Background: Sodium bicarbonate (SB) supplementation can enhance performance in short, high-intensity movements. However, its effectiveness in team sports such as rugby remains insufficiently explored. Methods: In this double-blind, parallel, controlled trial, 17 male professional rugby players ingested SB (0.3 g/kg) or a placebo 90 min before a high-intensity, rugby-specific training session monitored via GPS. The training session was conducted under real-world conditions to enhance ecological validity. Physical performance (countermovement jump, CMJ), fatigue markers (capillary lactate and ratings of perceived exertion, RPE), and gastrointestinal (GI) symptoms were assessed pre- and post-exercise. Results: No significant pre–post changes were observed in CMJ performance in either group. Lactate concentrations increased from pre- to post-exercise in both groups (both p < 0.001). The SB group showed higher GI symptom severity before, during and after exercise versus placebo, with several symptoms increasing over time solely in the SB group (p < 0.05). RPE increased similarly in both groups (SB: p = 0.012; PLA: p = 0.008). Due to the small sample size, only moderate-to-large within-group effects and very large between-group differences could be detected; therefore, the study was powered to detect moderate-to-large within-group effects but underpowered for detecting between-group differences. Conclusions: Acute SB ingestion at 0.3 g/kg did not result in detectable improvements in performance or fatigue markers during rugby-specific high-intensity training and was associated with a greater incidence of GI discomfort; however, the study was underpowered to detect small between-group differences. This study was registered on 23 May 2025 on ClinicalTrials.gov (NCT07017582).

Background: Body composition analysis in rugby is necessary for profiling athletes for ideal positioning, the establishment of standards and the development of nutritional and training strategies for improvement. This study aimed to assess the body composition of elite rugby players and examine seasonal variations between backs and forwards using dual-energy X-ray absorptiometry (DXA) and anthropometry. Methods: Thirty-two rugby players (25.97 ± 4.51 years; 93.00 ± 15.39 kg; 181.77 ± 6.27 cm) from First Spanish National league team had their body composition assessed using DXA and anthropometry before and after the season. Results: The main findings indicated that backs exhibited significant increases in total mass, lean mass and fat-free mass, whereas forwards showed and increased only in bone mass. In terms of somatotype, significant changes were observed only in backs, who demonstrated increased mesomorphy and ectomorphy. Within the forwards, front-row players experienced greater increases in LM and FFM compared to other forwards. Conclusions: Rugby players show differences in body composition based on the physical demands of their playing position. These positional differences in body composition are influenced by both training adaptations and genetic predispositions, which ultimately determine the suitability of players for specific roles on the field.