Several digestive conditions have overlap in common symptoms, making it rather common for one to be confused about which GI condition they may have.
Two particular conditions, small intestinal bacterial overgrowth (SIBO) and irritable bowel syndrome (IBS), not only have closely identical symptoms; they frequently occur together in the same person, with SIBO being a potential underlying factor behind many cases of IBS.
Despite this connection, these conditions require vastly different treatments, meaning relief starts with learning how SIBO vs. IBS differ, as getting the diagnosis right is key to treating symptoms in both.
Key differences between SIBO and IBS
While SIBO and IBS can look very similar and even coincide, they differ in many aspects, including primary cause, how symptoms surface, how they’re diagnosed, and how they’re treated.
| SIBO | IBS | |
| Cause | Excess bacteria in the small intestine | Gut-brain connection |
| Symptoms | Bloating, gas, bowel changes, possible malabsorption and nutrient deficiencies | Abdominal pain, bloating, and changes in bowel habits without malabsorption |
| Diagnosis | Breath testing alongside symptoms and medical history | Based on symptoms after other digestive conditions have been ruled out |
| Treatment | Antibiotics, treating underlying causes, nutrition support, and dietary changes | Diet and lifestyle changes, symptom-relieving medications, stress management, and psychological therapies when appropriate |
What is SIBO?
SIBO is a condition in which too many bacteria grow in the small intestine. Relatively few bacteria normally live in this part of the body, so when SIBO ushers in an imbalance, digestion and nutrient absorption can be disrupted.
SIBO often develops when food and bacteria move too slowly through the small intestine because of certain medical conditions or structural changes (through surgery, for instance), allowing bacteria to multiply.
Similar to IBS, SIBO can cause either constipation or diarrhea, depending on which of its three subtypes is present:
- Hydrogen-dominant SIBO, which can cause diarrhea due to hydrogen-producing bacteria speeding up digestion
- Methane-dominant SIBO, which can cause constipation due to methane-producing microbes (or methanogen overgrowth) slowing the movement of food through the gut
- Hydrogen sulfide SIBO, in which microbes break down sulfur to produce hydrogen sulfide gas, leading to a mix of both diarrhea and constipation
What is IBS?
IBS is a functional gastrointestinal disorder that affects how the digestive system works, changing your bowel movements. These effects on bowel habits dictate how the disorder is classified per subtype:
- IBS-C, in which constipation is predominant
- IBS-D, in which diarrhea is predominant
- IBS-M, in which bowel habits are mixed
- IBS-U, or unclassifiable, when symptoms don’t fit neatly into the first three
Unlike SIBO, IBS doesn’t involve an overgrowth of bacteria. The exact cause of IBS remains unknown, but it’s thought to result from changes in the way the brain and gut communicate.
Experts believe it develops from a combination of different factors, including altered gut motility, visceral hypersensitivity (a more sensitive gut), stress, genetics, changes in the gut microbiome, and sometimes an infection that triggers ongoing symptoms.
Overlapping symptoms of SIBO and IBS
Many people mistake SIBO and IBS for each other because they produce similar symptoms, including:
- Bloating
- Abdominal pain or cramping
- Excess gas and flatulence
- Nausea
- Fatigue
- Changes in bowel habits, including diarrhea, constipation, or an alternating mix of both
- Reduced quality of life due to persistent digestive symptoms
Given this overlap, it’s often not possible to diagnose either condition based on symptoms alone.
Symptoms linked to SIBO
How symptoms manifest tends to be highly individualized. Still, the following may be more suggestive of SIBO than IBS:
- Symptoms that develop after intestinal surgery or conditions that slow gut movement, like gastroparesis
- Unexplained weight loss
- Nutrient deficiencies, such as low vitamin B12 or iron
- Signs of malabsorption, including greasy or foul-smelling stools
- Belching and significant bloating or abdominal distension after eating
Symptoms linked to IBS
On the other hand, these signs are more characteristic of IBS than SIBO:
- Abdominal pain that improves or changes after a bowel movement
- Symptoms that flare during periods of stress
- A feeling of incomplete bowel movements
- Mucus in the stool
- Symptoms that come and go over months or years
Unlike SIBO, IBS does not typically cause vitamin deficiencies or unintended weight loss. If these symptoms are present when your prior diagnosis has been IBS, it’s best to seek further medical evaluation to rule out other possible conditions.
How are SIBO and IBS diagnosed?
No single test can effectively diagnose every digestive disorder, hence why a thorough medical evaluation is so important. Because SIBO and IBS share many symptoms, healthcare providers take a close look at these symptoms, along with your medical history and test results, before making a diagnosis.
How to diagnose SIBO
SIBO is most commonly evaluated using a hydrogen and methane breath test. After drinking a sugar solution, you breathe into a collection device at set intervals. High levels of hydrogen or methane may suggest bacterial overgrowth or intestinal methanogen overgrowth (IMO). Your healthcare provider will also consider your symptoms, prior abdominal surgery, medications, and possible conditions that slow gut movement.
While breath tests are useful, they are not perfect and can sometimes produce false-positive or false-negative results, so findings must be interpreted alongside your clinical history.
How to diagnose IBS
Unlike SIBO, there isn’t a specific test that confirms IBS. Instead, healthcare providers diagnose IBS based on a pattern of symptoms, especially recurring abdominal pain linked to changes in bowel habits.
Another step is elimination: providers would make sure that another condition isn’t causing the symptoms. Depending on the symptoms you’re experiencing, your healthcare provider may recommend blood tests, stool tests, or other investigations to rule out conditions such as celiac disease, inflammatory bowel disease (IBD), or other gastrointestinal disorders.
Can SIBO and IBS co-occur?
SIBO and IBS don’t just share similar symptoms; in many cases, they exist together, and this connection — particularly which one condition influences the other — isn’t always the easiest to uncover.
Research suggests that around 25% to 36% of people diagnosed with IBS may also have SIBO. There are instances where SIBO contributes to IBS symptoms developing or worsening. Meanwhile, other cases find that motility issues seen in IBS allow bacterial overgrowth to develop.
That being said, the common denominator between these two conditions — and what might explain frequent co-occurrences — is an imbalance in the gut microbiome, which can bring about various health issues, including ones that may impact the gut-brain axis.
Treatment approaches for SIBO and IBS
Treating SIBO and IBS depends entirely on which underlying diagnosis is behind your symptoms, as well as any contributing health conditions you may have. It’s also worth noting that no two people will have the same experience, so treatment plans are often personalized and may even need to be changed over time as symptoms evolve.
How to treat SIBO
Common approaches to managing SIBO include:
- Taking antibiotics, such as rifaximin, to reduce bacterial overgrowth
- Treating underlying conditions that slow gut movement or contribute to bacterial overgrowth
- Making key dietary changes, which may help relieve symptoms in some people under the guidance of a healthcare professional
- Correcting vitamin or nutrient deficiencies if malabsorption has occurred
It’s important to note that while SIBO is curable, it’s possible for it to return. Preventing recurrence often means addressing the root cause, such as impaired intestinal motility or structural problems in the digestive tract.
How to treat IBS
IBS, on the other hand, cannot be cured, since it primarily involves how the gut and brain interact. Instead, treatment focuses on improving symptoms and quality of life through:
- Following a low FODMAP diet guided by a registered dietitian
- Taking medications tailored to IBS-C, IBS-D, or IBS-M, as prescribed by a healthcare provider
- Adopting healthy lifestyle habits to improve gut-brain connection, including managing stress, regularly exercising, improving sleep, and diaphragmatic breathing
- Undergoing cognitive behavioral therapy or gut-directed hypnotherapy
The best treatment varies depending on your IBS subtype, symptom severity, and individual triggers, so working closely with your healthcare provider is essential to alleviating symptoms and improving your day-to-day quality of life.
Find the right path to digestive relief with Health Loft
Both IBS and SIBO are common, but they tend to be underdiagnosed. Getting a good grasp of what these conditions are, how they differ, and where they overlap is an important first step to understanding which condition you may be experiencing and how it can best be treated.
Don’t just rely on online symptom checkers or self-diagnosis alone. A registered dietitian specializing in SIBO, IBS, or other gastrointestinal conditions can help to identify the true cause of your symptoms and recommend the most appropriate care, creating a personalized plan that allows you to maintain proper nutrition as you manage your diagnosis. When needed, they can also work closely with other healthcare providers to support your overall digestive health.
Discover expert, research-backed nutrition guidance that brings you closer to long-term relief. Get started by reaching out to a Health Loft dietitian.
This article was reviewed by Haleigh Mann, MS, RDN, LDN. It is intended for informational purposes only and does not replace personalized nutritional advice.
