Diverticulitis is inflammation or infection of small pouches (called diverticula) that form in the wall of your colon, most commonly in the lower left side. These pouches themselves are extremely common and usually harmless, a condition known as diverticulosis. But when one or more of them becomes inflamed, the result is diverticulitis, which causes pain, fever, and digestive symptoms that can range from mild to severe.
How Diverticula Form in the Colon
Your colon wall has natural weak spots where blood vessels pass through the muscle layer. Over time, pressure inside the colon can push the inner lining outward through these weak spots, creating small balloon-like pouches that bulge from the outer wall. These are diverticula, and most people who have them never know it.
Several changes in the colon make this more likely as you age. The muscle layers of the colon thicken, connective tissue builds up, and the wall loses elasticity. Collagen in the colon becomes more tightly cross-linked, making the tissue stiffer and less able to stretch. Together, these structural changes create a colon that generates higher internal pressure and is more prone to herniation at those weak points.
Low-fiber diets play a role by producing smaller, harder stools that move more slowly through the colon. This slower transit time increases the pressure your colon has to generate to move things along. That added pressure, applied repeatedly over years, pushes tissue outward at the vulnerable spots. This is why diverticulosis is far more common in Western countries where fiber intake tends to be low.
What Triggers the Shift to Diverticulitis
Having diverticula is common. Having them become inflamed is not. Only about 1% of people with diverticulosis develop diverticulitis over an 11-year period. The transition happens when a diverticulum becomes blocked, typically by a small piece of stool. That blockage irritates the lining of the pouch, causing swelling, reduced blood flow, and bacterial overgrowth. The combination of inflammation, tissue damage, and possible infection is what produces the symptoms of diverticulitis.
Think of it like a clogged, infected pore on the skin, but inside your colon. The trapped material creates a breeding ground for bacteria, the tissue swells, and if the inflammation is severe enough, the pouch can develop a small tear or even rupture.
Risk Factors That Increase Your Chances
Genetics plays a surprisingly large role. Twin studies estimate that 40 to 53% of your susceptibility to diverticular disease comes from genetic factors. A large Danish study found that siblings of people with diverticular disease had roughly three times the risk compared to the general population. For identical twins, the risk was 14.5 times higher. Having a family history is also an independent risk factor for disease recurrence and complications, not just for developing it in the first place.
People with connective tissue disorders like Ehlers-Danlos syndrome or Marfan syndrome develop diverticulitis at higher rates, which makes sense given that the disease involves structural weakness in the colon wall. Certain genetic variants linked to diverticulitis also overlap with genes associated with hernias, organ prolapse, and vascular problems, pointing to a shared vulnerability in connective tissues throughout the body.
Beyond genetics, the well-established risk factors include:
- Low physical activity: Sedentary lifestyles are consistently associated with higher rates of diverticular disease.
- Obesity: Higher BMI and larger waist-to-hip ratio both correlate with increased risk.
- Low-fiber diet: Women consuming 25 grams or more of fiber daily had a 13% lower risk of diverticulitis compared to those eating less than 18 grams per day.
- Age: The disease has historically been most common in people over 65, though rates among adults under 40 have risen sharply in recent years.
The Role of Fiber
Fiber’s protective effect is real but more modest than you might expect. In a large study tracking women over time, those in the highest fiber group (averaging about 28.5 grams per day) had a 14% lower risk of developing diverticulitis compared to those in the lowest group (averaging about 12.5 grams per day). The benefit appears to come from fiber’s ability to increase stool bulk, soften stool consistency, and speed transit through the colon, all of which reduce the pressure that contributes to both diverticula formation and blockage.
Fiber from fruits and vegetables showed the strongest association with reduced risk. This doesn’t mean fiber is a guarantee against diverticulitis, but eating a plant-rich diet is one of the most actionable things you can do to lower your odds.
Nuts, Seeds, and Corn Are Not the Enemy
For decades, doctors told patients with diverticulosis to avoid nuts, seeds, popcorn, and corn, based on the theory that small particles could lodge in a diverticulum and trigger inflammation. This advice was never based on evidence, and a major study following men for 18 years found it to be flat-out wrong. Frequent consumption of nuts, corn, and popcorn showed no increased risk of diverticulitis. In fact, nut and popcorn consumption were associated with a slightly lower risk. Current guidelines no longer recommend avoiding these foods.
What Diverticulitis Feels Like
The hallmark symptom is pain in the lower left abdomen. It often comes on gradually, building over several days, though it can appear suddenly. The pain is typically constant rather than coming in waves, and it may worsen with movement or pressure on the area. Fever is common. You may also experience nausea, bloating, constipation, or less frequently, diarrhea. Some people notice a change in their bowel habits in the days leading up to a flare.
In more severe cases, you might have a high fever, intense abdominal tenderness, and a rigid abdomen. These symptoms suggest a possible complication like an abscess or perforation and require urgent medical attention.
Possible Complications
Most episodes of diverticulitis are uncomplicated, meaning the inflammation resolves without causing structural damage. But complications do occur. Abscesses, which are walled-off pockets of infection, develop in up to 30% of acute diverticulitis cases. These often need to be drained.
Fistulas, which are abnormal tunnels that form between the colon and a neighboring organ like the bladder or vagina, occur in about 14% of cases. Non-contained perforations, where the colon wall tears and leaks into the abdominal cavity, happen in 1 to 2% of acute cases. A perforation can lead to peritonitis, a serious and potentially life-threatening infection of the abdominal lining.
How It Is Diagnosed
CT scan with contrast is the standard diagnostic tool for suspected diverticulitis. Both the American College of Radiology and the American Society of Colon and Rectal Surgeons recommend it as the first-line imaging test. CT is highly accurate at confirming the diagnosis, identifying complications like abscesses, and ruling out other conditions that can mimic diverticulitis, such as colon cancer or ovarian problems. Ultrasound is sometimes used as an initial screening tool in some countries, but CT remains the go-to in the United States.
Treatment and Recovery
Treatment depends entirely on severity. For uncomplicated diverticulitis, the approach has shifted significantly in recent years. Current guidelines now favor monitoring without antibiotics for mild cases, based on evidence showing that patients with uncomplicated disease recover just as well with supportive care alone. This typically means a temporary shift to easily digestible foods, rest, and pain management while the inflammation subsides. Most uncomplicated episodes resolve within a week or two.
Complicated cases involving abscesses, perforation, or fistulas often require antibiotics, drainage procedures, or surgery. The specific approach depends on the severity, graded by clinicians on a scale from localized abscess to widespread abdominal infection.
Recurrence and Prevention
The good news about recurrence is that it is less common than most people assume. In a study of over 3,100 patients hospitalized for diverticulitis, only 13.3% had a recurrence within nine years. The old belief that each episode makes the next one more likely and more dangerous has been largely revised.
The lifestyle factors associated with lower recurrence and reduced risk mirror those for prevention: eating a plant-rich, high-fiber diet, staying physically active, maintaining a healthy weight, and managing stress. Regular bowel habits matter too, since reducing the time stool spends in the colon helps keep internal pressure low. Vegetarian diets, regular exercise, and yes, eating nuts and corn are all associated with reduced risk.