A 30% high-fiber diet mitigates oxidative stress and inflammation while modulating cecal microbiota in rats undergoing high-intensity exercise

High-intensity training (HIT) can induce inflammation and oxidative stress, posing challenges for athletes, while dietary fiber may offer protective benefits. This study examined the impact of a 30% increase in dietary fiber intake on oxidative stress, inflammation, hematological parameters, and gut microbiota in male Wistar rats subjected to HIT. Thirty-two rats (250 ± 20 g) were randomly assigned to four groups (n = 8 each): Control-NF (no training, normal diet), Control-Fib (no training, high-fiber diet), HIT-NF (HIT, normal diet), and HIT-Fib (HIT, high-fiber diet). HIT was performed at 90% VO₂max, five days per week for eight weeks. The high-fiber diet was enriched with wheat bran, providing 37-52% total dietary fiber, predominantly insoluble fractions (cellulose, hemicellulose, and lignin) with minor soluble components (arabinoxylans and β-glucans). Antioxidant enzyme activities, inflammatory cytokines, hematological indices, and cecal bacterial profiles were evaluated using two-way ANOVA (diet×training) with Tukey's post-hoc test. HIT significantly reduced catalase, SOD, and GPx activities and elevated TGF-β, IFN-γ, IL-17, and hsCRP levels, whereas fiber supplementation reversed these effects, restored leukocyte balance, and improved phagocytic activity. HIT also disturbed microbial composition by reducing Lactobacillus spp. and increasing coliform counts, while fiber partially normalized these alterations. Collectively, the findings provide novel evidence that a 30% high-fiber diet intake can serve as a practical nutritional strategy to mitigate physiological stress, inflammation, and microbiota imbalance induced by high-intensity exercise.