How do I know if it's coeliac and not FODMAP?

The blood test (tTG-IgA, plus total IgA to rule out IgA deficiency) is the only reliable way. It must be done while you're still eating gluten daily. If positive, expect a referral for confirmatory biopsy. Coeliac requires strict lifelong gluten-free; FODMAP sensitivity does not.

Can I just go gluten-free instead of bothering with the FODMAP test?

Possible but suboptimal. About 80 percent of 'gluten sensitivity' is actually fructan sensitivity. If you go gluten-free without testing, you can't tell which it is. You'll often find you can have sourdough wheat bread or small amounts of pasta on holiday with no issue, because the fructans were the problem, not gluten.

Will I feel worse before I feel better on low-FODMAP?

Some people do for the first 2-3 days as the gut adjusts. Most feel meaningfully better in week 1 or week 2. If you feel worse for the full 4 weeks, FODMAPs probably aren't the issue and you can stop.

Do I need a dietitian?

Strongly recommended for the reintroduction phase. Phase 1 is straightforward; phase 2 has the highest dropout rate when done alone. NHS gastro referral can include a Monash-trained dietitian; private dietitians charge £80-150 per session, usually 3-4 sessions across 3 months.

Are you actually intolerant or is it FODMAPs?

4 min readUpdated 2026-04-26By Dr Chad Okay

Most people who say they're intolerant to dairy, gluten or onions are actually reacting to FODMAPs (fermentable carbs) in those foods, not the dairy or gluten itself. The difference matters: a true intolerance means lifelong avoidance; a FODMAP issue means you can probably tolerate moderate amounts of the same foods if you find your dose. A 4-week elimination then structured reintroduction tells you which it is.

Almost everyone has at least one food they think they react to. The vast majority of those reactions aren't classic allergies and they're not even classic intolerances. They're FODMAPs, fermentable short-chain carbohydrates that pull water into the gut and ferment into gas. Sorting which is which saves years of unnecessary food avoidance.

What FODMAPs are

FODMAPs stands for Fermentable Oligo-, Di-, Mono-saccharides And Polyols. The main groups are fructans (in wheat, garlic, onion), galacto-oligosaccharides (in legumes, soy), lactose (in milk products), excess fructose (in honey, mango, apple), sorbitol (in apple, stone fruit, sugar-free gum) and mannitol (in cauliflower, mushrooms).

In a normal gut, FODMAPs cause harmless gas. In a sensitive gut, the same gas plus the water FODMAPs pull into the small intestine causes pain, bloating and altered bowel habit.

Real intolerances

Lactose intolerance: real, common, easy to test (hydrogen breath test). Symptoms within 30 minutes to 2 hours of dairy. Most people who are lactose intolerant tolerate hard cheese and live yoghurt fine.
Coeliac disease: an autoimmune reaction to gluten. Diagnosis with blood tTG-IgA test and biopsy. Strict gluten-free for life.
True wheat allergy: rare, IgE-mediated, with hives or breathing symptoms minutes after eating. Allergist diagnosis.
Histamine intolerance: contested as a diagnosis, but the symptom cluster (flushing, headache after wine and aged cheese) is real for some.
Caffeine sensitivity: real but often mistaken for general anxiety.

What 'intolerance' usually actually is

Wheat intolerance → usually fructans (a FODMAP), not gluten. Sourdough often tolerated because long fermentation breaks down fructans.
Onion/garlic intolerance → fructans again. Cooked vs raw rarely matters; dose matters.
Apple/stone fruit intolerance → sorbitol or excess fructose.
Bean intolerance → galacto-oligosaccharides. Soaking and slow cooking helps. Beano enzyme can help significantly.
Cauliflower/mushroom intolerance → mannitol.
Sugar-free gum/sweets intolerance → sorbitol or mannitol again.

How to find out which one you have

Rule out coeliac first. Blood tTG-IgA test from the GP, while still eating gluten. Don't cut gluten before testing or you'll get a false negative.
Try a 4-week strict low-FODMAP elimination. Use the Monash app for the food list. Symptoms should improve within 1-2 weeks if FODMAPs are involved.
Reintroduce ONE FODMAP group at a time, in increasing doses across 3 days, with rest days between groups.
Keep a daily symptom log. Patterns are clearer in writing than in memory.
After 6-8 weeks of structured reintroduction you'll know which groups you tolerate, which you tolerate at moderate doses, and which trigger symptoms.
Most people end up tolerating 4-5 of the 6 groups in normal portions and being careful with 1-2.

Why this matters

Long-term avoidance of foods you're not actually intolerant to costs you. It restricts diet diversity, which reduces microbiome diversity. It increases anxiety around eating. It makes social eating harder. And it usually doesn't reflect what's actually happening in your gut. Spending 10 weeks (4 elimination + 6 reintroduction) to find your real picture saves years of unnecessary restriction.

When to skip the DIY route

Severe IBS that disrupts work or sleep, see a gastroenterologist.
Suspect coeliac (anaemia, weight loss, family history), get the test before any dietary change.
Eating disorder history, restrictive diets are risky and need supervised support.
Vegetarian/vegan and worried about nutrient gaps during elimination, work with a registered dietitian.
Pregnancy or trying to conceive, wait until later or do this with dietitian supervision.

Common questions

How do I know if it's coeliac and not FODMAP?: The blood test (tTG-IgA, plus total IgA to rule out IgA deficiency) is the only reliable way. It must be done while you're still eating gluten daily. If positive, expect a referral for confirmatory biopsy. Coeliac requires strict lifelong gluten-free; FODMAP sensitivity does not.
Can I just go gluten-free instead of bothering with the FODMAP test?: Possible but suboptimal. About 80 percent of 'gluten sensitivity' is actually fructan sensitivity. If you go gluten-free without testing, you can't tell which it is. You'll often find you can have sourdough wheat bread or small amounts of pasta on holiday with no issue, because the fructans were the problem, not gluten.
Will I feel worse before I feel better on low-FODMAP?: Some people do for the first 2-3 days as the gut adjusts. Most feel meaningfully better in week 1 or week 2. If you feel worse for the full 4 weeks, FODMAPs probably aren't the issue and you can stop.
Do I need a dietitian?: Strongly recommended for the reintroduction phase. Phase 1 is straightforward; phase 2 has the highest dropout rate when done alone. NHS gastro referral can include a Monash-trained dietitian; private dietitians charge £80-150 per session, usually 3-4 sessions across 3 months.