The main cause of diverticulosis is a low-fiber diet, which leads to increased pressure inside the colon and gradually pushes small pouches (diverticula) through weak spots in the intestinal wall. This explanation has been the foundation of gastroenterology thinking for decades, though newer research shows genetics play a surprisingly large role as well. Understanding both factors helps explain why diverticulosis affects some people and not others, even when their diets look similar.
How Low Fiber Creates Colon Pressure
When you consistently eat too little fiber, your stool tends to be smaller and harder. Your colon has to squeeze more forcefully to push that compact stool forward. This repeated high-pressure squeezing is what drives diverticula to form over time.
The physics behind this involve a basic principle: the narrower a tube, the more internal pressure it needs to generate to move material through it. The sigmoid colon, the S-shaped section just before the rectum, has the smallest diameter of any part of the large intestine. That means it consistently operates under the highest pressure, which is exactly why the sigmoid is where diverticula appear most often. Over years of straining against small, firm stools, the muscular wall of the colon thickens, and the lining gets pushed outward through tiny gaps where blood vessels pass through the muscle layer. Those bulges are diverticula.
This process is slow. Most people develop diverticula over decades without any symptoms. By age 60, roughly half of adults in Western countries have them. By age 80, the number climbs to around two-thirds.
How Much Fiber You Actually Need
The general recommendation is about 14 grams of fiber per 1,000 calories you eat. For someone on a standard 2,000-calorie diet, that works out to roughly 28 grams per day. Most Americans get only about half that amount.
Fiber works by absorbing water and adding bulk to stool, which makes it softer and easier for the colon to move along without generating excessive pressure. Good sources include beans, lentils, whole grains, vegetables, fruits, and nuts. Increasing fiber intake gradually (rather than all at once) helps avoid bloating and gas as your gut adjusts.
Genetics Matter More Than Expected
A 2013 population-based study of twins and siblings estimated that 53% of the susceptibility to diverticular disease comes from genetic factors. That’s a striking number, and it means your inherited biology contributes at least as much as your diet does.
Researchers don’t yet have a clear map of which specific genes are responsible. What likely matters are inherited traits like the natural thickness and elasticity of your colon wall, the strength of connective tissue, and the baseline motility patterns of your intestines. Some people may simply have colons that are structurally more vulnerable to the effects of pressure over time. This helps explain why not everyone who eats a low-fiber diet develops diverticulosis, and why some people who eat plenty of fiber still do.
Other Factors That Raise Your Risk
Aging
The colon wall naturally loses elasticity and muscular strength as you get older. Connective tissue weakens, making those blood vessel entry points more prone to giving way under pressure. This is why diverticulosis is rare before age 40 and nearly universal in older adults.
Obesity and Weight Gain
Carrying excess weight increases your risk, particularly for complications like diverticulitis (when a pouch becomes inflamed or infected). Women with a BMI of 35 or higher have about a 42% greater risk of diverticulitis compared to women with a BMI under 22.5. Gaining 20 kilograms (roughly 44 pounds) or more from age 18 onward is associated with a 73% increased risk. The mechanism likely involves chronic low-grade inflammation that excess body fat promotes, combined with dietary patterns that tend to accompany weight gain.
Physical Inactivity
Regular physical activity stimulates normal colon contractions and helps move stool through more efficiently. A sedentary lifestyle slows transit time, which means the colon has to work harder and generate more pressure to move things along.
Certain Medications
NSAIDs (like ibuprofen and naproxen), corticosteroids, and opioid painkillers are all associated with an increased risk of diverticular perforation, the most serious complication. These medications don’t cause diverticula to form in the first place, but they can make existing pouches more likely to become inflamed or tear. NSAIDs and corticosteroids thin the intestinal lining, while opioids slow gut motility and increase internal pressure.
Diverticulosis vs. Diverticulitis
Diverticulosis simply means you have diverticula. Most people with them never know it. The pouches are typically discovered incidentally during a colonoscopy or CT scan done for other reasons. Diverticulosis itself doesn’t require treatment.
Diverticulitis happens when one or more pouches become inflamed or infected. This causes sudden pain (usually in the lower left abdomen), fever, nausea, and changes in bowel habits. About 10 to 25% of people with diverticulosis eventually develop diverticulitis at some point. The progression from harmless pouches to an inflamed pouch is what makes the underlying causes worth understanding: reducing the factors that create diverticula in the first place also reduces the chance of complications down the line.
What This Means Practically
The single most actionable thing you can do to reduce your risk is eat more fiber. Aiming for that 28-gram daily target through whole foods (not supplements alone) gives your colon the bulk it needs to function without excessive pressure. Staying physically active and maintaining a healthy weight further lower the odds, especially for complications. You can’t change your genetics or stop the aging process, but the modifiable factors, particularly diet, are the ones researchers have consistently linked to protection against diverticular disease across populations.