The Gut Under Evidence: SIBO, Calprotectin, and Gluten Without the Hype
Three of the best-selling labels in gut health — bacterial overgrowth, inflammation, and gluten — examined against what the tests actually measure, what they do not, and where the indexed literature separates association from causation.
Why the Gut Became a Showcase for Fads
A nonspecific symptom is not a diagnosis, and a positive test is not a cause.
Bloating, gas, abdominal pain, and changes in bowel habits are among the most common complaints in the clinic and, precisely because they are nonspecific, they have become fertile ground for seductive explanations: an 'overgrowth' to be eradicated, a 'silent inflammation' to be detected, a villain food to be banished. The problem is not the interest in gut health — it is the speed with which one jumps from association to cause and from hypothesis to prescription.
The aim of this text is more modest and more useful: to show, with high-quality indexed evidence, what three popular tools — the SIBO breath test, fecal calprotectin, and gluten testing — can actually establish. In all three cases, the central message is the same: the test informs, but rarely closes the diagnosis on its own, and interpreting it outside the clinical context produces more harm than clarity.
It is worth fixing a principle before getting into the details. A positive test in someone with symptoms proves only that the two things coexist. Establishing that one causes the other requires demonstrating a mechanism, temporality, and, ideally, that correcting the finding corrects the symptom in a reproducible way — criteria that, as we will see, are not always present where marketing presumes them.
SIBO: What the Breath Test Measures — and What It Does Not
A positive result with lactulose often reflects accelerated transit — not a bacterial colony in the small bowel.
Small intestinal bacterial overgrowth (SIBO) is defined as an increase in load or an alteration in the bacterial profile of the small intestine, associated with symptoms. The difficulty begins with measurement. The historical 'gold standard' — jejunal aspirate culture, with a classic threshold between 10³ and 10⁵ CFU/mL — is invasive, poorly reproducible, and subject to oropharyngeal contamination. The ACG guideline explicitly acknowledges that no single test is definitive, which supports the note of caution against overdiagnosis (Pimentel et al., 2020).
The hydrogen and methane breath test is the noninvasive alternative, and the North American Consensus standardized it while also exposing its weaknesses: a glucose (75 g) or lactulose (10 g) substrate, with a positive result for SIBO defined by a rise in H₂ ≥20 ppm above baseline within 90 minutes; methane ≥10 ppm at any point indicates methanogen overgrowth (IMO), not SIBO from hydrogen-producing bacteria (Rezaie et al., 2017).
The decisive limitation is low specificity, especially with lactulose: in people with accelerated orocecal transit, the substrate reaches the colon quickly and produces a hydrogen peak that mimics that of a small-bowel overgrowth. In other words, a large share of 'positives' may reflect transit speed, not bacterial density. Because the symptoms of SIBO overlap almost entirely with those of irritable bowel syndrome (IBS), the isolated finding does not authorize concluding that 'SIBO causes IBS,' nor does it justify treating every positive test with an antibiotic.
The prudent reading, therefore, treats the breath test as support for clinical reasoning in selected contexts (for example, anatomical alterations, known dysmotility, post-surgical), and not as a label to be applied to any abdominal bloating.
| Dimension | What the test does | What the test does NOT do |
|---|---|---|
| Substrate | Uses glucose (75 g) or lactulose (10 g) with serial measurement of H₂/CH₄ | Does not reproduce aspirate culture or quantify CFU/mL |
| Positivity criterion | H₂ ≥20 ppm above baseline within 90 min (North American Consensus) | Does not reliably distinguish overgrowth from accelerated transit, especially with lactulose |
| Methane | CH₄ ≥10 ppm signals methanogen overgrowth (IMO) | Does not characterize SIBO from hydrogen-producing bacteria |
| Diagnostic role | Supports the hypothesis in selected contexts | Does not close the diagnosis on its own or prove that SIBO causes IBS |
| Target | Suggests an altered small-bowel environment | Does not assess inflammation, detect celiac disease, or food sensitivity |
Rifaximin: A Real, Modest Effect — That Does Not Confirm SIBO
An antibiotic that helps in IBS-D is not retroactive proof of overgrowth.
The most common circular argument is: 'the antibiotic worked, therefore it was SIBO.' The response to treatment, however, does not validate the causal hypothesis. The TARGET 1 and TARGET 2 trials tested rifaximin 550 mg three times a day for 14 days in patients with IBS without constipation and showed adequate relief of global symptoms superior to placebo — about 41% versus 32%, a real but modest effect, with a number needed to treat in the range of 10 to 11 (Pimentel et al., 2011).
TARGET 3 delimited its use judiciously: in responders who relapse, retreatment with rifaximin is safe and effective, which characterizes an occasional, repeatable use — not continuous antibiotic therapy (Lembo et al., 2016). Nothing in these trials presupposes or confirms SIBO; the benefit occurs in a population defined by IBS symptoms, not by a breath test.
The ACG guideline on IBS management positions rifaximin and the low-FODMAP diet among the evidence-supported options, while advising against routine testing without indication — a clinical framing against diagnostic excess (Lacy et al., 2021). Extrapolating this modest and transient effect to justify prolonged or repeated courses outside the trial criteria adds risk of resistance and cost without proven gain.
Fecal Calprotectin: It Measures Inflammation, Not Cause
It separates inflammatory disease from functional disorder. It does not diagnose SIBO or food sensitivity.
Fecal calprotectin is a neutrophilic protein that quantifies inflammation of the intestinal mucosa. It is one of the few truly validated markers in this discussion: a classic meta-analysis reported sensitivity of around 93% and specificity of 96% in adults for distinguishing inflammatory bowel disease (IBD) from functional disorder, reducing unnecessary endoscopies (van Rheenen et al., 2010).
In patients with an IBS pattern, the practical value is above all exclusionary: a meta-analysis showed that calprotectin ≤40 µg/g, associated with low CRP, makes IBD highly unlikely, with a post-test probability of around 1% (Menees et al., 2015). This is why the ACG guideline recommends measuring fecal calprotectin or lactoferrin to exclude IBD in suspected IBS with diarrhea (Lacy et al., 2021).
What is often misinterpreted is the other side. Calprotectin measures neutrophilic inflammation — and nothing beyond that. It does not diagnose SIBO, does not detect food sensitivity, and does not confirm IBS (which remains a clinical diagnosis). Moreover, it is not specific to IBD: elevated values also appear with the use of non-steroidal anti-inflammatory drugs (NSAIDs), infections, and neoplasms. A high result calls for investigation of the cause of the inflammation, not the automatic adoption of a label.
| Situation | Typical interpretation | Reading limit |
|---|---|---|
| Suspected IBS-D, alarm signs absent | A low value (≈≤40–50 µg/g) makes IBD unlikely and helps avoid colonoscopy | Does not confirm IBS or exclude SIBO |
| Elevated value | Signals mucosal inflammation; requires investigation (endoscopy/colonoscopy) | Not specific to IBD (NSAIDs, infection, neoplasm raise it) |
| Uncertainty between organic and functional | A screening tool with good accuracy in adults | Does not measure food sensitivity or 'leaky gut' |
| Monitoring in established IBD | Tracks inflammatory activity over time | Does not replace endoscopic evaluation when indicated |
Gluten: Three Distinct Entities and One Diagnostic Prerequisite
Testing requires the patient eating gluten. Removing it beforehand erases the evidence.
Under the generic label of 'a problem with gluten' hide three entities that require different approaches. Celiac disease is an autoimmune enteropathy triggered by gluten in genetically predisposed individuals (HLA-DQ2/DQ8), with villous atrophy. Wheat allergy is an IgE-mediated reaction with immediate onset. Non-celiac gluten sensitivity (NCGS) is a diagnosis of exclusion, with no validated biomarker.
For celiac disease, the updated ACG guideline defines the path: initial screening with IgA anti-tissue transglutaminase (anti-tTG) accompanied by measurement of total IgA, confirmation by duodenal biopsy — and, crucially, testing with the patient on a gluten-containing diet (Rubio-Tapia et al., 2023). This is the most costly error in the field: starting a gluten-free diet before investigating normalizes serology and histology and makes the diagnosis unfeasible, leaving the person on a lifelong restriction without confirmation — or, worse, without the family screening and follow-up that celiac disease requires.
NCGS is the most contested territory. The first double-blind trial to suggest that gluten could cause symptoms in people without celiac disease came from a group that later qualified its own hypothesis (Biesiekierski et al., 2011). On blinded re-exposure, after prior reduction of FODMAPs, there was no specific dose-dependent effect of gluten and a strong nocebo response was observed — indicating that a large part of the symptoms attributed to gluten stems from wheat fructans (FODMAPs) and expectation, not from gluten itself (Biesiekierski et al., 2013).
This does not zero out the entity. A double-blind crossover trial with a low dose of gluten (4.375 g/day) found small but significant symptomatic worsening in a subgroup (Di Sabatino et al., 2015). The honest synthesis: NCGS is heterogeneous, a minority phenomenon, and can only be affirmed by a blinded, controlled challenge — never by self-report of 'improvement without gluten,' which is precisely the design most vulnerable to the nocebo effect and to confusion with FODMAPs.
| Characteristic | Celiac disease | Non-celiac gluten sensitivity | Wheat allergy |
|---|---|---|---|
| Mechanism | Autoimmune (HLA-DQ2/DQ8) | Unclear; no biomarker | IgE-mediated |
| Diagnosis | Anti-tTG IgA + total IgA; duodenal biopsy | Exclusion + controlled blinded challenge | IgE tests, clinical history |
| Test prerequisite | Patient ON a gluten-containing diet | Exclude celiac disease and allergy first | Allergy evaluation |
| Objective finding | Villous atrophy, positive serology | No validated marker | Immediate IgE-mediated reaction |
| Role of FODMAPs | Not the mechanism | Often explain the symptoms (fructans) | Not applicable |
The Anti-Pseudoscience Yardstick
What lacks validity should not guide diet, fear, or prescription.
Three practices circulate with a scientific appearance and do not survive scrutiny. First: 'leaky gut' as a diagnosis. Intestinal permeability is a real physiological phenomenon and a legitimate object of research, but it does not constitute a validated clinical diagnosis or a defined therapeutic target — using it as an explanatory stamp for varied symptoms is to go beyond what the evidence allows.
Second: IgG or IgG4 panels for 'food intolerance.' IgG antibodies against foods reflect exposure and normal immunological tolerance, not disease. They have no validity for guiding exclusion diets and, when used this way, produce broad restrictions, anxiety, and nutritional risk without demonstrated benefit.
Third: the gluten-free diet adopted without a diagnosis. Beyond erasing the possibility of confirming celiac disease, it often 'works' for the wrong reason — the concomitant reduction of FODMAPs — reinforcing a mistaken causal attribution to gluten.
The common thread is always the same. The breath test supports, but does not define, SIBO; rifaximin helps modestly without proving overgrowth; calprotectin measures inflammation, not cause; and gluten can only be incriminated after proper investigation and a blinded challenge. Rigor here is not coldness — it is the way to protect the person from lifelong labels, unnecessary restrictions, and unfounded treatments. Responsible practice begins by diagnosing before restricting, and by distinguishing, at each step, what the test really says from what we would like it to say.
Why this matters for your care
This content is educational and does not replace an individualized consultation, diagnosis, or treatment, in accordance with CFM rules. Decisions about SIBO testing, fecal calprotectin, celiac disease serology, and any dietary restriction should be made with a treating physician, considering each person's complete clinical picture.
If you want to organize your digestive symptoms before an appointment, the Functional Self-Assessment helps structure the history. To go deeper into other topics with the same rigor of indexed evidence, see the Library.
References
- Rezaie A, Buresi M, Lembo A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology. 2017. doi:10.1038/ajg.2017.46
- Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology. 2020. doi:10.14309/ajg.0000000000000501
- Pimentel M, Lembo A, Chey WD, et al. Rifaximin Therapy for Patients with Irritable Bowel Syndrome without Constipation. New England Journal of Medicine. 2011. doi:10.1056/NEJMoa1004409
- Lembo A, Pimentel M, Rao SS, et al. Repeat Treatment With Rifaximin Is Safe and Effective in Patients With Diarrhea-Predominant Irritable Bowel Syndrome. Gastroenterology. 2016. doi:10.1053/j.gastro.2016.08.003
- Lacy BE, Pimentel M, Brenner DM, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. American Journal of Gastroenterology. 2021. doi:10.14309/ajg.0000000000001036
- van Rheenen PF, Van de Vijver E, Fidler V. Faecal calprotectin for screening of patients with suspected inflammatory bowel disease: diagnostic meta-analysis. BMJ. 2010. doi:10.1136/bmj.c3369
- Menees SB, Powell C, Kurlander J, Goel A, Chey WD. A Meta-Analysis of the Utility of C-Reactive Protein, Erythrocyte Sedimentation Rate, Fecal Calprotectin, and Fecal Lactoferrin to Exclude Inflammatory Bowel Disease in Adults With IBS. American Journal of Gastroenterology. 2015. doi:10.1038/ajg.2015.6
- Rubio-Tapia A, Hill ID, Semrad C, et al. American College of Gastroenterology Guidelines Update: Diagnosis and Management of Celiac Disease. American Journal of Gastroenterology. 2023. doi:10.14309/ajg.0000000000002075
- Biesiekierski JR, Newnham ED, Irving PM, et al. Gluten Causes Gastrointestinal Symptoms in Subjects Without Celiac Disease: A Double-Blind Randomized Placebo-Controlled Trial. American Journal of Gastroenterology. 2011. doi:10.1038/ajg.2010.487
- Biesiekierski JR, Peters SL, Newnham ED, et al. No Effects of Gluten in Patients With Self-Reported Non-Celiac Gluten Sensitivity After Dietary Reduction of Fermentable, Poorly Absorbed, Short-Chain Carbohydrates. Gastroenterology. 2013. doi:10.1053/j.gastro.2013.04.051
- Di Sabatino A, Volta U, Salvatore C, et al. Small Amounts of Gluten in Subjects With Suspected Nonceliac Gluten Sensitivity: A Randomized, Double-Blind, Placebo-Controlled, Cross-Over Trial. Clinical Gastroenterology and Hepatology. 2015. doi:10.1016/j.cgh.2015.01.029
Educational and scientific content. It does not constitute diagnosis, prescription or individual clinical guidance, and does not replace a medical consultation. Management decisions must be individualized by a physician.