Short-term plant-based diet in recreational male runners and its associations with body composition and exercise-related physiological responses

Introduction Plant-based diets (PBDs) are characterized by minimal or no animal products and by high intake of complex carbohydrates, fiber, and antioxidants, and have been associated with improved metabolic and inflammatory profiles. This study investigated the effects of a short-term PBD and the acute response to exhaustive exercise in recreational runners. Methods Seven omnivorous male runners (25–45 years old), training ≥20 km/week for over 2 years, participated in a quasi-experimental within-subjects design. At baseline assessment (CON), body composition, an incremental treadmill test to exhaustion, and blood samples were collected before and after exercise to assess tumor necrosis factor-alpha (TNF-α), superoxide dismutase (SOD), nitric oxide (NO), and calprotectin. Participants then completed 21 days of their habitual omnivorous diet (OMD), followed by 21 days on a PBD, with standardized training intensity maintained. Results Post-PBD, body fat significantly decreased (−4.2%; −3.4 kg; p = 0.023), while no statistically significant reduction in fat-free mass was observed. SOD activity after the PBD phase was significantly lower than after the OMD assessment (adjusted p = 0.023), but did not differ significantly from CON. After exercise, NO and calprotectin increased during the PBD phase (unadjusted p < 0.05). TNF-α showed no significant differences between diets. Discussion These findings suggest that body fat was lower and selected exercise-related physiological responses differed following the PBD, with no statistically significant reduction in fat-free mass among recreational runners.