Across 16 trials pooled in this systematic review and meta-analysis, exercise interventions in older adults produced meaningful but modest gains in gut microbial α-diversity: the Shannon index improved by Hedges' g = 0.22 and Chao1 by g = 0.22, both statistically significant. Crucially, the benefit follows an inverted U-shaped dose-response curve, peaking at 700–900 METs per week — equivalent to roughly 150–200 minutes of moderate-intensity activity. At the genus level, Akkermansia rose substantially (g = 0.60) while pathobiont Escherichia fell (g = -0.64), with no meaningful shift in dominant phyla or Bifidobacterium. Moderate-intensity exercise, male sex, and higher BMI predicted stronger responses.
Akkermansia muciniphila has emerged over the past decade as a keystone gut bacterium linked to metabolic health, intestinal barrier integrity, and even longevity signals in animal models — making its exercise-driven increase particularly noteworthy. The inverted U-shaped dose-response is a genuinely useful clinical finding: it implies that excessive exercise volume may erode the diversity gains, a pattern consistent with overtraining-related gut permeability data. The effect sizes are small-to-moderate, and only 16 trials qualified, limiting statistical power for subgroup claims. The age-regression trend (β = -0.024, P = 0.079) hints that benefits may attenuate in the oldest old, but evidence remains inconclusive. This meta-analysis is confirmatory-plus — it quantifies what was suspected and adds an actionable dose target, though randomized longitudinal trials controlling for diet are still essential.