Run the List Podcast: SIBO Part I – Clinical Presentation

Date: November 3, 2025
Hosts: Ali Shebe, Dr. Kumar
Series: First episode in a three-part series on SIBO (Small Intestinal Bacterial Overgrowth)
Theme: Clinical presentation of SIBO—what it is, why it develops, and who is at risk.

Brief Overview

This high-yield, clinically-focused episode introduces SIBO, a condition increasingly seen in both primary care and gastroenterology settings. Dr. Kumar, a gastroenterologist at Brigham and Women's Hospital, explains the underlying mechanisms, common presentations, associated risk factors, and provides memorable clinical pearls on recognizing and reasoning through SIBO in patients.

Key Discussion Points and Insights

1. Case Introduction and Definition of SIBO

• [01:19] Clinical Case: Ms. R, a 45-year-old woman with obesity and a history of Roux-en-Y gastric bypass, presents with two months of bloating, flatulence, diarrhea, and postprandial abdominal distension.
• [01:48] Defining SIBO:
  Dr. Kumar explains SIBO as “a condition in which bacteria proliferate in the small intestine, which would otherwise be a largely sterile environment.”

  “We can contrast that with the colon, which is full of bacteria … think about how if these bacteria or other species take residence more proximally in the small intestine, how those bacteria can produce a variety of symptoms.”
  — Dr. Kumar [01:48]

2. Pathophysiology and Symptom Mechanisms

• [02:24] Three Main Mechanisms by Which SIBO Causes Symptoms:
  1. Fermentation & Gas Production:
     • Bacteria inappropriately present in the small intestine ferment nutrients, generating gas → bloating, cramping, distension, flatulence.
  2. Malabsorption & Diarrhea:
     • Bacteria lining mucosal surfaces disrupt absorption → unabsorbed substances draw water into the lumen (osmotic diarrhea).
     • Severe cases: steatorrhea from fat malabsorption (oily, hard-to-flush stool).
  3. Micronutrient/Vitamin Deficiencies:
     • Bacteria compete for and absorb B12 (leading to deficiency) and iron; paradoxically, some produce excess folate (leading to high serum folate).

  “A little pearl to share is that if you have a patient … with clinical signs or symptoms of SIBO … you may also see this hallmark sign on labs where they have B12 deficiency … and high folate levels often greater than assay because they are producing folate.”
  — Dr. Kumar [05:33]

3. Who Gets SIBO? – Key Risk Factors

• [06:18] Main Risk: Altered Motility

  • Most common is IBS; up to a third of IBS patients may also have SIBO.
  • Other motility disorders: diabetes mellitus, radiation enteritis, Crohn’s, scleroderma, amyloidosis.

  “The most common cause of altered GI motility ... is IBS … that’s largely driven by the altered GI motility that is characteristic of IBS.”
  — Dr. Kumar [06:31]

• [08:19] Anatomical Causes of Stasis:

  • Roux-en-Y gastric bypass: creates a blind pouch—stasis allows bacterial overgrowth.
  • Ileal stricture (Crohn’s): reduced migration = stasis.
  • Small bowel diverticula: bacteria collect in outpouchings.
  • Fistulas: allow colonic bacteria to enter the small bowel.
  • Loss of ileocecal valve (post-surgical): retrograde movement of colonic bacteria into small intestine.

  “With the Roux-en-Y gastric bypass, there is this formation of a blind pouch … stasis and hence an increased risk for bacteria to proliferate in that area.”
  — Dr. Kumar [08:23]

• [10:20] PPI Use/Low Gastric Acid:

  • Long-term PPIs decrease stomach acid—removes a critical defense against upstream bacterial growth.
  • Atrophic gastritis: loss of parietal cells → less acid → increased risk.

  “When patients are put on long-term PPI, the reduction in stomach acid impairs this host defense and allows bacteria to grow … in the small intestine.”
  — Dr. Kumar [10:34]

• [11:41] Other Important Categories:

  • Immunodeficiency (CVID, IgA deficiency, HIV/AIDS): greater risk for any bacterial proliferation, including in small intestine.
  • Altered bile/digestive enzymes: cirrhosis and chronic pancreatitis lead to changes in GI chemistry supporting overgrowth.

4. Clinical Pearls and Noteworthy Quotes

• If you see low B12 and high folate on labs, think SIBO, especially if symptoms fit [05:33].
• Consider motility, anatomic alterations (surgery, strictures, diverticula), low acid states, immunodeficiency, and enzyme deficiencies as predisposing factors [06:18][11:41].

5. Summary and Practical Takeaways

• [12:55] Dr. Kumar’s Three Key Takeaways:
  1. SIBO = Pathological increase of bacteria in a normally sterile small bowel.
  2. Symptoms primarily from “increased gas production and decreased mucosal absorption”: bloating, cramping, and (mostly watery/osmotic) diarrhea; sometimes steatorrhea.
  3. Major risk factors: altered motility (primarily IBS, but also other conditions), anatomic changes (strictures, surgery), and gastric hypochlorhydria (PPI use, parietal cell loss).

  “When you’re seeing patients who you believe may have SIBO … it gives you a sense of why they’re presenting with this condition at this time.”
  — Dr. Kumar [14:27]

Notable Quotes and Memorable Moments

• “There are three main pathways for bacterial overgrowth ... The first is [gas] from bacteria breaking down the food products … The second is watery diarrhea [from impaired absorption] … The third pathway ... is vitamin and mineral deficiencies.” — Dr. Kumar [02:24]
• “If you have a patient … with B12 deficiency and high folate, my index of suspicion for this being SIBO is even higher.” — Dr. Kumar [05:33]
• “There’s such a significant overlap between IBS and SIBO; recent data shows up to a third of IBS patients also had SIBO.” — Dr. Kumar [06:31]
• “With Roux-en-Y gastric bypass, there is a formation of a blind pouch … stasis and hence an increased risk for bacteria to proliferate.” — Dr. Kumar [08:23]
• “Long-term PPI … impairs [the] host defense and allows bacteria to grow more proximally in the small intestine.” — Dr. Kumar [10:34]

Timestamps for Important Segments

• [01:19] Clinical case introduction
• [01:48] Definition of SIBO
• [02:24] Symptom mechanisms (fermentation, malabsorption, deficiencies)
• [05:33] Lab pearl (low B12 and high folate)
• [06:18] Main risk factors and overlap with IBS
• [08:19] Anatomical and surgical risk factors
• [10:20] PPI use and low acid states
• [11:41] Additional categories: immunodeficiency; bile/enzyme alterations
• [12:55] Three key takeaways summarizing SIBO

Episode Summary

This episode delivers a foundational review of the clinical presentation of SIBO, providing listeners with a robust understanding of the disease mechanisms, classic symptoms, and risk profiles to watch for. By highlighting both well-known and underappreciated risk factors—and sharing practical pearls for clinical practice—Dr. Kumar sets the stage for the following episodes on SIBO diagnosis and treatment. Suitable for medical trainees and practitioners at any level aiming for high-yield and applicable internal medicine knowledge.