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Understanding IBS, SIBO, IMO, and SIFO: What You Need to Know!

By: Dahlia Marin, RDN, LD, CGN

Lead Integrative Dietitian & Co-Founder of Married to Health

IBS, SIBO, IMO, SIFO, married to health, integrative nutrition, gut health, microbiome

What you will learn:

  • Irritable Bowel Syndrome (IBS)

  • Small Intestinal Bacterial Overgrowth (SIBO)

  • Intestinal Methanogenic Overgrowth (IMO)

  • Small Intestinal Fungal Overgrowth (SIFO)

  • How they connect in your body

  • How to overcome them using an integrative approach!


What is IBS?

Irritable Bowel Syndrome (IBS) is a chronic functional disorder characterized by the collection of symptoms occurring together in the digestive system with no visible signs of damage or disease to the digestive tract. It can be considered a disorder of the gut-brain axis.  It’s one of those conditions in which no other diagnoses can be given, nothing is seen to be abnormal in blood work or in scopes such as endoscopies or colonoscopies, so the changes in abdominal pain, gas, and bloating are all chalked up to being IBS. About 10-15% of the population in the United States is living with IBS, with most patients being women and under the age of 501. So, in a group of twenty friends, on average, at least two of those friends have Irritable Bowel Syndrome. 

How is IBS Diagnosed?

First, there must be recurrent abdominal pain at least once per week on average in the last 3 months. On top of that, one must also be simultaneously dealing with at least two other criteria such as bowel movements, change in stool frequency, and change in stool form or appearance. There are also 4 major forms of IBS:

  • IBS-D, which is a tendency towards diarrhea

  • IBS-C, a tendency towards constipation 

  • IBS-M, having both tendencies of diarrhea and constipation 

  • IBS-U, that will be diagnosed when an individual doesn’t fit into the first three categories

Please note that it is important to know what kind of IBS someone may have in order to properly manage the symptoms or investigate further1.

What are common IBS Symptoms?

This disease can be very individualistic. It can manifest within a person as a result of different causes and with different sets of symptoms. The major symptoms are:

  • Pain or discomfort in the lower abdominal region

  • Change in stool frequency or the form of the stool

  • Mucus passed through the rectum

  • Abdominal pain reduction specifically coinciding with a bowel movement  

There may also be the sensation of bloating, gas, or the presence of diarrhea or constipation (depending on the type of IBS). As abdominal pain is the primary symptom of IBS, it comes as no surprise that the disease greatly decreases an individual’s quality of life. That pain, along with other uncomfortable symptoms, contributes to the majority of those with IBS willing to give up 10-15 years of their life expectancy for an immediate cure of their condition, according to the American College of Gastroenterology2.

IBS and the Gut Microbiome 

Our gut microbiome plays a huge part in the gut-brain axis, in other words, the connection between our gut and brain. This connection seems to be responsible for the majority of symptoms that are present with this disease. The gut-brain axis is made up of both the human nervous system and our gut microbes. Rather than the brain sending the signals and the gut listening, the gut also appears to have its own “voice” mainly through the microbes that live in it, sending more signals up to the brain than are sent from the brain to the gut.

Essentially, when these microbes are unhappy because of poor dietary choices, excess antibiotic, medication, supplement use, genetic influences, environmental factors, or other conditions, this can cause the gut to become irritated or inflamed. This then causes the symptoms that are largely associated with IBS. This is one major theory of how IBS symptoms have come to be. However, there is still new research literature appearing every day about the relationship between the gut microbiome and the brain. 

In December of 2020, the American College of Gastroenterology (ACG) established 25 recommendations for the clinical guidelines in the management of irritable bowel syndrome. The guidelines include, testing to rule out Celiac Disease and irritable bowel syndrome, routine colonoscopies, anorectal physiology testing for pelvic floor disorders, a limited trial of low FODMAP foods, intake of soluble fiber, use of peppermint oil, and the use of gut-directed psychotherapies 1


"Feeling confused about this large topic? It's ok, we totally understand. That is why we made our FREE Gut Action Assessment!"



A potential cause of IBS is SIBO, or small intestinal bacterial overgrowth. Dysbiosis, AKA imbalance of microbes, within the gut generated by SIBO causes IBS symptoms. Therefore, it can be established that all SIBO is IBS, however, not all IBS is SIBO. SIBO occurs when the common fermenting bacteria and microbes that live normally in the large intestine move up into the small intestine where undigested and unabsorbed food is present, causing unwanted bloating and gas production.While a healthy GI tract does include gut flora of the small intestine, the vast majority of bacteria reside in the large intestine/ colon.

Up to 78% of those with IBS can attribute their condition, partly or entirely, to Small Intestinal Bacterial Overgrowth (SIBO)3. Though, a 2020 meta-analysis (highest quality of evidence) of 25 case-control studies found that the prevalence of SIBO in IBS was 31% 4. In another 2012 study, 60% of those with diagnosed IBS tested positive for SIBO via small bowel aspirate culture (sampling of fluid from the Small Intestine to culture for bacterial overgrowth)5

SIBO, IBS, IMO, SIFO, gas, bloating, risk factors for sibo

Additional Small Intestinal Fungal Overgrowth (SIFO) Risk Factors:
  • Mold exposure 

  • Nutrient deficiencies, especially those which support mitochondria and detoxification (B vitamins, copper, iron, Vitamin D, etc)

  • Liver and gallbladder dysfunction 

  • Regular steroid use 

  • Excessive and/or prolonged antibiotic use 

  • Decreased digestive secretions

  • Impaired immune system and underlying disease states

  • Altered bowel flora

IBS, SIBO, IMO, SIFO, Married to Health, Gut Health, Microbiome, Bloating, Gas, Diarrhea, Constipation

Types of Small Intestinal Overgrowth (SIBO), Intestinal Methanogenic Overgrowth (IMO), and Fungal Overgrowth (SIFO)

Different types of microbes produce different types of gas. The latest types of SIBO testing have made it easier to understand which type of bacteria or other microbe is producing the gas to focus on safe and effective treatment for the specific type of microbe that is over-abundant in the small intestine. This can help prevent inappropriate and unnecessary treatment, antibiotic resistance, and decreased time spent suffering through painful symptoms. 

The four main types of microbial overgrowth the latest science has identified are:

  • Hydrogen SIBO (H2)- Hydrogen SIBO is characterized by an overproduction of hydrogen gas. The microbes most closely associated with hydrogen SIBO are E. Coli and Klebsiella. Hydrogen SIBO is most commonly associated with diarrhea and excess gas. Per the guidelines, Hydrogen SIBO is diagnosed with a level of hydrogen gas in the breath over 20 parts per million above their baseline/ starting value within 90 minutes of breath testing. (6,7)

  • Methane IMO (CH4)- Methane is a gas produced not by bacteria, but a type of gut microbe called archaea. These ancient gut microbes/ bugs often use the hydrogen produced by other types of bacteria that overgrow in the gut. Common methane producing gut bugs that produce methane gas in the small intestine & large intestine include Methanobrevebacter Smithii (M. Smithii), Methanomassiliicoccus, and Methanosphaera. Methane IMO is most commonly associated with constipation, bloat, and stuck gas. Methane IMO is diagnosed with a level of Methane gas in the breath over 10 parts per million at any given time of breath testing. (6,7,11)

  • Hydrogen Sulfide SIBO (H2S)- Hydrogen Sulfide SIBO is a newly identified type of SIBO characterized by production of hydrogen sulfide gas in the small intestine. Those with Hydrogen Sulfide SIBO usually test for higher amounts of Desulvibrio Piger, Fusobacteriaceae Varium (F. Varium), and E. Coli in their small intestine. The microbes responsible for producing Hydrogen Sulfide SIBO are often responsible for Inflammatory Bowel Disease (IBD). Hydrogen Sulfide SIBO is commonly associated with alternating constipation and diarrhea, bloat, sulfury smelling stool and gas that can smell like rotten eggs, painful defecation, and feelings of malaise immediately before, during, or after a bowel movement.  Hydrogen Sulfide SIBO is diagnosed with a level of Hydrogen Sulfide gas in the breath over 3 parts per million at any time during breath testing. (8)

  • SIFO- Small Intestinal Fungal Overgrowth is a type of gut dysbiosis characterized by an overabundance of yeast or fungus in the small or large intestine. Common types of overgrowth include Candida Albicans,  SIFO is commonly associated with belching, indigestion, nausea, gas, bloating, and itchy rectum, oral thrush, vaginal yeast infection, vaginal itch, and white specks in stool. The gold standard to currently test for SIFO is via small bowel aspirate performed during an endoscopy or via urine Organic Acid Test (OAT) looking for a by-product of yeast overgrowth called D-Arabinitol. Because of the low accessibility to this type of testing, many providers will treat this condition empirically, or based on symptoms. (9,10,11)

What Does this All Mean and What can be Done?

The easiest way we have found to explain all of this and help our thousands of patients understand what is happening, is by explaining that inside of each us lives a thriving ecosystem that is only rivaled by the microbial ecosystem in the soil. This ecosystem makes the Amazon Rainforest look like you backyard garden.

As with all ecosystems everything is connected. As you have read your SIBO is your IBS, your IBS can be various types of overgrowth with various ways to treat it. What's important is to understand that your inner-ecosystem and these microbes are not bad. They aren't there to hurt you, but really they are there to protect you.

We challenge our patients to understand what this inner-ecosystem is trying to tell you. We help our patients find what this inner-ecosytem needs with the optimal goal being to rebalance these connected ecosystem thereby alleviated your negative symptoms.

"Still feeling a bit overwhelmed about this large topic? It's ok, that is why we made our FREE Gut Action Assessment!"

married to health, plant based nutrition, plant based dietitian, integrative dietitian, SIBO, IMO, IBS, SIFO
James Marin, RD, EN & Dahlia Marin, RDN, LD, CGN

We hope you continue to ‘Heal with Each Meal’TM

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  1. Lacy, B. E., Pimentel, M., Brenner, D. M., Chey, W. D., Keefer, L. A., Long, M. D., & Moshiree, B. (2020). ACG clinical guideline: Management of irritable bowel syndrome. American Journal of Gastroenterology, 116(1), 17–44. 

  2. American College of Gastroenterology. About Irritable Bowel Syndrome. Accessed on June 23, 2022, from

  3. Ghoshal UC, Shukla R, Ghoshal U. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy. Gut Liver. 2017;11(2):196-208. doi:10.5009/gnl16126 

  4.  Treatment options for SIBO, IMO, and excess hydrogen sulfide: Trio-SMART®. trio.,term%20isn't%20quite%20accurate. Accessed June 30, 2022. 

  5. Pyleris, E., Giamarellos-Bourboulis, E.J., Tzivras, D. et al. The Prevalence of Overgrowth by Aerobic Bacteria in the Small Intestine by Small Bowel Culture: Relationship with Irritable Bowel Syndrome. Dig Dis Sci 57, 1321–1329 (2012). 

  6. Pimentel, Mark MD, FRCP(C), FACG1; Saad, Richard J. MD, FACG2; Long, Millie D. MD, MPH, FACG (GRADE Methodologist)3; Rao, Satish S. C. MD, PhD, FRCP, FACG4. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. The American Journal of Gastroenterology: February 2020 - Volume 115 - Issue 2 - p 165-178. doi: 10.14309/ajg.0000000000000501 

  7. Rezaie, A., Buresi, M., Lembo, A., Lin, H., McCallum, R., Rao, S., Schmulson, M., Valdovinos, M., Zakko, S., & Pimentel, M. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology, 2017. 

  8. Pimentel, M., Hosseini, A., Chang, C., Mathur, R., Rashid, M., Sedighi, R., Fowler, H., Torosyan, J., Wang, J., & Rezaie, A. Exhaled Hydrogen Sulfide Is Increased in Patients With Diarrhea: Results of a Novel Collection and Breath Testing Device. AGA Abstracts, 2021.

  9. Erdogan A, Rao SS. Small intestinal fungal overgrowth. Curr Gastroenterol Rep. 2015 Apr;17(4):16. doi: 10.1007/s11894-015-0436-2.  

  10. Abdulla, Hamza M. MD1; Yu, Siegfried MD2; Rattanakovit, Kulthep3; Badger, Collier3; Rao, Satish MD4. Small Intestinal Bacterial Overgrowth (SIBO) and Fungal Overgrowth (SIFO): A Frequent and Unrecognized Complication of Colectomy: ACG Category Award: Presidential Poster: 2396. American Journal of Gastroenterology: October 2015 - Volume 110 - Issue - p S995 

  11. Shah A, Talley NJ, Holtmann G. Current and Future Approaches for Diagnosing Small Intestinal Dysbiosis in Patients With Symptoms of Functional Dyspepsia. Front Neurosci. 2022 May 6;16:830356. doi: 10.3389/fnins.2022.830356. PMID: 35600619; PMCID: PMC9121133. 

  12. Martins, N., Ferreira, I. C. F. R., Barros, L., Silva, S., & Henriques, M. (2014). Candidiasis: Predisposing Factors, Prevention, Diagnosis and Alternative Treatment. Mycopathologia, 177(5-6), 223–240. doi:10.1007/s11046-014-9749-1 

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