In a 12-week randomized, double-blind trial of 102 adults aged 50-75, daily Lactiplantibacillus plantarum LP28 (two capsules, 5 x 10^9 CFU each) was compared with placebo. Cold incidence was 34.78% in the probiotic group versus 52.00% on placebo, with lower cumulative Wisconsin Upper Respiratory Symptom Survey scores. The probiotic group also showed higher serum interleukin-12, greater natural killer cell cytotoxicity, and lower C-reactive protein.
The immune signature is biologically coherent: IL-12 drives NK activity, and NK decline is a recognized feature of immunosenescence. Lower CRP would fit reduced low-grade inflammation, a target of interest in healthy aging. Still, caution is warranted. This is a preprint and has not been peer reviewed, so the results may change. With only 102 participants, the trial is small, and the analysis relied on a per-protocol approach plus outlier-removed sensitivity checks rather than a clean intention-to-treat result, which can inflate effects. The cold rates quoted (34.78% vs 52.00%) imply dropouts, since 51 per arm do not divide into those percentages. Cold incidence is also self-reported, and the 17-point gap may not hold up as a statistically robust difference given the sample size. Meta-analyses of probiotics for respiratory infections show modest, strain-specific benefits of uncertain quality, and a single-strain, single-site trial cannot be generalized to probiotics broadly. Nor does it show anything about lifespan or serious infection. Treat this as an incremental, hypothesis-generating result that needs replication in larger, preregistered trials before it informs supplement choices.