Diverticulitis develops when small pouches that form in the wall of your colon become inflamed or infected. These pouches, called diverticula, are extremely common and usually harmless. But when one gets blocked or irritated, the result is pain, fever, and sometimes serious complications. Understanding how this process unfolds, and what raises your risk, can help you take practical steps to lower your chances.
How the Pouches Form in the First Place
Before diverticulitis can happen, you first develop diverticulosis, which simply means having these small bulging pouches in your colon. They form gradually over years at naturally weak spots in the colon wall, places where blood vessels pass through the muscle layer. When pressure inside the colon rises from straining, spasms, or hard stool pushing through, the inner lining gets pushed outward through those weak points, creating a small sac. Most people with diverticula never know they have them. The pouches are typically discovered incidentally during a colonoscopy or CT scan done for another reason.
What Triggers the Inflammation
Having pouches is one thing. Getting diverticulitis is another. The shift from harmless pouch to painful inflammation happens through a few possible pathways, and the trigger may differ depending on your age.
In older adults, the most common explanation involves a small piece of stool getting trapped inside a diverticulum. This trapped material irritates the lining, causes local swelling, and creates a breeding ground for bacteria. As bacteria multiply, the waste and gas they produce can damage the pouch wall further, sometimes leading to a tiny tear or perforation.
In younger adults, a different mechanism may be at work. Rather than stool getting stuck, the colon itself may contract too forcefully or too long, compressing the blood vessels at the neck of the pouch. This cuts off blood flow, causing tissue damage and small perforations. Research suggests some people have heightened sensitivity to nerve signals that trigger these contractions, which could explain why diverticulitis is increasingly showing up in people under 50.
The Rising Rate in Younger Adults
Diverticulitis is no longer just an older person’s disease. An analysis of 5.2 million hospitalizations for diverticulitis between 2005 and 2020 found that about 16% of cases occurred in patients younger than 50. More concerning, the proportion of younger patients admitted with complicated diverticulitis (cases involving abscesses, perforations, or other serious problems) jumped from 18.5% to 28.2% over that period, a 52% relative increase. The reasons aren’t fully understood, but rising obesity rates, more sedentary lifestyles, and dietary shifts in younger generations likely play a role.
Risk Factors You Can Control
Low Fiber Intake
A diet low in fiber is one of the most consistently identified risk factors. Fiber softens stool and helps it pass through the colon more easily, reducing the pressure that forms pouches and the stagnation that inflames them. Not all fiber is equal here. Research published in Gastroenterology found that insoluble fiber, the kind found in whole grains and fruit skins, reduced diverticulitis risk by about 14% when comparing the highest intake to the lowest. Soluble fiber (the type in oatmeal and beans) did not show the same benefit.
Specific foods mattered too. Whole fruits, especially apples, pears, and prunes, were associated with lower risk. Fruit juice was not. Interestingly, vegetable fiber showed no protective effect either, suggesting that simply eating more salad isn’t enough. The practical takeaway: prioritize whole fruits and high-fiber cereals or whole grains over juices and refined carbohydrates.
Excess Body Weight
Carrying extra weight, particularly around the midsection, increases your risk significantly. A large study of women found that those with a BMI of 35 or higher had a 42% greater risk of developing diverticulitis compared to women with a BMI below 22.5. Waist circumference and waist-to-hip ratio showed similar patterns: women in the highest category for either measurement had roughly 35 to 40% higher risk. The connection likely involves chronic low-grade inflammation that excess fat tissue promotes throughout the body, including in the gut wall.
Certain Medications
Regular use of anti-inflammatory painkillers like ibuprofen and naproxen is linked to higher odds of diverticulitis complications, including perforation and abscess formation. A systematic review found that the increased odds with NSAIDs ranged from about 1.5 to 10 times higher, depending on the study and the specific complication. Corticosteroids carried even steeper risk, with odds ratios reaching as high as 32 times greater for perforation. Opioid painkillers also raised the risk roughly 2 to 4.5 times. These medications can mask early pain signals, slow gut motility, or weaken the immune response in the colon wall, all of which can make a minor pouch problem become a serious one.
Risk Factors You Can’t Control
Genetics
Your genes play a meaningful role. Genome-wide studies have identified dozens of genetic variants linked to diverticular disease, clustered into three main categories: genes involved in the immune system, genes that control connective tissue and the structural integrity of the colon wall, and genes related to the nervous system’s control of gut movement. If close family members have had diverticulitis, your own risk is elevated, though how much is hard to quantify precisely because genetic and dietary habits tend to run together in families.
Age
The colon wall naturally weakens with age, and the muscle layer becomes less elastic. This makes pouch formation more likely the older you get. While diverticulosis is relatively uncommon before age 40, it’s present in more than half of people over 60. More pouches means more opportunities for one to become inflamed.
The Role of Gut Bacteria
Your gut microbiome appears to influence whether diverticula stay quiet or flare up. Research has found that people who develop diverticulitis tend to have lower diversity of gut bacteria overall, with declines in beneficial species like Faecalibacterium and Ruminococcus. At the same time, potentially harmful bacteria such as Fusobacteria tend to increase. This imbalance may weaken the protective mucus layer inside the colon and promote the kind of inflammation that turns a stable pouch into an acute problem. A high-fiber diet is one of the best-known ways to support a diverse, healthy microbiome, which may partly explain why fiber is protective.
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 small particles could lodge inside a pouch and trigger inflammation. This advice has been thoroughly debunked. No specific foods are known to trigger diverticulitis attacks, and there is no evidence that seeds or nuts cause flare-ups. In fact, nuts are now recommended as part of a high-fiber diet for people with diverticular disease. If you’ve been avoiding these foods out of caution, you can stop.
What a Flare-Up Feels Like
Diverticulitis typically announces itself with steady pain in the lower left side of your abdomen, though in some people (particularly those of Asian descent) it occurs on the right. The pain often comes on gradually over a few hours and worsens over a day or two. You may also have fever, nausea, a noticeable change in bowel habits (constipation or diarrhea), and tenderness when pressing on your belly. Mild cases can often be managed at home with a temporary shift to easily digestible foods and a course of antibiotics, though some doctors now treat uncomplicated cases without antibiotics. Severe cases involving perforation, abscess, or blockage require hospitalization and sometimes surgery.
A first episode of diverticulitis doesn’t guarantee recurrence. Roughly 20 to 35% of people who have one episode will have another, but the majority do not. After recovery, increasing your fiber intake gradually, maintaining a healthy weight, staying physically active, and limiting NSAID use are the most evidence-backed strategies for keeping future flare-ups at bay.