Growing literature but limited evidence: A systematic review regarding prebiotic and probiotic interventions for those with traumatic brain injury and/or posttraumatic stress disorder

•TBI & PTSD are highly prevalent & often co-morbid conditions.•Inflammation may play key role in symptom onset & perpetuation in both conditions.•Efficacious treatments for co-occurring TBI & PTSD symptoms are extremely limited.Identified studies (3 severe TBI, 1 PTSD, 0 TBI & PTSD) suggested some promise.•Conclusions limited by variability in methods, supplements & outcomes of interest.•Larger trials are needed to explore biological signatures & clinical outcomes.