If you have been dealing with bloating, irregular bowel movements, or abdominal pain, you have likely heard the terms IBS and even SIBO. You may have even been told you have one or both. But what is SIBO vs. IBS? While they are often grouped together, they are not the same, and understanding the difference can be a critical step in healing.
As an IBS and SIBO Dietitian, I help my clients peel back all the layers to truly understand what is actually driving their digestive issues. IBS and SIBO are often used interchangeably and share many of the same symptoms, but they are not the same and need to be treated differently. Let’s break down how these two conditions differ and why that distinction matters.
First, What Is IBS?
Irritable Bowel Syndrome, or IBS, is considered a functional gastrointestinal disorder. This means the digestive tract looks normal on imaging and scopes, but it is not functioning as it should. IBS is often influenced by stress, nervous system dysregulation, disruption in the gut-brain communication, food sensitivities, and motility issues. It affects an estimated 10 to 15 percent of the population.
Common Symptoms of IBS include:
- Abdominal pain or cramping
- Bloating and gas
- Diarrhea, constipation, or both
- Incomplete bowel movements
How is IBS Diagnosed?
IBS is diagnosed based on exclusion of other conditions as well as by a specific set of symptom patterns rather than a single lab test or imaging finding. Clinicians use what’s called the Rome IV criteria, which is the current gold standard for diagnosing functional gastrointestinal disorders like IBS.
Under the Rome IV criteria, IBS is defined by recurrent abdominal pain, on average at least one day per week over the past three months, that is associated with two or more of the following:
- Pain related to bowel movements
- A change in stool frequency
- A change in stool form or appearance
How is IBS Classified?
Once IBS is identified, it is further categorized into subtypes based on predominant bowel patterns:
- IBS-D when diarrhea is most common
- IBS-C when constipation is most common
- IBS-M when both diarrhea and constipation occur
- IBS-U when symptoms don’t clearly fit one category
An important part of diagnosing IBS is ruling out other conditions that could explain symptoms. This is why many people undergo blood work, stool testing, imaging, or endoscopy/colonoscopy before receiving an IBS diagnosis. These tests are used to exclude inflammatory bowel disease, celiac disease, infections, structural abnormalities, or other medical conditions.
When all testing appears normal but symptoms persist, IBS is often diagnosed. While this can feel dismissive to patients, it doesn’t mean symptoms aren’t real. It simply means the issue lies in how the gut is functioning rather than visible damage. This is also where underlying contributors like motility issues, gut-brain signaling, stress, and conditions such as SIBO may be missed if further evaluation isn’t investigated.
What Is SIBO?
Small Intestinal Bacterial Overgrowth, or SIBO, occurs when bacteria builds up in the small intestine, an area that should be vastly sterile. This bacteria feeds off of consumed carbohydrates and other nutrients too early in digestion, fermenting and producing gas and triggering inflammation, which is why symptoms often overlap with IBS.
Common SIBO symptoms include:
- Bloating that worsens as the day goes on
- Excessive gas that may be foul-smelling or sulfur-like
- Abdominal discomfort or pressure
- Diarrhea, constipation, or both
- Fatigue and brain fog
- Nutrient deficiencies such as B12, iron, or fat-soluble vitamins
How is SIBO Diagnosed?
Unlike IBS, SIBO is a condition that can be objectively tested. The gold standard of diagnosis at this time is a SIBO breath test that measures gases produced by bacteria in the small intestine.
During a SIBO breath test, a person drinks a sugar solution, most commonly lactulose or glucose. Breath samples are then collected at 10-15 minute intervals to measure levels of hydrogen, methane, and in some tests, hydrogen sulfide gas. These gases are produced when bacteria ferment carbohydrates, and elevated levels within a specific time frame suggest bacterial overgrowth in the small intestine.
A positive SIBO breath test helps confirm that symptoms are being driven by bacterial fermentation and gas production rather than functional gut changes alone. While breath testing is not perfect, it remains the most widely used and clinically relevant tool for identifying SIBO. A biopsy can also be taken to measure for bacteria levels in the small intestines but is not often used due to the difficulty and invasive nature of the procedure.
How Is SIBO Classified?
SIBO is classified based on the type of gas produced, which often correlates with symptom patterns and helps guide treatment decisions.
- Hydrogen-dominant SIBO is commonly associated with diarrhea, urgency, bloating, and abdominal cramping.
- Methane-dominant overgrowth (IMO) is often linked with constipation, slowed motility, weight gain, and harder or less frequent stools.
- Hydrogen sulfide–dominant SIBO (ISO) may present with diarrhea, urgency, nausea, and foul-smelling gas or stools. It is often under diagnosed due to limited testing availability.
- Mixed gas patterns can lead to alternating bowel habits and more complex symptom presentations.
Understanding which gases are elevated is important because different patterns may require different treatment strategies and varying levels of motility support.
SIBO vs. IBS Comparison
IBS and SIBO are often discussed together because their symptoms can look very similar, but they are not the same condition. Clinicians diagnose IBS based on symptom patterns and how the gut functions, while SIBO involves bacterial overgrowth in the small intestine that clinicians can identify through breath testing.
In many cases, SIBO can exist underneath an IBS diagnosis, which is why understanding the distinction is so important. Comparing how each is diagnosed, what drives symptoms, and how treatment is approached helps create a clearer, more targeted path toward relief rather than relying on trial and error.
Why the IBS vs SIBO Distinction Matters
Research suggests that a up to 78% of people diagnosed with IBS may actually have underlying SIBO. When this goes unrecognized, treatment can miss the mark.
Treating SIBO as IBS, for example by increasing fiber or probiotics, and focusing only on symptom reduction, can sometimes worsen bloating and discomfort. On the other hand, using antibiotics or antimicrobials without confirmation of bacterial overgrowth may disrupt the gut microbiome unnecessarily.
Getting clarity allows for a more targeted, effective, and sustainable plan.
How I Approach Healing as an IBS and SIBO Dietitian
When I work with clients, the goal is not just symptom management or “killing the SIBO”. Understanding why symptoms are happening in the first place is key to truly heal. Depending on the individual, this may include:
- Identifying root causes through functional testing when appropriate
- Guiding clients through SIBO breath testing and interpretation
- Customizing nutrition approaches such as Low FODMAP or SIBO-specific strategies
- Supporting gut motility and digestion for long-term results
- Reducing symptom flares while rebuilding confidence with food
There is no one-size-fits-all approach, and that is especially true when IBS and SIBO overlap.
Final Thoughts
Many people question whether their symptoms truly point to IBS or whether something deeper, like SIBO, plays a role. These conditions are often confused, but they are not the same. Understanding the difference is often the first step toward meaningful relief.
Ready to Get to the Root of Your Gut Issues?
Inside my Bloating Breakthrough group program, I help clients move beyond guesswork and finally get clarity on their symptoms. Whether you suspect SIBO or have been living with an IBS diagnosis for years, we build a personalized, science-based plan that supports healing and long-term confidence.
