Symptoms and conditions
Abdominal bloating and distension: causes, mechanisms and when to see a gut specialist
Bloating is the feeling of fullness, pressure or trapped gas in the abdomen. Distension is a visible increase in its size. Most bloating comes from how the gut handles and senses gas, not from an excess of it. Constipation, food intolerances and bacterial overgrowth are the commonest treatable causes. Persistent, progressive distension always needs an examination.
In short
- Bloating that comes and goes, is worse by evening and better by morning is usually a functional problem.
- People with bloating often have normal amounts of gas but a gut that senses it more, or abdominal muscles that respond to it abnormally.
- Constipation is the most overlooked cause.
- Bloating that is constant, progressive or accompanied by weight loss needs prompt assessment.
What is the difference between bloating and distension?
Bloating is a sensation. Distension is a measurable change in the size of the abdomen. Many people have both; some have only one. The distinction matters because they have partly different mechanisms.
Why does bloating happen?
A sensitive gut
In IBS and functional indigestion, the gut’s nerves register ordinary amounts of gas and stretching as discomfort.
Constipation
Stool held in the colon slows everything behind it and gives bacteria more time to ferment. See constipation.
Fermentable foods
Lactose, fructose and other FODMAPs are fermented by bacteria and draw water into the bowel. See the low FODMAP diet.
Bacterial overgrowth
Bacteria in the small intestine ferment food early. See SIBO.
An abnormal muscle reflex
Normally, when the gut fills, the diaphragm rises and the abdominal wall tightens. In some people the opposite happens: the diaphragm descends and the abdominal wall relaxes, so the abdomen protrudes.
Swallowed air
From eating fast, fizzy drinks, chewing gum or mouth breathing.
Less often, bloating comes from coeliac disease, slow stomach emptying, disease of the pancreas, or a partial blockage.
When is bloating a warning sign?
See a doctor promptly if you have
- Distension that is constant and increasing over weeks
- Weight loss, poor appetite or feeling full very early
- Vomiting
- Blood in the stool or a low blood count
- Swelling of the legs or yellow eyes
- In women, persistent bloating with pelvic pain or a change in urination, particularly after the age of 50
Fluid in the abdomen from liver disease and growths of the ovary or bowel can present as “bloating”.
Which tests help?
Tests are chosen by the pattern:
- Blood tests for anaemia, coeliac disease and thyroid function
- An ultrasound when distension is persistent
- A breath test for bacterial overgrowth and sugar intolerance
- Endoscopy or colonoscopy when there are warning signs
- Anorectal manometry when constipation with straining is part of the picture
What helps?
Treat constipation first
Even mild constipation worsens bloating. A soft daily stool often reduces it more than any other single step.
Adjust food
Eat slowly, at regular times. Reduce fizzy drinks. A supervised low FODMAP diet for two to six weeks, followed by reintroduction, identifies your own triggers.
Treat overgrowth or intolerance
If a breath test is positive, a gut-specific antibiotic or a reduction in the offending sugar.
Move after meals
A ten-minute walk helps gas to move on.
Retrain the muscles
Slow abdominal breathing, taught with biofeedback, corrects the abnormal diaphragm reflex and reduces visible distension.
Calm the gut’s nerves
For persistent bloating in a sensitive gut, low doses of medicines that act on gut nerves, and gut-directed psychological therapies.
Remedies sold for gas have little evidence. Probiotics help some people and worsen bloating in others; a four-week trial is reasonable.
Questions patients ask me
Why is my abdomen flat in the morning and swollen by evening?
This pattern is typical of functional bloating. Food, gas and fluid accumulate through the day, and the abdominal muscle reflex tires. It settles overnight when the gut is empty and you are lying down.
Do I produce too much gas?
Usually not. Studies show that most people with bloating have normal amounts of gas. The difference lies in how the gut moves and senses it.
Should I stop eating dal and vegetables?
Not permanently. Some pulses and vegetables are high in FODMAPs and can be reduced for a few weeks. They are important foods, and most people can bring them back in suitable portions.
Is bloating a sign of a serious disease?
Most often it is not. Bloating that comes and goes, with a normal weight and normal blood tests, is rarely serious. The warning signs listed above are the reason to be examined.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- American College of Gastroenterology guideline on irritable bowel syndrome. Lacy BE et al. Am J Gastroenterol. 2021. doi:10.14309/ajg.0000000000001036
- Rome IV criteria for bowel disorders. Lacy BE et al. Gastroenterology. 2016. PubMed 27144627
- North American consensus on hydrogen and methane breath testing. Rezaie A et al. Am J Gastroenterol. 2017. PubMed 28323273
- American College of Gastroenterology guideline on small intestinal bacterial overgrowth. Pimentel M et al. Am J Gastroenterol. 2020. PubMed 32023228
- Controlled trial of a low FODMAP diet in irritable bowel syndrome. Halmos EP et al. Gastroenterology. 2014. PubMed 24076059