The short answer
Low-FODMAP works for 70-75 percent of IBS patients. For the remaining 25-30 percent, one of five things usually explains the failure: hidden FODMAPs in processed foods, methane-dominant SIBO (IMO) needing different treatment, visceral hypersensitivity that amplifies normal sensations, pelvic floor dysfunction, or pre-existing gastroparesis. Each needs a different intervention.
Reason 1: Hidden FODMAPs
The most common reason. Medication binders, "natural" flavorings, sauces, spice blends, toothpaste sweeteners, and even some protein powders contain FODMAPs that are easy to miss. Inulin, chicory root, and fructooligosaccharides (FOS) show up in surprisingly many "healthy" processed foods as prebiotic additives. Read the full ingredient list on anything you consume for a week, cross-reference against the Monash FODMAP app, and identify hidden sources. If you eliminate these and symptoms resolve within 2 weeks, this was your issue. If not, move to reason 2.
Reason 2: SIBO, particularly methane-dominant
SIBO affects 30-80 percent of IBS patients and is frequently misdiagnosed as FODMAP sensitivity because the symptoms overlap[2]. Low-FODMAP helps SIBO patients partially (less fermentable substrate means less gas) but rarely resolves it. Methane-dominant SIBO (intestinal methanogen overgrowth, IMO) is especially common in the constipation-predominant IBS subgroup. The signature: bloating within 30-90 minutes of most meals regardless of FODMAP content, often with slow stool transit. Diagnosis is a hydrogen-methane breath test through a GI clinician. Treatment is typically rifaximin, sometimes combined with neomycin for methane cases. See why bloat comes back after cutting foods for the broader SIBO-versus-food-elimination picture.
Reason 3: Visceral hypersensitivity
Visceral hypersensitivity means your nervous system amplifies normal gut sensations. The amount of gas is normal, but your brain registers it as pain or bloating. Low-FODMAP reduces gas production, so it helps modestly, but the core issue is central, not dietary. Gut-directed hypnotherapy has 65-70 percent response rates in randomized trials for this specific subtype. Low-dose tricyclic antidepressants (nortriptyline or amitriptyline at 10-25 mg) work for some by modulating nerve sensitivity, not treating depression. Cognitive behavioral therapy specifically for IBS is effective[3]. If your pattern is "I feel awful but my labs look normal and low-FODMAP barely moved the needle," this is likely your category.
Reason 4: Pelvic floor dysfunction
Often overlooked because it does not sound digestive. The pelvic floor muscles coordinate stool evacuation, and dysfunction here produces incomplete emptying, straining, and paradoxical contraction that feels like chronic bloating. It is more common in women, particularly post-childbirth. Diagnosis requires anorectal manometry or defecography, ordered by a GI or pelvic floor specialist. Treatment is pelvic floor physical therapy, which has strong evidence but is geographically under-available. If you have chronic bloating with significant constipation, a sensation of incomplete emptying, or straining, this is worth ruling out before concluding your gut just "does not respond" to dietary change.
Reason 5: Gastroparesis or motility issues
Delayed gastric emptying means food sits in the stomach longer than it should, producing early fullness, bloating, and nausea. Causes include diabetes, vagus nerve damage, connective tissue diseases, and idiopathic cases. Diagnosis is a gastric emptying study (nuclear medicine test). Treatment includes smaller meals, prokinetic medications, and sometimes dietary modification to lower-fat foods that slow emptying less. Low-FODMAP does not address gastroparesis because the issue is mechanical, not bacterial or fermentation-based. If your pattern is severe early fullness after small meals, nausea, and sometimes vomiting, gastroparesis workup is appropriate.
The order to investigate
If you have completed 4-6 weeks of strict low-FODMAP with less than 50 percent symptom improvement, the investigation order is: (1) audit for hidden FODMAPs for 2 weeks, (2) breath test for SIBO, (3) trial of gut-directed hypnotherapy or CBT if visceral hypersensitivity is suspected, (4) pelvic floor assessment if bloating pairs with straining, (5) gastric emptying study if early fullness and nausea are dominant. Work through them in order rather than jumping straight to expensive testing. Each step rules out a common cause that would have been missed by continuing the food-only approach. A food-to-feeling log during each trial (manually, or automated through tools like Aloe AI) catches partial responses that a pass/fail judgment misses.
