Posttraumatic stress disorder (PTSD) is a debilitating psychiatric disorder, which results from exposure to traumatic events. Veterans are more than twice as likely to suffer from PTSD compared to civilians. Current interventions for the treatment of PTSD are evidence-based psychotherapies, such as Cognitive Processing Therapy (CPT). Therefore, we plan to compare Cognitive Processing Therapy (CPT) with and without a novel prebiotic bar designed to beneficially modulate the microbiota to determine the utility of this combined approach to positively impact PTSD symptoms. We will conduct a 12-week randomized, double-blind, placebo-controlled, cross-sectional trial in veterans with PTSD. Participants will be randomized to receive CPT + prebiotic or CPT + placebo. They will be provided a supply of prebiotic or non prebiotic bars for 12 weeks. At three different time points, participants will complete questionnaires and complete questionnaires and produce a stool sample ( weeks 1,2, and 12). All participants will be recruited from the two-week in-person Intensive Outpatient Program (IOP) at the Rush Road Home Program, where they will be receiving CPT.

Inclusion Criteria:

• Veterans who are participating in the Rush Road Home Program 2-week Intensive Outpatient Program (IOP) and have the ability to consent.
• Ability and willingness to consume up to two prebiotic bars daily and collect stool samples.

Exclusion Criteria:

• Active suicidality or homicidality, current engagement in significant non-suicidal self harm, history of mania or psychosis, current eating disorders, and/or current substance use that would require medical observation if discontinued.
• Gastrointestinal disease: (1) prior intestinal resection, (2) history of GI disease (except for hiatal hernia, nonproton pump inhibitor requiring gastroesophageal reflux disease, or hemorrhoids), (3) known renal disease or abnormal liver function, (4) antibiotic use within the previous 12 weeks, (5) a plan to have a major change in dietary habits during the study.
• Allergy to almonds, flax seed or coconuts