Diverticulitis develops when small pouches that have formed in your colon wall become inflamed or infected. These pouches, called diverticula, are extremely common and usually harmless. But when one gets blocked or injured, the result is a painful inflammatory flare that can range from mild to severe. Understanding how those pouches form and what triggers them to become inflamed is key to understanding how diverticulitis happens.
How Diverticula Form in the First Place
Before diverticulitis can occur, you first develop diverticulosis, which simply means you have diverticula in your colon. These pouches form when increased pressure inside the colon pushes the inner lining outward through weak spots in the muscular wall. Those weak spots tend to be located where blood vessels pass through the muscle layer, creating natural points of vulnerability.
Think of it like a weak spot in an inner tube. When pressure builds, the softest point bulges outward. In the colon, this pressure comes from the effort of moving stool through the digestive tract. When stool is hard and compact, the colon has to squeeze harder, which raises pressure against the walls. Over time, repeated high-pressure contractions push the lining through those vulnerable points, forming small sac-like pockets that typically range from 5 to 10 millimeters across.
Most people with diverticula never know they have them. The pouches are usually discovered incidentally during a colonoscopy or imaging for something else. They become a problem only when something goes wrong inside one of them.
What Triggers the Inflammation
The shift from harmless diverticulosis to painful diverticulitis happens when a pouch becomes injured or infected. The exact trigger isn’t always clear, but the leading explanation involves a small piece of stool or hardened material getting lodged in a diverticulum, blocking its narrow opening. This blockage traps bacteria inside, leading to infection, swelling, and in some cases a tiny tear (microperforation) in the pouch wall.
That microperforation is what makes diverticulitis potentially serious. A small tear can allow bacteria to leak into the surrounding tissue, creating localized infection or, in worse cases, an abscess. Most flares stay contained and resolve with treatment, but the initial mechanism is essentially the same: obstruction, bacterial overgrowth, inflammation, and sometimes perforation.
Low Fiber and Colon Pressure
A low-fiber diet is one of the most well-established risk factors for developing diverticula and, eventually, diverticulitis. Fiber softens stool and helps it pass through the colon more quickly. This reduces the pressure the colon has to generate to move things along. Less pressure means fewer opportunities for the lining to get pushed through those weak spots.
Without enough fiber, stool is harder and more compact. The colon segments have to contract with greater force, and that sustained high pressure over years gradually creates the pouches. Most guidelines recommend 25 to 30 grams of fiber per day for adults, but the average intake in Western countries falls well short of that. The correlation between low-fiber diets and diverticular disease is one reason the condition is far more common in industrialized countries than in parts of the world where traditional diets are rich in whole grains, vegetables, and legumes.
Age, Genetics, and Rising Rates in Younger Adults
Age is the single biggest risk factor. The muscular wall of the colon weakens over time, making diverticula increasingly common after age 40 and nearly universal by age 80. But genetics play a larger role than many people realize. Twin studies estimate that roughly 40 to 50 percent of the susceptibility to diverticular disease is inherited. Certain connective tissue disorders, including Ehlers-Danlos syndrome and Marfan syndrome, significantly increase the likelihood of developing diverticula at a younger age because they affect the structural integrity of the colon wall.
If a first-degree family member has had diverticulitis, you’re more likely to need surgery for the condition, with one study finding 1.7 times higher odds of elective surgery compared to people without a family history. The risk of having a flare isn’t necessarily higher, but the course of the disease tends to be more involved.
What’s changed in recent decades is the age profile. Diverticulitis is no longer just a condition of older adults. The incidence among people aged 18 to 44 rose from 0.15 to 0.25 per 1,000 people within just seven years. Among those aged 40 to 49, acute diverticulitis rates climbed 132 percent between 1980 and 2007. Rising obesity rates, more sedentary lifestyles, and shifts in diet are the most commonly cited explanations for this trend.
The Role of Your Gut Bacteria
Your gut microbiome appears to influence whether diverticula stay quiet or become inflamed. People with diverticular disease tend to show specific imbalances in their bacterial populations. One pattern that shows up repeatedly in research is a decline in bacteria known for their anti-inflammatory effects, combined with an increase in potentially harmful species.
Studies have found that people with diverticular disease have notably higher levels of certain bacteria in the Firmicutes group, sometimes more than double what’s seen in the general population. At the same time, they tend to have reduced populations of beneficial bacterial families that help maintain the gut lining and keep inflammation in check. A drop in overall bacterial diversity, paired with a rise in inflammatory species, appears to increase the likelihood that a quiet diverticulum becomes an active problem. This is still an evolving area of understanding, but it helps explain why two people with identical diverticula can have very different outcomes.
Medications That Raise the Risk
Certain common medications can increase the chance that existing diverticula will perforate or become inflamed. Corticosteroids (like prednisone) are the most significant offender. Meta-analyses have found that corticosteroid use raises the odds of diverticular perforation anywhere from 2 to 32 times, depending on the study and dosage. Steroids suppress the immune response and weaken tissue integrity, making it harder for the body to contain a developing infection in a diverticulum.
NSAIDs, the class of pain relievers that includes ibuprofen and naproxen, are also linked to increased diverticular complications. These drugs can irritate the gut lining and impair its ability to heal. Opioid pain medications and certain blood pressure drugs (calcium channel blockers) have also appeared in research as potential contributors, though the evidence is strongest for steroids and NSAIDs. If you already have diverticulosis, these medications don’t guarantee a flare, but they do shift the odds.
Other Contributing Factors
Obesity is a consistent risk factor, particularly for severe diverticulitis requiring hospitalization. Excess abdominal fat increases pressure within the abdomen and promotes chronic low-grade inflammation, both of which can stress the colon wall. Smoking also raises the risk, likely through its effects on blood flow and tissue healing in the gut.
Physical inactivity contributes as well. Regular exercise promotes healthy bowel motility, which keeps pressure in the colon lower. Sedentary lifestyles slow transit time, leading to harder stools and more straining, which feeds the same pressure cycle that creates diverticula in the first place.
Red meat consumption has been associated with higher diverticulitis risk in large observational studies, though the mechanism isn’t entirely clear. It may relate to changes in gut bacteria, lower fiber intake in meat-heavy diets, or both.
Seeds, Nuts, and Popcorn: The Outdated Warning
For years, people with diverticulosis were told to avoid seeds, nuts, and popcorn out of fear that small particles could lodge in a diverticulum and trigger a flare. This advice has been abandoned by major medical organizations. There is no evidence that these foods increase diverticulitis risk. In fact, nuts and seeds are high-fiber foods that likely help reduce colon pressure. If you’ve been avoiding them based on old advice, you can stop.