Diverticular disease is caused by small bulges in the large intestine (diverticula) developing and becoming inflamed. If any of the diverticula become infected, this leads to symptoms of diverticulitis.

The exact reason why diverticula develop is not known, but they are associated with not eating enough fibre.

Fibre makes your stools softer and larger, so less pressure is needed by your large intestine to push them out of your body.

The pressure of moving hard, small pieces of stools through your large intestine creates weak spots in the outside layer of muscle. This allows the inner layer (mucosa) to squeeze through these weak spots, creating the diverticula.

There is currently no clinical evidence to fully prove the link between fibre and diverticula. However, diverticular disease and diverticulitis are both much more common in Western countries, where many people do not eat enough fibre.

Diverticular disease

It is not known why only one in four people with diverticula go on to have symptoms of diverticulitis. Diverticular disease may be chronic low-level diverticulitis. The symptoms are very similar to irritable bowel syndrome (IBS) and may overlap.

However, factors that appear to increase your risk of developing diverticular disease include:

• smoking
• being overweight or obese
• having a history of constipation
• use of non-steroidal anti-inflammatory drugs (NSAIDs) painkillers, such as ibuprofen or naproxen
• having a close relative with diverticular disease, especially if they developed it under the age of 50

Exactly how these lead to developing diverticular disease is unclear.

Diverticulitis

Diverticulitis is caused by an infection of one or more of the diverticula.

It is thought an infection develops when a hard piece of stool or undigested food gets trapped in one of the pouches. This gives bacteria in the stool the chance to multiply and spread, triggering an infection.