Psyllium, a soluble fiber derived from the seeds of Plantago ovata, is commonly used as a bulk-forming laxative. Its use in diverticulitis is primarily based on its well-documented ability to improve bowel regularity and stool consistency. Traditionally, patients with diverticular disease (including diverticulosis and diverticulitis) were advised to increase dietary fiber to reduce symptoms and prevent complications. Psyllium became a favored option due to its efficacy in increasing stool bulk and facilitating easier passage.

However, scientific evidence specifically supporting psyllium's benefit in acute diverticulitis is limited. Most studies and guidelines focus on its role in diverticulosis and the prevention of recurrence or progression rather than treatment during an active episode. Some randomized controlled trials and meta-analyses suggest that high-fiber diets, including psyllium supplementation, may reduce symptoms and prevent recurrence of diverticular disease, but these findings are not robust or exclusive to psyllium. The American Gastroenterological Association suggests a high-fiber diet for chronic management and prevention but acknowledges a lack of direct, high-quality evidence for fiber supplementation in acute diverticulitis.

In summary, while psyllium is commonly recommended and scientifically validated for overall bowel health and chronic diverticular disease, there is only modest evidence supporting its use specifically in the management or prevention of diverticulitis episodes, reflected in the evidence rating provided.