Live and Heat-Treated Lacticaseibacillus paracasei NB23 Improve Selected Functional and Physiological Outcomes in Naturally Aged Mice

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.