When IBS is a Symptom, Not the Cause

Bradley Bush, ND
6 days ago
4 min read

For millions of people, a diagnosis of Irritable Bowel Syndrome (IBS) becomes a long-term label—one that explains symptoms but often fails to explain why they are happening. Bloating after meals, unpredictable bowel habits, abdominal discomfort, and food sensitivities are commonly grouped under the IBS umbrella. Yet for a substantial portion of patients, these symptoms may not originate from IBS itself, but from an underlying and frequently overlooked condition: Small Intestinal Bacterial Overgrowth (SIBO). And, targeting SIBO, can reduce symptoms.

Over the past two decades, research has increasingly pointed to a strong connection between IBS and SIBO. Depending on the population studied and the diagnostic methods used, approximately 30–80% of individuals diagnosed with IBS test positive for SIBO. This wide range reflects differences in testing techniques and diagnostic criteria, but the overarching trend is clear—SIBO is highly prevalent among IBS sufferers and may be a primary driver of symptoms in many cases.

Understanding SIBO

SIBO occurs when bacteria normally confined to the large intestine begin to colonize the small intestine. While these microbes are not inherently harmful in the right location, their presence in the small intestine disrupts normal digestion.

The small intestine is designed for nutrient absorption, not fermentation. When bacteria are present in excess, they begin fermenting carbohydrates, producing gases such as hydrogen and methane. This process leads to hallmark symptoms including:

Bloating and visible abdominal distention
Excess gas and belching
Abdominal discomfort or pain
Altered bowel habits (diarrhea, constipation, or both)
Food sensitivities and intolerance

This symptom profile closely mirrors IBS, which is why SIBO is so often missed.

SIBO and IBS: Symptom Overlap

The overlap between SIBO and IBS is not subtle—it is substantial. Many of the defining features of IBS are also hallmark features of SIBO.

Symptom	IBS	SIBO
Bloating	Very common	Very common
Gas / Flatulence	Common	Common
Abdominal Pain	Core diagnostic feature	Very common
Diarrhea	IBS-D subtype	Common (hydrogen-associated)
Constipation	IBS-C subtype	Common (methane-associated)
Alternating bowel habits	IBS-M subtype	Can occur
Food sensitivities	Common	Very common
Fatigue /brain fog	Occasionally reported	More frequently reported
Nutrient deficiencies	Less common	More common

This overlap highlights an important clinical reality: IBS may describe the symptoms, but SIBO may explain the cause.

How SIBO Influences IBS Subtypes

One of the most clinically relevant insights is that the type of gas produced in SIBO often correlates with the pattern of IBS symptoms.

IBS with Diarrhea

Hydrogen-dominant SIBO is commonly associated with diarrhea-predominant IBS (IBS-D). Hydrogen gas tends to accelerate intestinal transit, leading to loose stools, urgency, and more frequent bowel movements.

IBS with Constipation

In contrast, methane-dominant overgrowth—technically linked to methanogenic archaea rather than bacteria—is associated with constipation-predominant IBS (IBS-C). Methane has been shown to directly slow intestinal motility, contributing to harder stools and infrequent bowel movements.

This distinction is critical because it moves IBS from a vague symptom-based diagnosis toward a mechanism-based understanding, where treatment can be more targeted and effective.

What the Research Shows

A growing body of literature supports the relationship between SIBO and IBS:

A meta-analysis found that individuals with IBS were approximately 3–4 times more likely to have SIBO compared to healthy controls.
Another systematic review reported a pooled prevalence of SIBO in IBS patients of ~38%, with higher rates in certain subgroups.
Earlier studies using lactulose breath testing reported positivity rates approaching 70–80% in IBS populations, though these higher estimates remain debated due to testing limitations.
Methane production has been consistently associated with constipation and delayed intestinal transit, reinforcing its role in IBS-C.

While the exact prevalence varies, the consistency across studies supports a meaningful association—one that has significant implications for diagnosis and treatment.

Why SIBO Changes the Approach to IBS

Traditional IBS management often focuses on symptom suppression—using dietary modification, fiber, laxatives, or anti-diarrheal medications. While these approaches may provide temporary relief, they do not address an underlying microbial imbalance if SIBO is present. That means symptoms will continue and may even worsen.

When SIBO is identified and appropriately treated, many patients experience:

Reduced bloating and gas
Improved bowel regularity
Decreased food sensitivities
Better overall digestive function

In some cases, symptoms previously labeled as IBS resolve to the point where the diagnosis no longer applies.

Testing for SIBO: A Practical First Step

The most widely used clinical tool for evaluating SIBO is breath testing, which measures hydrogen and methane gas production after ingestion of a carbohydrate substrate such as lactulose or glucose.

This testing provides actionable information, including:

Presence or absence of bacterial overgrowth
Type of gas produced (hydrogen vs methane)
Insight into how symptoms may be generated

When interpreted correctly, breath testing can be a powerful tool for guiding personalized treatment.

A Root-Cause Approach to IBS

IBS is often not treated, but managed as a lifelong condition. However, for many patients, IBS symptoms may be a signpost pointing toward something more specific and treatable.

SIBO represents one of the most important—and often missed—contributors to chronic digestive symptoms. Identifying it can be the turning point between ongoing symptom management and meaningful resolution.

At Natural Medicine of Stillwater, the focus is not simply on labeling symptoms, but on identifying and addressing the underlying drivers of digestive dysfunction.

Our IBS clinical model emphasizes:

Root-cause investigation rather than symptom suppression
Advanced testing, including SIBO breath testing
Individualized treatment strategies based on patient physiology
Integration of gut health with broader systems, including brain–gut interactions

Our doctors bring decades of clinical experience to caring for GI health, providing a level of depth and precision that goes beyond conventional IBS care. Find out if our clinic is a good fit with a free 10-minute consult.