Is it Really SIBO? Or just faecal loading?

The Overlooked Cause of IBS Symptoms

As a dietitian working closely with clients navigating complex gut health concerns, I often see people arrive at their consultation frustrated, confused, and sometimes burnt out by the time they land on the diagnosis of Small Intestinal Bacterial Overgrowth (SIBO). But what if the true root cause isn’t SIBO at all?

In this post, I’ll share a clinical observation that’s becoming more common in my practice: faecal loading or impaction as an under-recognised cause of persistent gut symptoms — and potentially, SIBO misdiagnosis.

What Is SIBO?

Small Intestinal Bacterial Overgrowth (SIBO) occurs when bacteria normally found in the large intestine migrate into the small intestine, where they ferment food too early in the digestive process. This produces excessive gas, bloating, risks to nutrient deficiencies, and more.

Common SIBO Symptoms Include:

• 🚩 Bloating (especially after meals)

  🚩 Abdominal discomfort or cramping

  🚩 Diarrhoea, constipation, or both

  🚩 Reflux and nausea

  🚩 Burping or gas

  🚩 Fatigue and brain fog

  🚩Nutrient deficiencies (e.g., iron, B12)

Can you test for SIBO?

Hydrogen and methane breath testing using lactulose is a common method used to detect SIBO. During this test, the person consumes a sugar solution (like lactulose), and breath samples are collected every 15–20 minutes over a 2–3 hour period to detect gases produced by bacteria (primarily hydrogen and methane). A rise in hydrogen before 90 minutes or the presence of a "double peak" (the first from small intestinal overgrowth, the second from colonic fermentation) is often interpreted as a positive result [1].

However, the validity of this test is debated. Sensitivity ranges from 17% to 89%, and specificity between 44% and 100%, indicating a high potential for both false positives and false negatives [1]. For example, a rapid intestinal transit can lead to an early hydrogen rise, mimicking SIBO even in its absence [2]. Conversely, delayed gastric emptying or slow intestinal transit can lead to false negatives, as gas may not be detected in the expected window [2]. These limitations may be further amplified because lactulose itself can accelerate intestinal transit [3].

Due to these limitations, lactulose breath testing may not always reliably predict who will respond to treatment, and it should not be used in isolation to diagnose SIBO [2,4].

Bacterial overgrowth in the small intestine. Majority of bacteria should be in the large intestine.

What Is Faecal Loading?

Faecal loading refers to retained stool along the length of the colon, often in the caecum and ascending colon — leading to symptoms that mimic both IBS and SIBO. In some cases, the impacted stool creates pressure on the ileocecal valve, allowing colonic bacteria to translocate retrogradely into the small intestine.

Not only is faecal loading often the underlying cause of SIBO manifestation, it’s also likely the cause of your IBS symptoms.

Symptoms of Faecal Loading or Impaction:

• Constipation (e.g., Type 1 on the Bristol Stool Chart)

• Sensation of incomplete emptying

• Alternating constipation and loose stools

• Early satiety and post-meal fullness

• Bloating and nausea

• Sometimes reflux or abdominal discomfort

• Fatigue and low mood

• Abdominal Pain

• Brain Fog

• Histamine intolerance

  
  

I won’t deep dive here into the other complications of chronic constipation — like gut dysbiosis, intestinal permeability, and gut-brain axis dysfunction, but these are real and clinically relevant issues I address in practice.

How Does Faecal Loading Cause SIBO?

The mechanism is straightforward: due to the backlog of stool, pressure builds in the large intestine and forces bacteria and stool contents backwards through the ileocecal valve into the small intestine — where they don’t belong. This creates an environment ripe for bacterial overgrowth, leading to the classic SIBO symptom picture.

Anatomy of Large intestine. Small intestine connects to the large intestine at the bottom right of abdomen.

Clinical Insight: Don’t Skip the Basics

Before jumping to a SIBO protocol or breath test, it’s essential to rule out faecal loading and screen for iron and B12 deficiency to help confirm diagnosis. But just as importantly, a thorough diet and menstrual history is needed to ensure nutrient deficiencies aren’t simply due to:

• Inadequate Iron and B12 intake (e.g., vegetarian/vegan diet)

• Restrictive eating or Disordered eating

• Heavy menstrual bleeding

Case Study: When It Wasn't SIBO After All

A female client recently presented to me after being diagnosed with SIBO and prescribed antibiotics based on a breath test. Prior to this, a gastroenterologist had advised a low FODMAP diet, which provided little relief. Despite treatment, her symptoms persisted:

• Incomplete emptying: 10/10

• Constipation: Type 1 stools

• Diarrhoea: Occasionally (known as overflow diarrhoea

• Bloating: 6/10

• Nausea, reflux, early satiety (feeling full quickly), and post-prandial fullness

• No abdominal pain

• Fatigue, low iron levels, and a recent iron infusion

I made a referral for an abdominal x-ray and the results speak for themselves.

This is a powerful screening question I use often:

"Did your symptoms improve after your colonoscopy?" If yes, it’s often a sign that faecal impaction is the key issue, not SIBO.

Treatment Focus

1. Resolve the faecal loading

   • This may require an aggressive laxative protocol, which I tailor based on severity and individual needs.

2. Address the root cause of why faecal loading is happening:

   • Slowed gut motility

   • Suboptimal eating or hydration behaviours

   • Gut-brain axis dysfunction or autonomic imbalance

   • Restrictive eating (undernourishment)

   • Malnutrition

3. Replenish nutrient deficiencies

   • Iron, B12, magnesium, sodium, etc.

4. Rebuild a healthy gut environment

   • Gradual fibre reintroduction

   • Mindful eating strategies

   • Bowel retraining if needed

If you're wanting to get to the root cause of your gut symptoms, I’d love to help you find clarity.

Book Consultation Here

Final Thoughts

While SIBO can be real and debilitating, it’s critical we don’t overlook simple, correctable issues like faecal loading. Therefore, an abdominal x-ray is the best place to start. In many cases, addressing the constipation first resolves the symptoms entirely. No antimicrobials or restrictive diets required.

note: It is important to assess this on an individual cases as in some instances you still need to treat SIBO.

I hope that helps. Brent.

References:

1. Abu-Shanab A, Quigley EM. Diagnosis of small intestinal bacterial overgrowth: the challenges persist! Expert Rev Gastroenterol Hepatol. 2009;3(1):77–87.

2. Yu D, Cheeseman F, Vanner S. Combined oro-caecal scintigraphy and lactulose hydrogen breath testing demonstrate that breath testing detects oro-caecal transit, not small intestinal bacterial overgrowth in patients with IBS. Gut. 2011;60(3):334–340.

3. La Brooy SJ, Male PJ, Beavis AK, Batt RM, Silk DB. Assessment of the reproducibility of the lactulose H2 breath test as a measure of mouth to caecum transit time. Gut. 1983;24(10):893–896.

4. Yao CK, Rotstein DA, Pang T, Irving PM, Gibson PR. Poor reproducibility of breath hydrogen testing: Implications for its application in functional bowel disorders. United European Gastroenterol J. 2017;5(2):284–292.

Are you struggling with IBS?

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