Changes in gastrointestinal (GI) microbiome composition and metabolism have been identified in many patients with gastrointestinal diseases, and the GI microbiome may play an important role in promoting and maintaining GI health. This study evaluated the efficacy of a fiber‐supplemented Control Food (CF) and a therapeutic food (TF) with specialized plant fibers known to contain antioxidant and polyphenol compounds in cats with constipation or diarrhea. A prospective clinical study was conducted with 44 adult cats, recruited from private veterinary practices across the United States. Cats exhibiting clinical signs of diarrhea or constipation were enrolled in the study. Cats were excluded from this study if they were under 1 year of age, had systemic disease including kidney disease (IRIS stage 3 or greater), had a Body Fat Index less than 20, were unwilling or unable to exclusively consume the study food, had megacolon or were otherwise unlikely to benefit from a fiber supplemented food, chronically used colonic motility drugs, or were currently receiving oral antibiotics or probiotics and were unwilling to discontinue. Enrolled cats were randomized to one of two complete and balanced dry therapeutic foods (25 CF, 19 TF). Fiber sources included CF: cracked pearled barley, ground whole grain corn, dried beet pulp, fructooligosaccharides (FOS) & psyllium seed husk; TF: ground whole grain corn, ground pecan shells, cracked pearled barley, whole grain oats, dried beet pulp, pea fiber, flaxseed, dried citrus pulp, pumpkin, pressed cranberries, FOS & psyllium seed husk. Veterinarians performed physical examinations, evaluated clinical signs of constipation or diarrhea and rated the cat’s response to the study food at days 2, 3, 4, and 28 using a defined scale (Negative Response, Non‐Response, Positive Response, and Complete Response). This study was reviewed & approved by the Institutional Animal Care & Use Committee, Hill’s Pet Nutrition, Inc. Veterinarian evaluations were analyzed using a Cochran‐Mantel‐Haenszel test with modified ridit scores. Results significant at p<0.05 are reported. The proportion of cats receiving CF rated by the veterinarian as Positive or Complete Response vs. study start was 36% at day 2, 48% at day 3, 60% at day 4, and 69% at day 28 (p=NS); the proportion of cats receiving TF rated by the veterinarian as Positive or Complete Response vs. study start was 42% at day 2, 63% at day 3, 65% at day 4, and 100% at day 28 (p<0.01). TF rapidly and effectively resolved veterinarian‐assessed clinical outcomes in cats with constipation or diarrhea. Fiber sources rich in antioxidant and anti‐inflammatory compounds were more effective than traditional fiber sources in improving clinical response in cats with diarrhea or constipation.Support or Funding InformationThis study was funded by Hill’s Pet Nutrition, Inc.