Diverticulosis can cause diarrhea, though for years it was considered a silent condition. A large population study found that people with diverticulosis were about twice as likely to report loose stools and high stool frequency compared to people without it, regardless of age. In people over 60, the association was even stronger, with a nearly tenfold increase in the odds of diarrhea-predominant irritable bowel syndrome.
Why Diverticulosis Was Once Considered Harmless
Diverticulosis refers to small pouches that form in the wall of the colon, most commonly on the left side. Over half of people older than 65 have them. For decades, the medical view was simple: the pouches themselves don’t cause symptoms unless they become inflamed (diverticulitis) or bleed. Diarrhea, abdominal pain, and bloating were attributed to other conditions like irritable bowel syndrome.
That view has shifted. Research now shows that even without inflammation or infection, diverticulosis is linked to loose stools, urgency, mucus in the stool, and abdominal pain. The connection is strong enough that doctors now recognize a distinct condition sitting between silent diverticulosis and full-blown diverticulitis.
Symptomatic Uncomplicated Diverticular Disease
The in-between condition is called symptomatic uncomplicated diverticular disease, or SUDD. It’s defined as persistent pain in the lower left abdomen along with changes in bowel habits, including diarrhea, constipation, and bloating, in someone who has diverticula but no signs of body-wide inflammation like fever or elevated white blood cell counts. SUDD is considered a chronic condition that can significantly reduce quality of life.
What makes SUDD tricky is that it involves low-grade, localized inflammation. A marker called fecal calprotectin, which signals intestinal inflammation, is often mildly elevated. So while nothing dramatic is happening (no abscess, no perforation), the colon wall around those pouches is quietly irritated. That ongoing irritation appears to drive the diarrhea and pain many patients experience.
What’s Happening Inside the Colon
Several biological mechanisms explain how diverticula contribute to diarrhea. The pouches can trap stool and bacteria, creating pockets where the microbial balance shifts. Studies of patients with diverticular disease show a reduction in bacteria that have anti-inflammatory effects, along with overgrowth of other species that promote inflammation. This imbalance, sometimes called dysbiosis, can irritate the colon lining and speed up the movement of stool through the intestines.
There’s also a nerve component. The inflammation around diverticula can make the nerves in the colon wall hypersensitive, a phenomenon called visceral hypersensitivity. Once those nerves become overreactive, even normal amounts of gas or stool passing through can trigger pain, urgency, and loose bowel movements. This is one reason diverticular symptoms overlap so heavily with irritable bowel syndrome.
Bacterial overgrowth in the small intestine (SIBO) may also play a role. One study of 90 patients with uncomplicated diverticulitis found that nearly 59% also had SIBO. Bacterial overgrowth in the small intestine causes bloating, abdominal pain, and diarrhea on its own, and when layered on top of diverticular disease, it can make symptoms considerably worse.
Diverticular Diarrhea vs. IBS
The symptoms of SUDD and irritable bowel syndrome are nearly identical: abdominal pain, bloating, diarrhea or constipation, mucus in the stool. The key difference is timing. IBS typically develops earlier in life, while diverticular symptoms tend to appear in the sixth or seventh decade. If you’re over 55 and developing new-onset diarrhea with left-sided abdominal discomfort, diverticulosis is a more likely contributor than IBS presenting for the first time.
Distinguishing between the two is genuinely difficult even for specialists. Standard blood tests and imaging look normal in both conditions. Fecal calprotectin may be slightly elevated in SUDD but not in IBS, which can help point in the right direction. Still, there’s significant diagnostic overlap, and some researchers believe the two conditions may share underlying mechanisms like altered nerve signaling and changes in gut bacteria.
A Less Common Cause: Segmental Colitis
A rare condition called segmental colitis associated with diverticulosis (SCAD) can cause more persistent diarrhea. It affects roughly 1.5% of people with diverticulosis, mostly older men. SCAD involves inflammation of the colon wall between the diverticula, typically in the sigmoid colon on the lower left side. Symptoms include chronic diarrhea, lower abdominal pain, and occasional blood in the stool.
SCAD is diagnosed through colonoscopy, which reveals red, inflamed patches of colon between non-inflamed diverticula. One distinguishing feature is that SCAD spares the rectum and the end of the small intestine, which helps differentiate it from Crohn’s disease and ulcerative colitis. If you have diverticulosis with chronic diarrhea that isn’t responding to dietary changes, SCAD is worth investigating.
Managing Diarrhea With Diverticulosis
Fiber is the cornerstone of managing diverticular symptoms, though the type matters. A daily intake between 20 and 30 grams is recommended for all patients with diverticular disease who aren’t in the middle of an acute flare. Soluble fiber from sources like oats, psyllium, and cooked vegetables tends to be better tolerated than insoluble fiber from raw vegetables and bran, which can worsen diarrhea in some people. Starting low and increasing gradually helps your gut adjust without making things worse.
Probiotics have shown mixed results. Some small studies found that specific bacterial strains reduced symptom recurrence in patients with symptomatic diverticular disease. One trial found that combining an anti-inflammatory medication with a probiotic kept 100% of participants symptom-free at 12 months, compared to about 77% on either treatment alone. But a larger study found no added benefit from probiotics. The evidence is encouraging but inconsistent, and there’s no single probiotic strain with strong enough data to recommend universally.
Beyond fiber and probiotics, managing diverticular diarrhea often involves the same strategies used for IBS: identifying food triggers, managing stress (which worsens visceral hypersensitivity), staying hydrated, and in some cases using medications that target gut motility or inflammation. Because the overlap with IBS is so significant, treatments that work for one condition frequently help the other.