A diverticulum is a small pouch or sac that bulges outward from the wall of a hollow organ, most commonly the large intestine. These pouches form at naturally weak spots in the organ wall where blood vessels pass through the muscle layer. Most people who have diverticula never know it, because the pouches themselves rarely cause symptoms.
How a Diverticulum Forms
The walls of your colon are made up of several layers, including an inner lining and an outer muscle layer. Where small arteries penetrate through the muscle to supply blood to the inner lining, the wall is structurally weaker. When pressure inside the colon rises, those weak spots can give way, and the inner lining pushes outward through the muscle to form a small balloon-like pocket.
The sigmoid colon, the S-shaped section in your lower left abdomen, is the most common site for these pouches. It has the smallest diameter of any colon segment, which means it generates the highest internal pressures. Think of it like squeezing a tube of toothpaste: the narrower the tube, the more force the contents exert on the walls. Abnormal muscle contractions or spasms in the colon can amplify this pressure further, essentially pushing the lining through those vulnerable points.
True vs. False Diverticula
Not all diverticula are built the same way. The pouches that form in the colon are technically “false” diverticula because only the inner lining and a thin connective tissue layer bulge outward. The muscle layer stays behind. A “true” diverticulum, by contrast, contains every layer of the organ wall. The distinction matters more to doctors than to patients, but it explains why colonic diverticula are relatively fragile and can sometimes bleed or become inflamed.
The best-known true diverticulum is a Meckel diverticulum, a congenital pouch in the small intestine that about 2% of people are born with. Unlike colonic diverticula, which develop over time, a Meckel diverticulum is present from birth. It forms when a structure from fetal development doesn’t fully close, leaving a small sac that includes all normal layers of the intestinal wall. It’s usually found within about 100 centimeters of where the small intestine meets the large intestine, and it sometimes contains tissue normally found in the stomach or pancreas, which can occasionally cause bleeding or pain.
Where Diverticula Can Occur
Although the colon gets the most attention, diverticula can develop in other hollow organs. The esophagus can form pouches, particularly near the throat or just above the diaphragm. The bladder can develop diverticula, especially in men with chronic urinary obstruction. The stomach and duodenum (the first part of the small intestine) are less common sites but not unheard of. When someone says “diverticula” without specifying a location, they almost always mean the colon.
Diverticulosis vs. Diverticulitis
Having diverticula is called diverticulosis. It’s extremely common, especially with age, and the vast majority of people with diverticulosis have no symptoms and need no treatment. It’s often discovered incidentally during a colonoscopy or CT scan done for another reason.
Diverticulitis is what happens when one or more of those pouches becomes inflamed or infected. The process typically starts when a small piece of stool gets trapped in the pouch, blocking its opening. This can cause a tiny tear and trigger inflammation. The hallmark symptoms are pain in the lower left abdomen (sometimes called “left-sided appendicitis”), tenderness when that area is pressed, and signs of inflammation like fever or elevated blood markers. Diagnosis requires both evidence of inflammation in blood tests and confirmation through imaging, usually a CT scan of the abdomen with contrast dye. Clinical symptoms alone aren’t enough to confirm it.
The distinction between uncomplicated and complicated diverticulitis depends on whether the inflamed pouch has perforated. Doctors look for signs of free air, abscesses, or abnormal connections between the colon and other organs on imaging. Uncomplicated cases, which are the majority, typically resolve with a short period of dietary modification and sometimes antibiotics. Complicated cases may require more involved treatment.
Risk Factors for Developing Diverticula
Age is the single biggest risk factor. The colon wall naturally weakens over decades of use, and the cumulative effect of internal pressure takes its toll. By age 60, a significant portion of people in Western countries have at least a few diverticula.
Genetics play a measurable role. Research using polygenic risk scores (which estimate inherited risk across thousands of genes) has confirmed that some people are genetically more prone to developing diverticula and diverticulitis. People with connective tissue disorders like Marfan syndrome or Ehlers-Danlos syndrome are also at higher risk, because the structural proteins that reinforce the intestinal wall are altered in these conditions.
Lifestyle factors matter independently of genetics. Higher body mass index consistently increases risk across all genetic categories. Physical inactivity contributes as well. A large integrated analysis of four prospective studies found that each unit increase in a healthy lifestyle score (which factored in BMI and physical activity, among other variables) reduced diverticulitis risk by 11 to 14%, and this benefit held regardless of whether someone carried low, medium, or high genetic risk. In other words, healthy habits help even if your genes put you at a disadvantage.
Fiber, Hydration, and Prevention
A high-fiber diet is the most widely recommended strategy for preventing diverticula from forming and for keeping existing ones from causing trouble. Fiber adds bulk and softness to stool, which reduces the pressure your colon has to generate to move things along. The current recommendation is 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 people fall well short of that.
If your current diet is heavy on refined grains, meat, and cheese, ramping up fiber too quickly can cause bloating and discomfort. A gradual approach works better: start with a few substitutions (whole grain bread instead of white, an extra serving of vegetables) and increase over weeks. Fiber needs water to do its job, so hydration matters just as much. A practical guideline is to drink roughly half your body weight in pounds as ounces of water. For a 160-pound person, that’s about 80 ounces per day.
How Diverticula Are Found
Because diverticulosis is almost always silent, most people learn about their diverticula through imaging done for something else. During a routine colonoscopy, the openings of diverticula are clearly visible along the colon wall. On a CT scan, the pouches appear as small outpouchings from the colon’s outer surface. If diverticulitis is suspected, a CT scan with intravenous contrast is the preferred test, as it shows both the inflamed pouch and any complications in the surrounding tissue. Many imaging centers also use oral contrast to make the intestinal tract easier to evaluate.