In 152 older stroke patients (mean age 78.4 years) undergoing inpatient rehabilitation, the benefit of dietary protein density on motor ADL recovery — measured by Functional Independence Measure motor effectiveness — depended significantly on the relative abundance of butyrate-producing gut bacteria (β=0.046, 95% CI 0.013–0.079; p=0.006). Patients with higher proportions of putative butyrate producers appeared to extract greater functional gains from higher protein density, while the association was attenuated in those with fewer such bacteria. Notably, the interaction did not extend to handgrip strength or skeletal muscle mass index, suggesting the effect operates through ADL-specific functional pathways rather than gross muscle anabolism.
This finding reframes a longstanding clinical assumption: that protein intake reliably supports post-stroke recovery. The microbiome — specifically butyrate-producing taxa — may act as a critical gatekeeper. Butyrate enhances intestinal barrier integrity, modulates systemic inflammation, and influences muscle protein synthesis signalling, offering plausible mechanistic pathways. However, this is a small registry-based observational study with 16S rRNA sequencing, which identifies taxa rather than confirmed metabolic function, and cannot establish causality. The 156-patient cohort, drawn from a single Japanese rehabilitation centre, limits generalizability across ethnicities and healthcare settings. That said, the interaction is statistically robust across sensitivity analyses and directionally consistent. For clinicians, this is an incremental but genuinely important signal: dietary protein prescriptions after stroke may need to be paired with gut microbiome support — particularly fermentable fibre to nourish butyrate producers — before meaningful functional gains are realized.