Diverticulitis flares happen when small pouches in the colon wall become inflamed or infected, typically causing sharp pain in the lower left abdomen. The triggers range from everyday habits like diet and medication use to deeper factors like genetics and gut bacteria imbalances. Understanding what sets off a flare can help you reduce the frequency and severity of future episodes.
How a Flare Actually Starts
Diverticula are small, sac-like pouches that form where weak spots in the colon give way under pressure. Having these pouches (diverticulosis) is common and usually harmless. A flare begins when something disrupts the lining of one or more pouches, allowing bacteria to invade and trigger inflammation. This can lead to tiny tears, called microperforations, which may progress to abscesses or larger perforations in severe cases.
The process often involves a combination of factors working together: increased pressure inside the colon, a weakened mucosal barrier, and shifts in the local bacterial environment. That’s why flares can seem unpredictable. Rarely is a single cause responsible.
Low Fiber Intake
A low-fiber diet is one of the most consistent risk factors. Fiber softens stool and helps it move through the colon with less pressure. Without enough fiber, the colon has to squeeze harder to push stool along, and that increased pressure is what creates diverticula in the first place and what inflames existing ones.
The Dietary Guidelines for Americans recommend about 14 grams of fiber per 1,000 calories you eat, which works out to roughly 28 grams per day on a 2,000-calorie diet. Most Americans fall well short of that. Gradually increasing your fiber through fruits, vegetables, whole grains, and legumes is one of the most straightforward ways to reduce flare risk. Increasing too quickly, though, can cause gas and bloating, so it helps to ramp up slowly over a few weeks.
Pain Relievers You Might Not Suspect
Over-the-counter pain medications, particularly ibuprofen, naproxen, and other NSAIDs, are a well-established trigger. A large prospective study found that regular NSAID users had a 72% higher risk of diverticulitis compared to non-users. Even more striking, the risk of complicated diverticulitis (the kind involving abscesses or perforations) was about 2.5 times higher in regular NSAID users.
Aspirin carries a smaller but still meaningful increase, roughly 25% higher risk. These drugs appear to damage the colon lining in two ways: direct irritation of the tissue, and suppression of protective compounds called prostaglandins that help maintain the mucosal barrier. With that barrier compromised, bacteria can more easily penetrate the diverticular wall and spark inflammation. If you rely on NSAIDs for chronic pain, it’s worth discussing alternatives with your provider.
Gut Bacteria Imbalances
The bacteria living in your colon play a direct role in whether diverticula stay quiet or flare up. People developing acute diverticulitis show a distinct shift in their gut microbiome: protective, anti-inflammatory bacterial species decline while potentially harmful ones increase. Specifically, researchers have observed drops in beneficial bacteria like Lactobacilli alongside overgrowth of inflammatory species.
This creates a vicious cycle. The bacterial imbalance promotes mucosal inflammation, which in turn drives further imbalance. Factors that disrupt gut bacteria, including antibiotic use, poor diet, and chronic stress, may therefore indirectly contribute to flares. An increase in a group of bacteria called Proteobacteria has been flagged as a particularly notable marker in people heading toward a flare.
Body Weight and Physical Inactivity
Carrying excess weight, especially around the midsection, raises your risk. Women with a BMI of 30 to 35 had about a 45 to 50% higher risk of diverticulitis compared to those with a BMI under 22.5, and the risk climbed further at higher BMIs. Waist-to-hip ratio matters too: women in the highest category for abdominal fat had a 40% increased risk, suggesting that visceral fat (the fat packed around organs) is particularly problematic, likely because it promotes systemic inflammation.
Physical activity, on the other hand, is protective. A large study of men found that vigorous exercise significantly lowered the risk of both diverticulitis and diverticular bleeding. Running was the only specific activity tied to a statistically significant reduction. Non-vigorous activity like casual walking didn’t show the same benefit. Men who were both obese and inactive faced the highest risk of all. Even modest increases in exercise intensity may help if you’re currently sedentary.
Genetics Play a Bigger Role Than Expected
If diverticulitis runs in your family, your risk is genuinely elevated. Twin studies estimate that 40 to 53% of an individual’s susceptibility to diverticular disease comes from genetic factors. That’s a surprisingly large share for a condition most people think of as purely lifestyle-driven.
A Danish population study found that siblings of people with diverticular disease had about three times the risk of the general population. For identical twins, the relative risk jumped to 14.5 times. Siblings of people who needed surgery for diverticulitis had an even higher risk (about 5.4 times), suggesting that genetics may influence not just whether you develop the condition but how severe it becomes. None of this means flares are inevitable if you have a family history, but it does mean the lifestyle factors described here matter even more for you.
The Seeds and Nuts Myth
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. There is no evidence that these foods cause diverticulitis. The Mayo Clinic and other major medical institutions now state clearly that you do not need to avoid them. In fact, many nuts and seeds are high in fiber and may actually be beneficial.
What a Flare Feels Like
The hallmark symptom is pain in the lower left abdomen, which can range from a dull ache that builds over a few days to a sudden, severe episode. Other common symptoms include fever, nausea and vomiting, a bloated or tender abdomen, constipation (or sometimes diarrhea), and occasionally rectal bleeding. A chronic flare tends to build gradually, while a first acute attack often comes on more abruptly.
Mild, uncomplicated flares can sometimes resolve on their own within about a week. Recent guidelines from the American Gastroenterological Association note that antibiotics aren’t always necessary for mild cases in otherwise healthy people. A review of over 2,500 patients found no difference in recovery time, complications, or need for surgery between those treated with antibiotics and those managed without them. However, antibiotics are still recommended when there are signs of more significant infection, complications, or if you have other health conditions that weaken your immune response.
Reducing Your Risk of Recurrence
About one in five people who have a first episode of diverticulitis will experience another flare. The most actionable steps to lower that risk are eating a high-fiber diet (aiming for at least 25 to 30 grams daily), maintaining a healthy weight, getting regular vigorous exercise, and avoiding routine NSAID use when possible. These aren’t guarantees, especially given the genetic component, but they address the modifiable triggers that research has most consistently linked to flares.