Crohn’s disease causes diarrhea through several overlapping mechanisms, not just one. The inflammation that defines Crohn’s damages the intestinal lining in ways that disrupt how your gut absorbs water, nutrients, and salts. But inflammation is only the starting point. Depending on where in the digestive tract the disease is active and what structural damage it has caused, diarrhea can stem from leaky barriers, malabsorbed bile acids, bacterial overgrowth, or even fistulas that reroute food past sections of bowel entirely.
Inflammation Disrupts Water Absorption
The most direct cause is what inflammation does to the cells lining your intestine. Normally, these cells absorb water and electrolytes from digested food as it passes through. In Crohn’s, immune cells release inflammatory signaling molecules, primarily TNF-alpha and others like interferon-gamma and IL-1 beta, that interfere with this absorption process. These signals don’t just cause swelling. They actually shut down the molecular pumps that pull sodium and water out of your intestine and into your bloodstream.
TNF-alpha, for example, causes sodium-absorbing transporters on the surface of intestinal cells to be pulled inside the cell where they can’t function. Without sodium absorption, water stays in the intestine. The result is loose, watery stool, even when the intestine isn’t actively secreting extra fluid. This is an important distinction: the diarrhea in Crohn’s is largely caused by a failure to absorb, rather than an overproduction of fluid.
The Intestinal Barrier Breaks Down
Your intestinal lining is held together by tight junctions, protein structures that seal the gaps between cells and control what passes through. In Crohn’s, inflammatory signaling triggers a process where these tight junction proteins are pulled from the cell surface and internalized. This happens through an inflammatory pathway called NF-kB, which essentially loosens the seal between cells.
Once those junctions open up, the intestinal wall becomes permeable in ways it shouldn’t be. Water, electrolytes, and plasma proteins leak from your bloodstream into the intestinal space, adding volume to stool. At the same time, inflammatory cytokines can trigger cell death in the intestinal lining itself, creating gaps that worsen the leakiness. This is sometimes called exudative diarrhea, because the stool contains leaked blood, mucus, and proteins from the damaged wall. In severe cases, enough protein can be lost through these erosions and ulcerations to lower protein levels in the blood, a condition called protein-losing enteropathy.
Bile Acid Malabsorption
Crohn’s most commonly affects the terminal ileum, the last section of the small intestine. This happens to be the exact spot where your body reabsorbs bile acids, the compounds your liver produces to help digest fat. When the terminal ileum is inflamed, scarred, or has been surgically removed, bile acids pass through without being recaptured.
Those unabsorbed bile acids then enter the colon, where they stimulate the colon to secrete water and speed up its contractions. This produces a watery, urgent type of diarrhea that can persist even when Crohn’s inflammation is otherwise well controlled. It’s one of the reasons some people with Crohn’s continue to have diarrhea after their disease appears to be in remission. Bile acid malabsorption is treatable with specific medications that bind bile acids in the gut, so it’s worth identifying as a separate contributor.
Unabsorbed Nutrients Pull Water Into the Gut
Crohn’s can damage the absorptive surface of the small intestine, including the tiny finger-like projections called villi that are responsible for breaking down and absorbing nutrients. When these are flattened or destroyed by inflammation, carbohydrates like lactose and sucrose may not be properly digested. The enzymes that normally break them down sit on those villi, so when the villi are damaged, the enzymes go with them.
Undigested sugars create an osmotic effect, meaning they draw water into the intestine the same way salt draws moisture from food. This influx of fluid distends the small bowel, speeds up transit, and pushes a large volume of liquid into the colon. Once there, bacteria ferment the unabsorbed carbohydrates, producing gas, short-chain fatty acids, and lactate. This explains why diarrhea in Crohn’s often comes with bloating, cramping, and flatulence, not just loose stools.
Bacterial Overgrowth Adds to the Problem
Small intestinal bacterial overgrowth, or SIBO, is significantly more common in people with Crohn’s than in the general population. Normally, a valve between the small and large intestine (the ileocecal valve) keeps colonic bacteria from migrating backward into the small bowel. Crohn’s can damage this valve through inflammation, scarring, or surgery. Strictures and fistulas further alter the anatomy in ways that slow motility and allow bacteria to colonize areas they shouldn’t.
A meta-analysis found that patients with stricturing or penetrating Crohn’s disease had more than three times the odds of developing SIBO compared to those without these complications. Those with a history of abdominal surgery had about twice the odds. The overgrown bacteria interfere with normal digestion and produce excess hydrogen gas, contributing to bloating (three times more likely in SIBO-positive patients) and flatulence (nearly five times more likely). Because SIBO symptoms overlap so heavily with active Crohn’s, it can be difficult to tell whether a flare is truly a flare or a bacterial overgrowth issue layered on top of the disease.
Fistulas and Strictures
Crohn’s disease can penetrate through the full thickness of the intestinal wall, creating abnormal tunnels called fistulas that connect one part of the bowel to another or to other organs. When a fistula links the small intestine to the colon (an ileosigmoid fistula, found in 16 to 26% of Crohn’s patients), partially digested food bypasses the remaining small bowel and dumps directly into the colon. The colon isn’t designed to handle this volume or composition of material, and the result is diarrhea, weight loss, and abdominal pain. Fistulas connecting the colon to the stomach, though rare, can also promote bacterial overgrowth that worsens diarrhea further.
Strictures present a different problem. As Crohn’s scars and narrows sections of bowel, partial blockages can develop. Liquid stool may squeeze past the narrowed segment while solid material backs up behind it. This creates a pattern sometimes called overflow diarrhea, where you experience frequent loose stools even though the underlying issue is actually an obstruction. It’s counterintuitive but important to recognize, because treating this type of diarrhea requires addressing the stricture rather than the diarrhea itself.
Why Diarrhea Can Persist in Remission
One of the more frustrating aspects of Crohn’s is that diarrhea doesn’t always resolve when inflammation comes under control. This happens because several of the mechanisms above operate independently of active disease. Bile acid malabsorption continues if the terminal ileum has been permanently damaged or removed. SIBO can persist if the anatomy has been altered by surgery or scarring. Carbohydrate intolerance may remain if villi haven’t fully regenerated. And changes in the gut microbiome, which shift during active disease, don’t always return to their pre-flare composition.
This is why treating Crohn’s-related diarrhea sometimes requires looking beyond inflammation control. The American College of Gastroenterology defines clinical remission as being free of symptoms like increased stool frequency, and achieving that often means identifying and addressing whichever combination of these mechanisms is active in a given person. For ileocecal Crohn’s, corticosteroids targeting the ileum are a first-line option for bringing active flares under control. But for diarrhea that lingers after mucosal healing, the cause is often one of these secondary mechanisms rather than persistent inflammation.