Lacticaseibacillus rhamnosus LM1019 Enhances Natural Killer Cell Activity in Defined Adult Subsets: A Double-Blind, Placebo-Controlled Trial

Probiotics offer a non-pharmacological approach to support immune function, yet clinical evidence for strain-specific benefits remains limited. We conducted an 8-week, randomized, double-blind, placebo-controlled trial of <i>Lacticaseibacillus rhamnosus</i> LM1019 in 121 generally healthy adults. Both the active and placebo arms produced comparable within-group increases in natural killer (NK) cell cytotoxicity and modest, non-differential declines in circulating cytokines; safety and tolerability were excellent, with mild adverse events evenly distributed. In a post-hoc subgroup defined by age ≥ 40 years, baseline white blood cell count ≥ 5.0 × 10³/µL, and LDL cholesterol < 130 mg/dL, the probiotic arm demonstrated statistically significant enhancements in NK activity (<i>p</i> = 0.021-0.008 across all effector-to-target ratios), whereas no change was observed in the placebo group. These findings suggest that this intervention may selectively boost NK-mediated immunity in individuals with preserved baseline immune and lipid profiles. Future larger trials using phenotype-driven enrollment and controlled dietary intake are warranted to confirm and extend these results.