What is diverticular disease?
Diverticula (singular: diverticulum) are small pouches that bulge outward through weak spots in the wall of the large bowel (colon). They form when the inner lining of the bowel pushes through gaps in the muscular layer — a process similar to the inner tube of a bicycle tyre bulging through a weak spot in the outer tyre. They are typically 5–10mm in diameter and most commonly develop in the sigmoid colon (the S-shaped lower section of the large bowel on the left side of the abdomen), though they can occur anywhere in the colon.
Diverticular disease is described using three related but distinct terms, and the distinction matters:
Diverticulosis
The presence of diverticula without any symptoms. This is the most common state — the vast majority of people with diverticula are entirely unaware of them. Diverticulosis is considered a normal feature of the ageing bowel in Western populations rather than a disease in its own right. No treatment is needed and no dietary restrictions are required.
Diverticular disease
When diverticula cause symptoms — typically intermittent lower left abdominal pain, bloating, and changes in bowel habit (either looser or more constipated stools). The pain may be triggered by eating and relieved by passing wind or stool. Symptoms can overlap significantly with irritable bowel syndrome (IBS — a functional gut disorder causing chronic abdominal pain and altered bowel habit without structural abnormality), and the two conditions can coexist.
Diverticulitis
When one or more diverticula become infected or inflamed — producing more persistent pain, fever, and tenderness, usually in the lower left abdomen. Diverticulitis can be uncomplicated (confined to the bowel wall) or complicated — involving a pericolic abscess (a collection of pus around the bowel), perforation (a tear through the bowel wall), fistula (an abnormal channel to an adjacent organ such as the bladder), or peritonitis (widespread abdominal infection from bowel contents entering the abdomen).
Why is it so common?
Diverticular disease is strongly associated with a low-fibre diet, physical inactivity, obesity, and smoking — all hallmarks of Western lifestyles. It is sometimes described as a "disease of Western civilisation" because it is rare in rural sub-Saharan Africa and parts of Asia where dietary fibre intake is high, but increasingly prevalent in populations that have adopted Western dietary patterns. The condition was rare before the 20th century and has increased markedly since industrialisation and the widespread adoption of processed, low-fibre foods.
- Diverticulum / Diverticula
- A small pouch (diverticulum) or pouches (diverticula) that bulge outward through weak spots in the wall of the large bowel.
- Diverticulosis
- The presence of diverticula without any symptoms — the most common state. Considered a normal feature of the ageing bowel in Western populations.
- Diverticular disease
- Diverticula that are causing symptoms — typically lower left abdominal pain, bloating, and altered bowel habit — without active infection or inflammation.
- Diverticulitis
- Active infection or inflammation of one or more diverticula, producing persistent pain, fever, and tenderness. Can be uncomplicated or complicated.
- Sigmoid colon
- The S-shaped lower section of the large bowel on the left side of the abdomen — the most common site for diverticula to develop in Western populations.
- Abscess
- A localised collection of pus. A pericolic abscess (around the bowel) is a complication of diverticulitis, sometimes requiring drainage.
- Peritonitis
- Widespread infection of the abdominal cavity, occurring if a diverticulum perforates (tears through the bowel wall) and gut contents enter the abdomen. A surgical emergency.
- Fistula
- An abnormal channel that forms between the bowel and an adjacent organ — most commonly the bladder (colovesical fistula) — as a complication of recurrent diverticulitis.
Why does it matter?
While most diverticula cause no trouble, the minority that do become symptomatic or inflamed can significantly affect quality of life. Acute diverticulitis accounts for a substantial number of emergency hospital admissions in England each year, and complicated diverticulitis — with abscess, perforation, or peritonitis — carries real risk.
The educational value for a lay audience lies in two areas: first, understanding the distinction between the three stages so that an incidental finding of diverticulosis does not cause unnecessary anxiety; and second, understanding the dietary evidence — which is strong — and the reversal of long-standing dietary advice about seeds and nuts.
What your doctor might do
Management in UK practice depends entirely on which stage of the spectrum a person is in — asymptomatic diverticulosis, symptomatic diverticular disease, or acute diverticulitis.
- Peritonitis — generalised abdominal pain, board-like rigidity of the abdomen, high fever: suggests bowel perforation — clinically recognised as a surgical emergency. 999 exists for this.
- Severe, worsening lower left abdominal pain with high fever and rigors (uncontrolled shivering) suggest complicated diverticulitis — clinically recognised as requiring urgent hospital assessment. 111 and urgent GP services exist for this.
- Frank rectal bleeding (bright or dark red blood from the back passage) that is significant in volume is a recognised feature of diverticular haemorrhage — clinically recognised as requiring assessment.
What the research shows
Diverticular disease is one of the most common digestive conditions in the UK — and one of the most misunderstood. The majority of people with diverticula (diverticulosis) will never develop symptoms and need no treatment, no dietary restrictions, and no follow-up. The anxiety that sometimes follows an incidental finding is not supported by the evidence.
The dietary evidence is strong: a high-fibre diet — 30g per day — is associated with a 41% reduction in the risk of developing diverticular disease. The average UK adult consumes around 18g per day, well below this level. The reversal of the long-standing advice to avoid seeds, nuts, and fruit skins is equally important — these foods may actually be beneficial, not harmful.
For those who do develop symptoms, management in UK practice is lifestyle-based. Antibiotics are no longer routinely used even for acute diverticulitis, and NSAIDs — still commonly taken for joint pain and other conditions — carry a recognised risk of perforation that is worth knowing about.
Most diverticula are silent and harmless. The evidence points clearly toward dietary fibre as the key modifiable factor — and the good news is that the bowel responds to changes in diet at any age.