SIBO Part I: Clinical Presentation

Summary

---

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.

---

Transcript

[A]

Welcome back to Run the List, a medical education podcast in internal medicine. As a quick disclaimer, this podcast is made for educational and informational purposes only and should not be understood as medical advice under any circumstances.

[B]

I am Ali Shebe, a current third year medical student at Harvard and a recent member of the RTL team. Today we will be talking about small intestinal bacterial overgrowth, also known as sibo, an increasingly discussed and diagnosed condition in the primary care office and gastroenterology clinic. I have with me Dr. Kumar, a gastroenterologist at Brigham and Women's Hospital and of course women of our fearless leaders of rtl.

[C]

Thank you so much Ali. It's so great to be back recording with you and especially on such a hot topic as sibo. I also want to thank you for all your outstanding work in bringing back our RTL handouts along with your co medical student Hannah Shapiro, who is also a third year student at Harvard Medical School. And if I may have the honors, it's been a little bit, so I'd love to say our token line here. Let's go ahead and run the list.

[B]

And as always, let's start with the case. Ms. R is a 45 year old female with a history of obesity status post roux en y gastric bypass who presents to primary care clinic with two months of worsening bloating, flatulence and diarrhea. She reports abdominal distension, is present upon waking up and worsens after meals. She's concerned that she might have underlying SIBO. Dr. Kumar, can you first explain what SIBO is?

[C]

Absolutely. So sibo, or small intestinal bacterial overgrowth, is a condition in which bacteria proliferate in the small intestine which would otherwise be a largely sterile environment where you do not have very much bacteria at all. So we can contrast that with the colon, which is full of bacteria, and think about how if these bacteria or other species take residence by more proximally in the small intestine, how those bacteria can produce a variety of symptoms. For patients dealing with this condition, that totally makes sense.

[B]

So how does the overgrowth of bacteria cause the symptoms our patient is experiencing?

[C]

That's an excellent question, Ali. And the way I think about this when I'm teaching or even when I'm in clinic and talking to my own patients, is that there's three main pathways for bacterial overgrowth of a small intestine to cause symptoms. So the first one is that the small intestinal bacteria that are in this area of the digestive tract where they shouldn't be end up breaking down the foods and the nutrients from a recently ingested meal. Right, because the first thing that happens after the food empties from the stomach is that it gets into the small intestine. And normally those nutrients would be broken down and completely absorbed in the small intestine. But when there's bacteria there, the bacteria actually break down these nutrients and. And in the process of breaking down these products, they release gas. And that gas leads to the classic symptoms of abdominal bloating, cramping, distension, as well as flatulence. So the first mechanism is from the bacteria breaking down the food products and releasing gas. Now, the second pathway that I think about is how these small intestinal bacteria, which again, are not in the location that they should be, they can actually take up resonance right along the mucosal surfaces of the small intestine. And as they line up on the mucosal surfaces, they can interfere with absorption. In these cases, what typically happens is a watery diarrhea ensues because the unabsorbed substances that would have otherwise been absorbed, but instead are remaining in the lumen because they're not getting absorbed, being blocked by the bacteria that are line of mucosa. Those unabsorbed substances draw water into the intestinal tract, a la osmotic diarrhea. So in even more severe cases, you don't only have an osmotic diarrhea, but you can also get steatorrhea because small intestinal bacteria actually disrupt absorption of fats. So some patients with SIBO actually complain more of steatorrhea where they see oil droplets in the stool, or they have the classic very difficult to flush stool from the toilet bowl of statorrhea. So think about what is happening at the mucosal level in terms of absorption and how that can lead to osmotic as well as statorrhea and the third pathway, and this is typically only in extreme cases, but these same small intestinal bacteria that are competing for nutrients and potentially interfering with absorption at the level of the mucosa, they can actually lead to vitamin and mineral deficiencies. So what can happen is that these bacteria can absorb B12 for themselves and take that away, essentially steal it from the host and cause B12 deficiency. This can also happen with iron deficiency as well, where the bacteria are consuming the iron that was ingested, and instead of the host being able to absorb that iron, the bacteria take it for themselves. And then inversely, an interesting phenomenon is that some of these bacteria actually produce folate, and so they can raise the levels of folate above assay when measured from the blood. So a little pearl to share is that if you have a patient who has clinical signs or symptoms of sibo, let's say they have the abdominal gas, distension and diarrhea. You may also see this hallmark sign on labs where they have B12 deficiency because the SIBO is stealing the B12 from the host. And then high folate levels, often greater than assay because they are producing folate. So I always, if the patients happen to have had recent anemia, labs with a B12 and folate, I look to look at those because it fits that pattern. Low B12, high folate. I'm thinking, okay, my index of suspicion for this being sibo is even higher.

[B]

Awesome. That's a great pearl. So now that we understand the pathophysiology explaining why the constellation of symptoms of sibo occurs, who actually gets sibo, and are there any risk factors for developing this disease?

[C]

Yes, for sure. So the overarching theme, as with any proliferation of bacteria in any part of the body, is altered motility. So the most common cause of altered GI motility, as we all know, is ibs, or irritable bowel syndrome. And that's why there's such a significant overlap between IBS and sibo. Actually, recent data and literature shows that up to a third of patients who have IBS also had sibo based on data obtained from those patients. So there's a very high overlap between IBS and sibo, and that's largely driven by the altered GI motility that is characteristic of ibs. There are also other diseases that affect gut motility that we should also consider in patients who are at risk for sibo. And that includes conditions such as diabetes mellitus, radiation, enteritis, Crohn's enteritis, scleroderma, and amyloidosis. So that's a bigger box and some are more common than others. But it's always important to just consider what is the underlying motility of my patient's digestive tract and does that if.

[D]

You'Ve tuned in before. You know we love sharing clinical pearls, practical tips and tricks for real world care you often won't find in your textbooks. While we're at it, we should mention one of our favorite hacks that completely changed how we show up for work. Figs. Figs make scrubs designed for performance, lightweight, breathable, with four way stretch and smart, functional details like moisture wicking fabric and pockets where you actually need them. The fit is also a huge upgrade from the boxy scrubs we started out in. Figs rethought them completely with better tailoring and tons of styles and colors to choose from. If you want to check them out, figs is offering 15% off your first purchase. Just go to Wearfigs.com and use the code FIGSRX at checkout. That's Wherefigs.com, code FIGSRX for 15% off.

[C]

Put them at risk for SIBO.

[B]

That makes sense. Peristalsis is not consistent. This provides an excellent environment for bacteria to stay in one place and replicate, right?

[C]

Exactly. And then stasis can occur not just because of altered motility, but also due to anatomical variation. And that's true for the patient that you described, Ms. M, who has a history of a Roux en y gastric bypass. And so with the Roux en y gastric bypass, there is this formation of a blind pouch within the newly altered GI tract. And within that blind pouch there is stasis and hence an increased risk for bacteria to proliferate in that area. Another example would be stasis due to an ileal stricture in Crohn's disease. We already mentioned Crohn's disease as a possible risk factor because of altered motility, but also if there is a narrowing, stool is not going to travel through that area and as readily. And so that decrease in motility and also just stasis from hanging above the stricture can lead to increased risk for sibo. Another common, actually fairly common, cause that I've found in my patients is identifying a small bowel of diverticula. We think about diverticulosis primarily in the large intestine, but it can also be present in the small intestine. And you can imagine if there's an out pouching in the small intestine that almost shields is shielded from the natural transit through the lumen. Bacteria can proliferate in that little out pouching within the small intestine. So the bacteria like to hang out there as well. So we talked mainly about stasis in this segment, but the other anatomical variants to consider are fistulas. So think about fistulizing Crohn's disease, where you're connecting the large intestine to the small intestine and how bacteria can move retrograde from the large to the small intestine via the fistula. Or also many patients who have had surgery such as a right hemicolectomy, they will lose the ileocecal valve that otherwise prevents backflow or reflux of colonic contents into the small intestine. So loss of that valve will allow bacteria to directly flow upwards from the large intestine to the small intestine and cause SIBO in those patients.

[B]

Great review of other comorbidities. We should be thinking about when SIBO is on the differential. I've also heard about there being a risk of SIBO when there is long term PPI use. What is the mechanism behind this relationship?

[C]

Right, so as you can imagine, stomach acid is one of the GI defenses against bacterial growth, that the lower ph is actually an environment in which bacteria do not grow well in. And so when patients are put on long term ppi, the reduction in stomach acid impairs this host defense and allows bacteria to grow more approximately in the small intestine. So it's important to think about SIBO in your patients who are on long term PPI and coming to you with symptoms like bloating or diarrhea. And then also it can occur outside of PPI use in patients who have extensive atrophic gastritis where they've lost or damaged many of their parietal cells from an autoimmune process. The lack of parietal cells producing acid will lead to this, again, low acid state that can put them at risk for bacterial overgrowth.

[B]

So to review, we've identified altered GI motility, altered anatomy, and low stomach acid production as causes for sibo. Are there any other categories we should be considering?

[C]

Yes, there are two more risk factors for SIBO that we should generally be aware of. The first, big bucket, is anyone with immunodeficiency. So things we think about here are patients with CVID or Common Variable Immune Deficiency, IgA deficiency and or HIV or AIDS. Just the general risk factor of being immunodeficient will of course put these patients at higher risk for any type of bacterial proliferation. And that includes that bacterial proliferation within the small intestine. The second category is includes patients who have altered bile or digestive enzyme composition. So the main disorders that we think about in this category are patients with cirrhosis as well as patients with chronic pancreatitis. Both of these patient categories, cirrhosis and chronic pancreatitis, have this issue with altered bile digestive enzyme composition, which can lead to the proliferation of bacteria in the small intestine.

[B]

Fantastic. We covered a lot today in terms of understanding why SIBO happens, why patients present with the symptoms they do, and who might be at risk. Before we go, can you summarize the most important takeaways for our listeners?

[C]

Yes, absolutely. Let's do three key takeaways here. So the first key takeaway is just understanding that sibo, or small intestinal bacterial overgrowth, is due to a pathological increase in in the amount of bacteria in the small bowel, which otherwise should be a sterile environment. And contrast that with the colon, which we discussed earlier, is full of bacteria. So the issue is when these bacteria start proliferating in the small intestine where they shouldn't be. The second main takeaway that I'd like to share is that these small intestinal bacteria can produce a variety of symptoms, but the main ones are due to increased gas production and decreased mucosal absorption. And the combination of the increased gas production and the decreased mucosal absorption can leads to the classic symptoms of bloating, abdominal discomfort and diarrhea, which is usually a watery diarrhea due to osmotic diarrhea from the osmolytes remaining in the intestine as opposed to being absorbed. But then as we talked about earlier, there are instances where patients will present more with steatorrhea due to fat malabsorption. My last key takeaway is that the major risk factor for SIBO is altered motility in the GI tract, and that's most commonly due to ibs. But also consider abnormal anatomy, whether that be strictures or fistulizing disease or small intestinal diverticula or post surgical, such as Roux en Y or loss of ileocecal valve. And then also the gastric hypochlorhydria, which basically means a low acid state which can be due to long term PPI use or excessive atrophic gastritis leading to parietal cell death. So those are the main risk factors that I want you to think about when you're seeing patients who you believe may have sibo, because it gives you a sense of why they're presenting with this condition at this time.

[B]

Thank you Dr. Kumar for all your SIBO insights. And to our audience, we hope you enjoyed the first episode of our three part series on sibo. We will be back soon to discuss SIBO diagnosis and treatment.

[C]

Thank you so much, Al, you were an awesome guest host. And thank you again for helping to resurrect our handouts. We'll definitely have one for this episode and like Ali said, we're looking forward to coming back to discuss the next two episodes on SIBO and diagnosis and then treatment.

[A]

Some content from this episode was generated with the assistance of artificial intelligence.