Diverticulitis happens when small pouches (diverticula) that form in the wall of your colon become inflamed or infected. The exact trigger isn’t fully understood, but the disease results from a combination of structural weakness in the colon wall, blockages inside those pouches, and several lifestyle and genetic factors that raise your risk. Here’s what we know about each one.
How the Pouches Form in the First Place
Before diverticulitis can develop, you first need diverticula, the small bulging sacs that push through weak spots in the colon wall. These form over time as waves of high pressure inside the colon cause the muscular wall to weaken, thin out, and eventually herniate outward. The sigmoid colon, the S-shaped section just before the rectum, is hit hardest because it has the narrowest diameter and therefore experiences the highest internal pressures.
Having these pouches is called diverticulosis, and it’s extremely common. Most people with diverticulosis never develop problems. But in roughly 10 to 25 percent of cases, one or more pouches become inflamed, and that’s diverticulitis.
What Triggers the Inflammation
The leading explanation is that a small piece of stool or undigested food gets trapped inside a diverticulum, blocking its narrow opening. Once blocked, mucus builds up inside the pouch and normal gut bacteria begin to multiply in the confined space. This combination of distension and bacterial overgrowth compromises blood flow to the thin wall of the pouch, leading to tiny tears (microperforation) or, in serious cases, a visible hole (macroperforation) that leaks bacteria into surrounding tissue.
A second theory suggests that hard food particles or elevated pressure inside the colon directly erode the wall of a diverticulum, causing localized inflammation, tissue death, and perforation without the blockage step. Both pathways likely contribute in different people and different episodes.
Low Fiber and Diet
A low-fiber diet is one of the most consistently identified risk factors. Fiber adds bulk and softness to stool, which reduces the pressure your colon has to generate to move things along. Without enough fiber, the colon contracts harder and more often, accelerating the formation of pouches and raising the chance that existing pouches become inflamed.
Adults need about 14 grams of fiber for every 1,000 calories consumed, which works out to roughly 28 grams per day on a standard 2,000-calorie diet. Most people fall well short of that. If your current diet leans heavily on refined flour, meat, and cheese, increasing fiber intake gradually gives your digestive system time to adjust without uncomfortable bloating or gas.
Diets high in red meat have also been linked to higher risk, though the relationship is less precisely quantified than fiber. The overall pattern is clear: a Western-style diet low in plant foods and high in processed meat creates conditions that favor diverticular disease.
Genetics Play a Larger Role Than Expected
Twin studies estimate that 40 to 53 percent of a person’s susceptibility to diverticular disease comes from genetic factors. That’s a surprisingly large contribution, roughly equal to the influence of all environmental and lifestyle factors combined. Researchers have identified specific gene variants involved in connective tissue structure and immune signaling that appear to increase risk. Some families also show patterns of early-onset, severe disease in otherwise healthy members, pointing to inherited vulnerabilities in the proteins that maintain colon wall integrity.
This doesn’t mean the disease is inevitable if it runs in your family, but it helps explain why some people who eat plenty of fiber and stay active still develop diverticulitis.
Changes in Gut Bacteria
The community of bacteria living in your colon appears to shift in ways that promote inflammation before and during diverticulitis episodes. People who go on to develop the disease tend to have lower diversity in their gut bacteria overall, with specific reductions in beneficial, anti-inflammatory species like Faecalibacterium and Ruminococcus. At the same time, potentially harmful bacteria such as Fusobacteria increase in number.
A decline in certain bacteria known for producing short-chain fatty acids, which help nourish the colon lining and keep inflammation in check, is a recurring finding. Whether these microbial shifts are a cause or a consequence of the disease is still being untangled, but they likely create a feedback loop: an imbalanced microbiome weakens the local immune environment, making inflamed pouches slower to heal and more prone to recurrence.
Body Weight and Physical Activity
Carrying excess weight substantially raises your risk. Compared to people at a normal weight, those who are overweight face roughly 2.9 times the odds of developing diverticulitis. Obesity likely contributes through chronic low-grade inflammation, higher intra-abdominal pressure, and changes to gut bacteria composition.
Physical activity, particularly vigorous exercise, has the opposite effect. A large prospective study of runners over age 50 found that those who ran 8 or more kilometers per day had a 48 percent lower incidence of diverticulitis compared to those running less than 2 kilometers per day. The risk dropped about 6 percent for every additional kilometer per day of running. You don’t need to become a distance runner to benefit, but consistent cardiovascular exercise that gets your heart rate up appears to be genuinely protective.
Pain Relievers and Medication Risks
Common over-the-counter pain relievers can increase your risk of both diverticulitis and diverticular bleeding. A 22-year study tracking thousands of men found that regular use of non-aspirin anti-inflammatory drugs (like ibuprofen or naproxen) raised the risk of diverticulitis by 72 percent and diverticular bleeding by 74 percent compared to nonusers. Regular aspirin use (two or more times per week) increased diverticulitis risk by 25 percent and bleeding risk by 70 percent.
These medications work by suppressing compounds that protect the gut lining, making the already-thin walls of diverticula more vulnerable to erosion and perforation. Corticosteroids and opioid painkillers have also been associated with increased complication rates, though the evidence is strongest for anti-inflammatory drugs. If you have known diverticular disease and rely on these medications for pain management, it’s worth discussing alternatives with your doctor.
Rising Rates in Younger Adults
Diverticulitis was once considered a disease of people over 65, but that profile has changed dramatically. Among adults aged 18 to 44, the incidence rose from 0.15 to 0.25 per 1,000 people in just seven years. Adults aged 40 to 49 saw a 132 percent increase in acute diverticulitis cases between 1980 and 2007. Rising obesity rates, declining fiber intake, more sedentary lifestyles, and possibly changes in the food supply all likely contribute to this shift.
Seeds, Nuts, and Popcorn Are Not the Problem
For decades, people with diverticula were told to avoid nuts, seeds, and popcorn based on the theory that these small, hard particles could lodge inside a pouch and trigger inflammation. There is no evidence supporting this. The Mayo Clinic and other major medical institutions now state clearly that these foods do not cause diverticulitis. In fact, nuts are recommended as part of a high-fiber diet that helps prevent flare-ups. If you’ve been avoiding these foods out of caution, you can stop.