What Actually Causes Bloating

Bloating is one of the most commonly reported digestive complaints, and one of the most loosely defined.¹ In the Rome Foundation Global Epidemiology Study, an internet survey of 51,425 adults across 26 countries, close to 18 percent reported bloating at least weekly over a three month period. Reported prevalence ranged from 11 percent in East Asia to 20 percent in Latin America, declined with age, and was roughly twice as high in women as in men.³

That is a very large number of people describing a symptom that does not have one cause. Understanding which mechanism is in play is what separates a response that makes sense from a response that is guesswork.

Two different symptoms, usually said in one breath

Clinicians separate two things most people run together. Bloating is the subjective sensation of trapped gas, pressure, or fullness. Distension is a measurable increase in abdominal girth. They frequently coexist, and both are recognized within the Rome IV framework for functional gastrointestinal disorders, but they are not interchangeable.¹

The distinction matters because the sensation and the visible swelling can arise through different routes.

The abdominal wall, not just the gas

The intuitive model is that a bloated abdomen contains more gas. Direct measurement complicates that.

Villoria and colleagues infused gas loads into the colon of 20 patients with bloating and 15 healthy controls while recording abdominal wall muscle activity. In healthy subjects the load increased girth, relaxed the diaphragm, and increased anterior wall tone. Patients with bloating showed the opposite pattern: paradoxical contraction of the diaphragm alongside relaxation of the internal oblique muscle, producing significantly greater distension from the same gas volume.⁴

This motor pattern, abdomino-phrenic dyssynergia, means the abdominal wall can push outward with no change in what is actually inside it. It is one reason that removing foods from the diet lands unevenly across people who describe the same symptom.

Fermentable carbohydrates

Some carbohydrates are incompletely absorbed in the small intestine and are fermented by bacteria further along, which generates gas. This group is often abbreviated as FODMAPs: fermentable oligosaccharides, disaccharides, monosaccharides, and polyols.

In a randomized, controlled, single-blind crossover trial, 30 adults with irritable bowel syndrome and 8 healthy controls ate either a diet low in FODMAPs or a typical Australian diet for 21 days each. Participants with IBS recorded a mean overall gastrointestinal symptom score of 22.8 mm on the low FODMAP arm versus 44.9 mm on the control arm. Bloating, abdominal pain, and gas passage were among the individual symptoms scored lower. Healthy controls showed minimal change on either diet.⁵

That last detail is the informative one. The same dietary change did not move symptom scores in people who did not have the underlying disorder.

A later network meta-analysis in Gut pooled 13 randomized trials covering 944 patients. A low FODMAP diet ranked first against habitual diet for global symptoms and ranked first for abdominal bloating or distension severity. The authors also noted a real limit on what that evidence describes: most trials were run in secondary or tertiary care, and they did not study the effects of FODMAP reintroduction and personalization.⁶ In other words, the trial evidence is about a restriction phase, not about a permanent way of eating.

Lactose is the single most familiar example of the same mechanism. NIDDK draws a line between lactose malabsorption, in which the small intestine cannot break down all the lactose consumed, and lactose intolerance, which is malabsorption accompanied by digestive symptoms such as bloating, gas, and diarrhea. Not everyone with malabsorption has symptoms, and most people with lactose intolerance can consume some amount of lactose without them.⁷

Sensation, microbes, and transit

Beyond gas volume and wall mechanics, a review in the Journal of Neurogastroenterology and Motility describes several candidate mechanisms that the evidence supports to varying degrees: gut hypersensitivity, impaired gas handling, altered gut microbiota, and abnormal abdominal-phrenic reflexes. The same review is direct that the pathophysiology remains ambiguous and that this incomplete understanding is why the available options are limited.²

The Clinical Gastroenterology and Hepatology review lists a broadly overlapping set of causes: food intolerances, disruption of the microbiota following infection, abnormal visceral sensation, delayed transit, and dysfunction of viscero-somatic reflexes. Its authors note that no regimen works consistently, and that useful management starts with identifying the underlying cause and setting realistic expectations.¹

Where SIBO fits

Small intestinal bacterial overgrowth is a frequent explanation offered online, so it is worth being precise about what the guideline actually says.

The 2020 American College of Gastroenterology guideline defines SIBO as the presence of excessive numbers of bacteria in the small bowel causing gastrointestinal symptoms. It reports that abdominal pain, bloating, gas, distension, flatulence, and diarrhea are the most commonly described symptoms in patients with SIBO. It is also explicit that no single symptom can be specifically attributed to SIBO, and that these symptoms often masquerade as other diagnoses including IBS and functional dyspepsia.⁸

The guideline is equally direct about the testing. It states that current breath tests have low sensitivity and specificity and that additional validation studies are needed for standardization. In one systematic review it cites, the sensitivity of lactulose breath testing ranged from 31 percent to 68 percent and specificity from 44 percent to 100 percent when compared against cultures of small bowel aspirates.⁸

A positive breath test, on those numbers, is a weaker piece of evidence than it is usually presented as being.

When bloating warrants a clinical evaluation

Most bloating is not a sign of serious disease, but two categories are worth naming because they are commonly missed.

Celiac disease is an immune response to gluten that occurs in people with particular gene variants. NIDDK lists bloating and gas among its digestive symptoms, while noting that digestive symptoms are more common in children than in adults.⁹

The National Cancer Institute lists gastrointestinal problems such as gas, bloating, or constipation, and pain, swelling, or a feeling of pressure in the abdomen or pelvis, among the signs and symptoms of ovarian epithelial, fallopian tube, and primary peritoneal cancers. It notes these signs may also be caused by other conditions, and advises checking with a doctor if signs or symptoms get worse or do not go away on their own.¹⁰

The practical read

Bloating is a symptom description, not a diagnosis. The published mechanisms include abdominal wall motor patterns, fermentation of poorly absorbed carbohydrates, altered sensation, transit changes, and microbial shifts, and they are not mutually exclusive within one person.¹² Because of that, the useful question is not which product to buy but which mechanism the pattern points to, and a persistent or worsening symptom belongs in front of a clinician rather than a search bar.