IBD and SIBO: Crohn's Disease, Ulcerative Colitis & Structural Risk

Inflammatory bowel disease and small intestinal bacterial overgrowth are two distinct digestive conditions that frequently occur together. Inflammatory bowel disease is a broad term for conditions like Crohn’s disease and ulcerative colitis, which involve chronic inflammation of the digestive tract. Small intestinal bacterial overgrowth is a condition where abnormally high numbers of bacteria build up in the small intestine.

While the two conditions have different causes, they share many outward signs. This overlap creates challenges for people experiencing symptoms and for the professionals trying to find the source of the digestive distress. The presence of inflammatory bowel disease directly increases the risk of developing a bacterial overgrowth. Research shows that people with inflammatory bowel disease have a 9.51 times higher risk of developing small intestinal bacterial overgrowth compared to healthy individuals¹. Across various studies, approximately 22% to 31% of people with inflammatory bowel disease test positive for bacterial overgrowth¹.

Shared Symptoms and Physical Predictors

From an outward perspective, a flare of inflammatory bowel disease and an episode of small intestinal bacterial overgrowth look nearly identical. Both conditions cause significant digestive distress. The shared symptoms include watery diarrhea, severe abdominal pain, bloating, and excessive gas¹, closely matching hallmark SIBO symptoms.

Because the symptoms are so similar, a bacterial overgrowth is easily mistaken for an active flare of inflammatory bowel disease³. A person might have their underlying inflammation well managed, yet they continue to experience daily pain, bloating, and diarrhea. When these problems persist even though the inflammatory bowel disease is under control, small intestinal bacterial overgrowth is often the actual cause of the discomfort³.

Certain physical signs predict the presence of small intestinal bacterial overgrowth in people with inflammatory bowel disease. Individuals who have a lower body mass index, report high levels of flatulence, and have a history of abdominal surgery are at the highest risk². Both conditions also interfere with the absorption of essential nutrients. When the small intestine is damaged by inflammation or overrun by bacteria, the digestion process breaks down. People with either condition frequently develop deficiencies in iron and vitamin B12, which leads to anemia, weakness, and fatigue⁴.

Symptom Feature	Mechanism in Inflammatory Bowel Disease	Mechanism in Small Intestinal Bacterial Overgrowth
Watery Diarrhea	Inflammation damages the intestinal lining, preventing water absorption and increasing fluid secretion.	Bacteria ferment carbohydrates early, drawing water into the intestine and accelerating bowel movements.
Bloating and Gas	Structural narrowing and altered gut movement cause gas to build up.	Excess bacteria consume sugars in the small intestine and produce large amounts of hydrogen and methane gases.
Vitamin B12 Deficiency	Inflammation or surgical removal of the ileum removes the physical site where B12 is absorbed.	Bacteria consume the B12 for their own growth before the human body can absorb it.
Weight Loss	Chronic inflammation increases the body's energy demands while making eating painful.	Bacteria interfere with the digestion of fats and carbohydrates, causing food to pass through unabsorbed.

Biological Differences in the Gut Environment

Despite the shared symptoms, the biological events happening inside the gut are completely different. Inflammatory bowel disease is primarily a problem of immune dysregulation and structural damage⁸. In a healthy body, the immune system protects the gut from dangerous infections. In a person with inflammatory bowel disease, the immune system mistakenly attacks harmless bacteria and the healthy cells of the digestive tract⁹. This attack creates ulcers, bleeding, and physical destruction of the tissue.

Small intestinal bacterial overgrowth is a problem of location and quantity. The human digestive tract has distinct zones. The large intestine, or colon, holds trillions of bacteria that help ferment leftover waste. The small intestine is meant to be relatively clean, holding fewer than 100,000 bacterial organisms per milliliter of fluid¹⁰ (under contemporary guidelines, over 1,000 CFU/mL is diagnostic). This low bacterial count allows the small intestine to absorb nutrients without competition. In small intestinal bacterial overgrowth, colonic bacteria migrate upward into the small intestine and multiply. These bacteria are normal gut bacteria living in the wrong place. When they live in the small intestine, they encounter food before it is fully digested. They ferment this food, creating gas, acid, and pressure that damage the lining of the small intestine⁵.

The bacterial populations themselves change differently in the two conditions. In inflammatory bowel disease, the intense inflammation reduces the overall diversity of bacterial life⁸. The inflammatory gut environment increases oxygen levels, which favors the growth of specific bacteria like Proteobacteria⁸. At the same time, beneficial bacteria like Faecalibacterium prausnitzii decrease⁸. This specific bacterium produces butyrate, a short-chain fatty acid that keeps the intestinal lining strong and healthy. When butyrate levels drop in inflammatory bowel disease, the intestinal wall becomes weak and permeable⁸. In small intestinal bacterial overgrowth, the issue is not just a lack of good bacteria, but a massive overpopulation of fermenting bacteria in a space that cannot handle them.

How Inflammatory Bowel Disease Causes Bacterial Overgrowth

The human body relies on several physical and chemical defenses to keep the small intestine clear of excess bacteria. These defenses include strong stomach acid, steady intestinal movement, and physical valves. Inflammatory bowel disease breaks down these specific defense systems. When these barriers fail, bacteria from the colon easily move up into the small intestine and multiply⁶, representing key structural causes of SIBO.

Structural Damage and Intestinal Surgery

One of the main reasons inflammatory bowel disease leads to bacterial overgrowth is the physical change it causes to the intestinal anatomy. Chronic inflammation causes scar tissue to build up inside the intestines. This scar tissue creates strictures, which are narrow points in the digestive tube². When food and fluid hit a stricture, they slow down or pool behind the narrowed area. This stagnant fluid creates a breeding ground for bacteria⁵. The presence of stricturing or penetrating disease behavior increases the odds of a patient developing bacterial overgrowth by 3.51 times².

Inflammation also creates fistulas, which are abnormal tunnels connecting one part of the intestine to another. A fistula bypasses the normal flow of digestion, allowing bacteria from the heavily populated colon to flow directly into the small intestine⁴.

Surgery is a common treatment for severe inflammatory bowel disease. Surgeons often remove heavily damaged sections of the bowel. While this stops the immediate inflammation, it permanently alters the anatomy. One specific anatomical structure frequently removed during these surgeries is the ileocecal valve¹. This valve sits at the exact junction between the small intestine and the large intestine. It functions as a one-way door, allowing waste to leave the small intestine while preventing the dense bacterial population of the colon from washing backward. When surgery removes the ileocecal valve, the physical barrier is gone. Colonic bacteria freely migrate upward, resulting in a very high risk of small intestinal bacterial overgrowth¹. Studies show that low pressure in the ileocecal valve strongly predicts bacterial overgrowth, with 65.2% of patients with low valve pressure testing positive for the condition¹.

Altered Intestinal Motility

The digestive tract has a built-in cleaning mechanism called the migrating motor complex (MMC). Between meals, the nerves and muscles of the stomach and small intestine create a series of strong sweeping waves. These waves push leftover food, debris, and stray bacteria down into the colon. This sweeping action is the primary way the body prevents bacteria from settling in the small intestine¹¹.

Inflammation directly disrupts the nerves and muscles of the gut wall. In people with inflammatory bowel disease, the migrating motor complex often becomes weak or irregular⁶. Because the sweeping waves are slow or absent, the small intestine does not clear itself properly. The delayed transit time allows bacteria to linger, attach to the intestinal walls, and multiply³.

The Role of Bile Acids and Fat Digestion

The intersection of inflammatory bowel disease and small intestinal bacterial overgrowth is highly visible in how the body processes dietary fat. The liver produces bile acids, which are stored in the gallbladder and released into the small intestine when a person eats. Bile acids act like a detergent. They break large fat droplets into tiny microscopic particles so the body can absorb them along with fat-soluble vitamins¹².

Bile acids are originally produced in a conjugated form, meaning they are chemically bound to an amino acid like taurine or glycine. This form is necessary for them to digest fat properly. In a healthy gut, these conjugated bile acids travel to the end of the small intestine, specifically the ileum, where they are reabsorbed into the bloodstream and recycled back to the liver¹².

Bacterial overgrowth completely disrupts this process. The excess bacteria in the small intestine chemically alter the bile acids through a process called deconjugation¹². The bacteria break the chemical bonds, changing the bile acids into an unusable form. Because the bile acids are destroyed early in the digestive process, the body cannot break down or absorb dietary fats¹⁴. The undigested fat travels down into the colon, resulting in steatorrhea, which is loose, foul-smelling, fatty diarrhea⁶, a complication often observed when patients attempt high-fat or ketogenic diets without adequate bile flow.

Inflammatory bowel disease complicates this further. Crohn’s disease frequently attacks the ileum, the exact location where bile acids are reabsorbed¹². If the ileum is inflamed or surgically removed, the body cannot recycle its bile acids. The combination of bacteria destroying the bile acids early and the damaged ileum failing to recycle them creates a severe cycle of fat malabsorption and persistent diarrhea⁶. This leads to severe deficiencies in fat-soluble vitamins, particularly vitamin D⁷.

Crohn’s Disease Versus Ulcerative Colitis

While both main types of inflammatory bowel disease increase the risk of bacterial overgrowth, the risk is not equal. Small intestinal bacterial overgrowth is significantly more common in people with Crohn’s disease than in people with ulcerative colitis². Research indicates that approximately 32.2% of Crohn’s disease patients develop bacterial overgrowth, compared to 27.8% or lower in ulcerative colitis patients².

This difference comes directly from how the two diseases behave physically. Crohn’s disease can attack any part of the digestive tract from the mouth to the anus, but it most frequently targets the small intestine and the ileocecal valve¹. Because it affects the small intestine, it causes the strictures, fistulas, and motility issues that directly invite bacterial overgrowth. Crohn’s disease penetrates deeply into the intestinal wall, causing thick scar tissue that requires surgical removal of the small bowel².

Ulcerative colitis behaves differently. It only affects the large intestine and the rectum. The inflammation is continuous but shallow, affecting only the innermost lining of the colon. Because ulcerative colitis does not physically damage the small intestine or the ileocecal valve, the normal defenses of the small intestine remain mostly intact⁴. A patient with ulcerative colitis usually only develops small intestinal bacterial overgrowth if the inflammation alters the overall movement of the entire digestive tract, or if they have widespread colon involvement known as pancolitis⁴.

Disease Characteristic	Crohn's Disease	Ulcerative Colitis	Impact on Bacterial Overgrowth Risk
Location of Inflammation	Can occur anywhere, highly favors the small intestine.	Strictly limited to the large intestine and rectum.	Crohn's disease directly alters the small intestine environment.
Type of Damage	Deep, full-thickness inflammation causing strictures and fistulas.	Shallow, surface-level inflammation without strictures.	Physical blockages in Crohn's disease trap bacteria.
Surgical Outcomes	Frequent removal of the small intestine and ileocecal valve.	Removal of portions of the colon; small intestine remains.	Loss of the ileocecal valve in Crohn's disease removes the bacterial barrier.
Overgrowth Odds Ratio	10.86 compared to healthy controls¹.	7.96 compared to healthy controls¹.	Crohn's disease presents a significantly higher structural risk.

The Impact of Hydrogen Sulfide

One of the specific biological connections between inflammatory bowel disease and bacterial overgrowth involves a gas called hydrogen sulfide. In the human gut, certain types of bacteria are classified as sulfate-reducing bacteria. These bacteria consume dietary sulfur and produce hydrogen sulfide gas as a byproduct of their digestion¹⁶.

In a healthy gut, the body handles small amounts of hydrogen sulfide easily. The cells lining the colon use small amounts of it for energy, and they convert excess gas into harmless chemicals like thiosulfate¹⁷. When there is an overgrowth of sulfate-reducing bacteria, the production of hydrogen sulfide overwhelms the body’s defenses¹⁷.

High concentrations of hydrogen sulfide are toxic to the cells lining the intestine. The gas interferes with the cells’ ability to use energy, breaking down the mucosal barrier and causing tissue damage and increased intestinal permeability (leaky gut)¹⁸. In inflammatory bowel disease, patients consistently show elevated levels of sulfate-reducing bacteria and hydrogen sulfide in their stool, which correlates directly with the severity of their symptoms¹⁶.

In the context of small intestinal bacterial overgrowth, a specific subtype called hydrogen sulfide overgrowth involves these same bacteria moving into the small intestine¹⁹. The small intestine does not have the same thick mucosal protection or detoxifying capacity as the colon. The hydrogen sulfide causes immediate symptoms by accelerating the movement of the intestines, resulting in severe urgency, abdominal pain, and watery diarrhea²¹. The bacteria also produce acetate, which inhibits the body’s enzymes from clearing the gas, creating a cycle that intensifies the damage to the intestinal wall¹⁶.

Diagnostic Overlap and Clinical Challenges

Because the daily symptoms of inflammatory bowel disease and bacterial overgrowth are so similar, medical professionals rely on specific testing to tell them apart. Identifying which condition is causing the symptoms dictates the management approach. If a patient is experiencing an inflammatory bowel disease flare, they require medications that suppress the immune system and stop the inflammation. If the patient has small intestinal bacterial overgrowth, immune-suppressing drugs do not stop the diarrhea or bloating. Instead, they require therapies specifically targeted to clear out the bacteria in the small intestine³.

To solve this problem, doctors use clinical activity scores, biological markers, and breath tests. The overlap of the two diseases makes these tests complicated to interpret.

Clinical Activity Scores and Functional Overlap

Doctors measure the severity of inflammatory bowel disease using standardized scoring systems. For Crohn’s disease, they use the Harvey-Bradshaw Index, which awards points based on general wellbeing, abdominal pain, the number of liquid stools per day, and physical complications³. For ulcerative colitis, they use the Partial Mayo Score, which tracks stool frequency and rectal bleeding³.

Because small intestinal bacterial overgrowth causes diarrhea and abdominal pain, it artificially inflates these activity scores. A patient might score high on the Harvey-Bradshaw Index, suggesting they are having a severe Crohn’s disease flare, when their symptoms are actually driven entirely by bacterial overgrowth³. Research shows that when patients with Crohn’s disease receive treatment to eradicate their bacterial overgrowth, their median Harvey-Bradshaw Index score drops significantly, moving from a score of 5 down to 3³. Similarly, eradicating the bacteria in ulcerative colitis patients lowers their Partial Mayo Score³. This proves that bacterial overgrowth heavily mimics the clinical signs of an inflammatory flare, much like it does in irritable bowel syndrome (IBS).

Fecal Calprotectin

Fecal calprotectin is a highly reliable marker used to measure inflammation in the gut, routinely ordered as part of comprehensive stool tests. Calprotectin is a protein found inside neutrophils, which are a type of white blood cell²⁵. When the immune system attacks the intestinal lining in inflammatory bowel disease, thousands of white blood cells rush to the area. These cells break down and release calprotectin into the stool.

Measuring the amount of calprotectin in a stool sample gives a direct, objective measurement of how much inflammation is present in the intestinal wall²⁵. A normal level of fecal calprotectin is below 50 micrograms per gram. In a person having an active flare of inflammatory bowel disease, the calprotectin levels spike massively, often reaching well over 500 or 1,000 micrograms per gram²⁶.

Fecal calprotectin is useful for distinguishing between the two conditions. Small intestinal bacterial overgrowth is a bacterial problem, not an autoimmune attack, so it does not trigger a massive white blood cell response²⁸. If a person has severe diarrhea and bloating, but their fecal calprotectin level is very low, it indicates that the symptoms are caused by bacteria, not an inflammatory flare²⁹.

Small intestinal bacterial overgrowth does cause a low grade of surface irritation in the small intestine. This mild irritation can cause a slight elevation in fecal calprotectin³. A patient with bacterial overgrowth might have a calprotectin level between 50 and 250 micrograms per gram²⁸. While this slight elevation complicates the diagnosis, it remains much lower than the massive spikes seen in active inflammatory bowel disease.

Patient Status	Typical Fecal Calprotectin Level	Clinical Interpretation
Healthy Individual	Below 50 µg/g	No active inflammation.
SIBO without active IBD	50 µg/g to 250 µg/g	Mild irritation from bacterial gas; no major immune attack²⁸.
Active IBD Flare	500 µg/g to \>1,000 µg/g	Severe mucosal damage; high white blood cell activity²⁶.

Breath Testing and Rapid Transit Times

The most common way to diagnose small intestinal bacterial overgrowth is through a hydrogen and methane breath test. Human cells do not produce hydrogen or methane gas. Only bacteria and archaea produce these gases when they ferment carbohydrates³⁰. During a breath test, a patient drinks a solution containing a sugar like lactulose or glucose. As the sugar travels through the digestive tract, any bacteria present consume it and release gas. This gas enters the bloodstream, travels to the lungs, and is exhaled in the breath³¹.

In a healthy person, the sugar travels through the relatively empty small intestine without causing gas. It eventually hits the colon, where the normal bacteria ferment it, creating a late spike in breath gas. If a person has small intestinal bacterial overgrowth, the sugar hits the misplaced bacteria early in the small intestine, creating an early spike in breath gas within 90 minutes³¹.

Using this test in people with inflammatory bowel disease is difficult because the disease alters how fast food moves through the gut. People with Crohn’s disease frequently have rapid orocecal transit times, meaning food and liquids race through their stomach and small intestine much faster than normal³.

If a Crohn’s disease patient drinks the lactulose sugar solution for a breath test, their rapid transit time might push the sugar all the way into the colon in under an hour. The normal colon bacteria ferment the sugar immediately, creating an early spike in gas. The test machine registers this early spike and flags it as a positive result for small intestinal bacterial overgrowth³¹. In reality, the small intestine was clear, but the food reached the colon too fast. This false-positive result frequently confuses the diagnosis, leading to patients appearing to have an overgrowth when their true issue is rapid motility³². Because of these false positives, interpreting breath tests for people with inflammatory bowel disease requires looking closely at transit times and using glucose breath tests, which absorb earlier in the gut and are less affected by rapid transit speeds³.

Works Cited & Scientific References 34

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