Irritable bowel syndrome doesn’t have a single cause. It develops from a combination of factors, including disrupted signaling between the brain and gut, changes in gut bacteria, immune activation, and heightened pain sensitivity in the intestines. Around 11% to 13% of people worldwide have IBS, with women roughly twice as likely to be affected as men. What makes IBS frustrating is that these causes overlap and reinforce each other, which is why the condition looks different from person to person.
The Brain-Gut Connection
Your brain and your digestive tract communicate constantly through a network of nerves, hormones, and chemical messengers. In people with IBS, this two-way communication system misfires. Signals from the gut get amplified on their way to the brain, and signals from the brain can trigger exaggerated responses in the intestines. Stress, anxiety, and emotional distress feed directly into this loop, which is why psychological strain so reliably worsens IBS symptoms.
One of the key chemical messengers involved is serotonin. About 95% of your body’s serotonin is produced in the gut, where it regulates motility and sensation. Cells lining the intestine release serotonin, which then activates nerve endings that control how fast food moves through you and how much discomfort you feel. In IBS, the system that reabsorbs serotonin after it’s done its job doesn’t work properly. The result is excess serotonin lingering in the gut lining. People with diarrhea-predominant IBS tend to have significantly elevated serotonin levels in their blood and urine compared to healthy controls, though elevated levels have also been found in constipation-predominant IBS.
Another signaling system that goes wrong involves GABA, the brain’s main calming chemical. People with IBS have lower levels of GABA and fewer GABA receptors in the gut, alongside increased activity of transporters that clear GABA away too quickly. This imbalance affects how the intestines contract and how the gut processes pain signals.
A Gut That Feels Too Much
One of the most consistent findings in IBS research is visceral hypersensitivity, a lowered pain threshold in the intestines. When researchers inflate a small balloon inside the rectum, people with IBS report pain at volumes that feel completely normal to healthy subjects. The gut is literally more sensitive.
This happens through a process called peripheral sensitization. Nerve endings in the intestinal wall become overly responsive, particularly after episodes of inflammation. Nerves that normally only detect strong stimuli start responding to gentle pressure. Some nerve fibers that were previously silent, not responding to any mechanical stimulus at all, can acquire sensitivity and begin firing. Chemical signals like ATP and bradykinin, released during even mild inflammation, drive this shift. Once these nerves are sensitized, normal digestive events like gas, stool movement, or mild stretching of the intestinal wall register as pain.
Immune Cells Sitting Too Close to Nerves
The intestinal lining of people with IBS contains more mast cells than normal, particularly in a tissue layer called the lamina propria. Mast cells are immune cells that release inflammatory chemicals when activated. What makes this especially relevant to IBS is their location: an estimated 90% of intestinal mast cells sit in direct contact with or very close to nerve fibers.
When these mast cells activate, they release chemicals that act on nearby nerve endings and hormone-producing cells. This triggers a cascade: the nerves become more excitable, send amplified pain signals to the brain, and stimulate changes in how the intestines contract. Animal studies confirm this relationship. Rats exposed to chronic stress develop mast cell overgrowth specifically near the nerve endings in the gut lining, mirroring what’s seen in human IBS tissue samples.
A Leaky Gut Lining
The intestinal barrier is a single layer of cells held together by protein complexes called tight junctions. These junctions act like gates, controlling what passes from the intestine into the bloodstream. In IBS, several of these gatekeeper proteins are disrupted.
A scaffolding protein called ZO-1, which holds tight junctions in place, is reduced and redistributed in the intestinal tissue of people with diarrhea-predominant IBS. This is linked to mast cell activation. Another protein, occludin, gets broken down faster than normal. Sealing proteins called claudin-1 and claudin-4 are decreased in people with diarrhea-predominant IBS, while constipation-predominant IBS shows elevated levels of claudin-1 and claudin-3.
The consequence of these protein changes is a gut lining that lets through substances it shouldn’t. Inflammatory chemicals perpetuate the cycle by further loosening the tight junctions, creating a self-reinforcing loop of barrier damage, immune activation, and nerve sensitization.
Gut Bacteria Out of Balance
The trillions of bacteria in your intestines play a direct role in IBS. Research published in the journal Gut identified a distinct microbiome pattern in a subset of IBS patients: an overgrowth of bacteria from the Firmicutes group (including known problem species like Clostridium difficile and Clostridium perfringens) and a depletion of Bacteroides species, which are generally associated with a healthy gut.
These bacterial shifts aren’t just bystanders. The altered microbiome in IBS patients shows enriched genes for fermenting sugars like lactose, fructose, and trehalose, as well as for producing certain short-chain fatty acids. It also shows changes in tryptophan production, which is the building block of serotonin. So the bacterial imbalance feeds directly into the serotonin dysregulation and fermentation-driven symptoms described above. Notably, when IBS patients with this abnormal bacterial profile followed a low-FODMAP diet, their Bacteroides levels increased and their levels of pathogenic bacteria decreased, shifting their microbiome closer to a healthy pattern.
Infection as a Starting Point
Some people develop IBS after a bout of food poisoning or gastroenteritis, a condition called post-infectious IBS. Prospective studies estimate that 3% to 36% of people who get a gut infection go on to develop persistent IBS symptoms. The risk depends on the pathogen involved. Campylobacter infections carry a 9% to 13% risk. A dual infection with Campylobacter and a particular strain of E. coli was associated with a 36% incidence of IBS symptoms still present two years later.
The infection itself resolves, but it leaves behind lasting changes: low-grade inflammation, altered nerve sensitivity, shifts in gut bacteria, and a compromised intestinal barrier. These are the same mechanisms found in people whose IBS developed without an obvious infectious trigger, which suggests that infection is one of several entry points into the same underlying condition.
Genetics and Inherited Risk
IBS tends to run in families, and researchers have identified specific genetic variants that help explain why. One well-studied example involves the SCN5A gene, which codes for a sodium channel found in the muscle and pacemaker cells of the intestine. These cells control the rhythmic contractions that move food through your digestive tract.
People carrying mutations in SCN5A reported significantly more gastrointestinal symptoms, especially abdominal pain, compared to family members without the mutation, with roughly six times the odds. One specific mutation reduced the electrical current through the channel by 49% to 77%, which would directly impair normal gut motility. This association was specific to SCN5A; a similar analysis of a potassium channel gene expressed in the gut showed no connection to symptoms.
Genetics don’t determine whether you’ll get IBS, but they can lower the threshold. Someone with a genetic predisposition toward sluggish gut motility or heightened nerve sensitivity may develop IBS after a trigger that wouldn’t affect someone else.
How Food Triggers Symptoms
Certain carbohydrates, collectively called FODMAPs (fermentable sugars found in foods like onions, wheat, apples, and dairy), cause problems through two distinct mechanisms. First, because they aren’t fully digested in the small intestine, they reach the large intestine intact, where bacteria ferment them and produce gases like hydrogen and methane. This gas causes intestinal distension, which in someone with visceral hypersensitivity registers as bloating and pain.
Second, these sugars have high osmotic activity, meaning they pull water into the intestinal space. The resulting fluid accumulation accelerates gut motility, stretches the intestinal wall, and stimulates the already-sensitized nerves. In a healthy gut, these effects are minor and easily tolerated. In an IBS gut with heightened sensitivity, an altered microbiome primed for fermentation, and a compromised barrier, they produce the full range of symptoms: pain, bloating, urgency, and diarrhea.
This is why dietary changes help some people with IBS but not others. The food itself isn’t the root cause. It’s the underlying gut environment that determines whether a particular meal becomes a problem.