Background: Exercise-induced fatigue arises primarily from energy substrate depletion and the accumulation of metabolites such as lactate and ammonia, which impair performance and delay recovery. Emerging evidence implicates gut microbiota modulation—particularly via probiotics—as a means to optimize host energy metabolism and accelerate clearance of fatigue-associated by-products. Objective: This study aimed to determine whether live or heat-inactivated Lacticaseibacillus paracasei NB23 can enhance exercise endurance and attenuate fatigue biomarkers in a murine model. Methods: Forty male Institute of Cancer Research (ICR) mice were randomized into four groups (n = 10 each) receiving daily gavage for six weeks with vehicle, heat-killed NB23 (3 × 10¹⁰ cells/human/day), low-dose live NB23 (1 × 10¹⁰ CFUs/human/day), or high-dose live NB23 (3 × 10¹⁰ CFUs/human/day). Forelimb grip strength and weight-loaded swim-to-exhaustion tests assessed performance. Blood was collected post-exercise to measure serum lactate, ammonia, blood urea nitrogen (BUN), and creatine kinase (CK). Liver and muscle glycogen content was also quantified, and safety was confirmed by clinical-chemistry panels and histological examination. Results: NB23 treatment produced dose-dependent improvements in grip strength (p < 0.01) and swim endurance (p < 0.001). All NB23 groups exhibited significant reductions in post-exercise lactate (p < 0.0001), ammonia (p < 0.001), BUN (p < 0.001), and CK (p < 0.0001). Hepatic and muscle glycogen stores rose by 41–59% and 65–142%, respectively (p < 0.001). No changes in food or water intake, serum clinical-chemistry parameters, or tissue histology were observed. Conclusions: Our findings suggest that both live and heat-treated L. paracasei NB23 may contribute to improved endurance performance, increased energy reserves, and faster clearance of fatigue-related metabolites in our experimental model. However, these results should be interpreted cautiously given the exploratory nature and limitations of our study.

Live and heat-treated microbial preparations are candidate nutritional ingredients for supporting function during aging. This study evaluated Lacticaseibacillus paracasei NB23 in naturally aged male C57BL/6NCrl mice. Ten young mice served as a reference group, and forty aged mice received vehicle, live NB23 at 2.05 × 10⁹ or 6.15 × 10⁹ colony-forming units (CFU)/kg/day, or heat-treated NB23 at 6.15 × 10⁹ cell equivalents/kg/day for 12 weeks. Relative to aged controls, NB23-treated mice had shorter water-maze acquisition latency, longer treadmill endurance, higher hepatic antioxidant enzyme activities and glycogen, and lower selected whole-brain oxidative-damage markers. High-dose live and heat-treated NB23 also increased grip strength and muscle glycogen. Gastrocnemius myofiber area and selected femoral microarchitectural outcomes improved, whereas rotarod performance and tibialis anterior myofiber area did not. Exploratory analyses of brain and intestinal transcripts, fecal metabolites, and microbiota–host associations did not establish a gut-mediated mechanism. Among 325 taxon–outcome pairs in the exploratory microbiome–host screen, 49 met nominal p < 0.05, but none survived Benjamini–Hochberg correction (minimum adjusted p = 0.08967). Assay-specific subsets comprised four mice per group for micro-CT and five per group for qPCR, fecal metabolites, and microbiome sequencing, with two gene-specific qPCR exclusions. These findings support further evaluation of NB23 as a functional food ingredient, while the small, male-only animal study does not establish clinical anti-aging efficacy or equivalence of the live and heat-treated preparations.