et al. [17], two experienced ultra-endurance swimmers participated in a 12 h continuous swim, during which no significant changes were observed in body mass, fat mass, or skeletal muscle mass. These results suggest that prolonged swimming, even at ultra-endurance durations, may not elicit substantial alterations in body composition when hydration and nutritional intake are adequately maintained.  
The authors hypothesized that the thermoregulatory effects of water, combined with the horizontal body position and low-impact muscular activity, may contribute to maintaining homeostasis in body tissues. Furthermore, the swimmers consumed fluids and carbohydrates at regular intervals, minimizing the risk of energy deficit and dehydration. Ultra-marathon swimming has seen an increased participation among adults and middle-aged athletes worldwide. However, misconceptions still exist regarding how anthropometric and body composition variables affect performance across different ages and genders. Therefore, this study aims to address the demands of a 10 h ultra-marathon swimming effort on body composition and physiological indices, investigating simultaneously how these variables behave before and after swim effort and their association with performance in regional-level male swimmers, orienting coaches, and ultra-marathon swimmers on the strategies that must be implemented before and during that kind of effort.