Elimination Diet for IBS: A Step-by-Step Guide to Finding Your Food Triggers

Elimination Diet for IBS: A Step-by-Step Guide to Finding Your Food Triggers

By Dr. Onikepe Adegbola, MD PhD — Johns Hopkins-trained physician-scientist and founder of Casa de Sante

Why an Elimination Diet Works When Nothing Else Does

IBS is not a one-size-fits-all condition. What triggers a flare in one patient may be perfectly tolerated by another. This is why general dietary advice ("eat more fiber," "avoid dairy") fails so frequently — it does not account for individual variation.

An elimination diet works by temporarily removing the most common trigger categories, observing whether symptoms improve, and then systematically testing each food group to create a personalized map of your specific triggers. It is the dietary equivalent of a diagnostic test.

Phase 1: Elimination (Weeks 1-6)

What to Remove

The low FODMAP elimination removes six categories of fermentable carbohydrates:

FODMAP Group	What It Is	Found In
Fructose	Fruit sugar (in excess of glucose)	Apples, pears, honey, mango, watermelon, HFCS
Lactose	Milk sugar	Milk, soft cheese, yogurt, ice cream
Fructans	Fructose chains	Garlic, onion, wheat, rye, inulin
GOS	Galacto-oligosaccharides	Legumes, lentils, chickpeas, cashews
Sorbitol	Sugar alcohol	Stone fruits (peach, plum, apricot), avocado, sugar-free gum
Mannitol	Sugar alcohol	Mushrooms, cauliflower, snow peas, watermelon

What You CAN Eat

Low FODMAP is not a starvation diet. Plenty of delicious foods are allowed:

• Proteins: All meat, fish, eggs, firm tofu, tempeh
• Vegetables: Carrots, zucchini, bell peppers, green beans, bok choy, spinach, lettuce, tomatoes, potatoes, eggplant
• Fruits: Strawberries, blueberries, oranges, grapes, kiwi, pineapple, banana (firm)
• Grains: Rice, oats, quinoa, corn, gluten-free bread and pasta
• Dairy alternatives: Lactose-free milk, hard cheeses (cheddar, Parmesan), almond milk
• Fats: Olive oil, butter, peanut butter, walnuts, pecans

Tools for Phase 1

• Monash University FODMAP App: The definitive resource. Traffic light system (green/yellow/red) for every food at specific serving sizes.
• Food diary: Track everything you eat and your symptoms daily. Rate bloating, pain, and stool consistency on a 0-10 scale.
• Casa de Sante Digestive Enzymes: Take with every meal during elimination to optimize digestion and reduce confounding variables.

When to Assess Results

Most patients see improvement within 2-3 weeks. If no improvement after 4 weeks of strict elimination, the diet is unlikely to be the answer — consider other causes (SIBO, functional dyspepsia, bile acid malabsorption, pelvic floor dysfunction).

If symptoms improve by 50% or more — congratulations, FODMAPs are a significant trigger. Proceed to Phase 2.

Phase 2: Reintroduction (Weeks 7-14)

This is the most important phase — and the one most patients skip or do poorly. The reintroduction phase determines WHICH specific FODMAPs are YOUR triggers, allowing you to create a personalized diet rather than staying unnecessarily restricted.

The Protocol

1. Continue the low FODMAP base diet throughout testing
2. Test ONE FODMAP group at a time
3. Each test lasts 3 days:
   • Day 1: Small dose (e.g., 1/2 cup milk for lactose)
   • Day 2: Medium dose (1 cup milk)
   • Day 3: Large dose (1.5 cups milk)
4. Monitor symptoms for 24-48 hours after Day 3
5. If symptoms return: that FODMAP group is a trigger. Note the threshold dose.
6. If no symptoms: that group is tolerated — you can include it in your diet!
7. Return to baseline low FODMAP for 3 days before testing the next group

Testing Order (Suggested)

1. Lactose (milk) — easiest to test, clear-cut results
2. Fructose (honey, mango)
3. Sorbitol (avocado, apricot)
4. Mannitol (mushrooms, cauliflower)
5. GOS (chickpeas, lentils)
6. Fructans — wheat (bread)
7. Fructans — garlic/onion (tested separately because tolerance often differs)

Phase 3: Personalization (Ongoing)

After reintroduction, you will know which FODMAP groups trigger your symptoms and at what doses. The goal of Phase 3 is to eat the BROADEST possible diet that keeps symptoms controlled:

• Freely eat FODMAP groups you passed in testing
• Avoid or limit only the groups you failed (at your threshold dose)
• Periodically re-test failed groups (6-12 months) — tolerance can improve as gut health improves
• Continue digestive enzyme support to maintain tolerance for borderline foods

Common Mistakes

1. Staying in elimination too long. The elimination phase is temporary (2-6 weeks). Staying on strict low FODMAP for months or years damages microbiome diversity by starving prebiotic-dependent bacteria.
2. Not doing reintroduction. Without reintroduction, you are unnecessarily restricted. Most patients tolerate 3-4 of the 6 FODMAP groups — that is a dramatically wider diet than full elimination.
3. Testing multiple foods at once. Reintroduce one FODMAP group at a time. Testing garlic and milk on the same day means you cannot determine which caused symptoms.
4. Not using the Monash app. Serving sizes matter enormously in FODMAP. A food can be low FODMAP at 1/4 cup and high FODMAP at 1/2 cup. The app provides precise serving size guidance.
5. Attributing all symptoms to food. Stress, poor sleep, hormonal changes, and anxiety all worsen IBS independently of diet. If symptoms persist despite dietary management, address these factors.

Frequently Asked Questions

Can I do low FODMAP while on GLP-1 medications?

Yes. Many GLP-1 patients with IBS benefit from combining low FODMAP with their medication. The reduced appetite may actually make the elimination phase easier. Ensure adequate protein and nutrient intake despite eating less overall.

Is low FODMAP safe for children?

Low FODMAP has been studied in children and adolescents with IBS with positive results. However, it should only be done under dietitian supervision to ensure adequate growth and nutrition.

Will I need to eat low FODMAP forever?

No. The goal is personalized FODMAP management — eating as broadly as possible while avoiding your specific triggers at your threshold doses. Many patients eventually tolerate moderate amounts of even their trigger FODMAPs, especially as gut health improves over time.