Low FODMAP Diet for IBS: The 3 Phases and What Comes Next

The low FODMAP diet for IBS is a three-step eating plan from researchers at Monash University, built to help manage the symptoms of medically diagnosed irritable bowel syndrome. Step 1 swaps high FODMAP foods for low FODMAP ones for 2 to 6 weeks. Step 2 tests one FODMAP group at a time. Step 3 keeps only the restrictions you personally need. The strict part is short, and Monash says to do it with a dietitian's guidance.

It often doesn't go that neatly. You cut dairy for a month, then bread, and the bloating is still there by late afternoon. Nobody said which foods to bring back, or when.

This post covers what FODMAPs are, the FODMAP diet phases as Monash sets them out, where the diet sits in IBS care, and what to check if you've tried it and it hasn't helped. It's general information. It isn't medical advice, and it doesn't replace a visit with your own doctor.

What FODMAPs Are, in Plain Language

FODMAP is an acronym for a set of short-chain carbohydrates: Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols. Monash University sorts them into four groups and gives these examples.

Group	The FODMAPs in it	Monash's examples
Oligosaccharides	Fructans and GOS	Wheat, rye, onions, garlic, legumes and pulses
Disaccharides	Lactose	Dairy products like milk, soft cheeses and yogurts
Monosaccharides	Fructose	Honey, apples, high fructose corn syrup
Polyols	Sorbitol and mannitol	Some fruit and vegetables, and artificial sweeteners

Why would these cause trouble? Monash describes two things happening. FODMAPs move slowly through the small intestine and draw water in as they go. Then they reach the large intestine, where gut bacteria ferment them and produce gas. The extra water and gas stretch the intestinal wall.

FODMAPs Are Not Gluten

This mix-up sends a lot of people down the wrong aisle. Wheat and rye are on Monash's list because of their fructans, which are carbohydrates. Gluten is a protein in wheat, barley and rye, and it's a separate question. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) notes that some people with IBS have more symptoms after eating gluten even though they don't have celiac disease.

So a gluten-free label tells you nothing about the onion or the honey in the same product.

The Three Phases of the Low FODMAP Diet, According to Monash University

These phases belong to Monash, and Monash calls them steps. Its own page describes "a 3 step diet." What follows is a summary of how Monash lays them out.

Step 1: Low FODMAP Swaps (2 to 6 Weeks)

In Step 1 you swap high FODMAP foods for low FODMAP alternatives. Monash says to follow this step for 2 to 6 weeks. That's the whole elimination phase. It has an end date built in.

Step 2: FODMAP Reintroduction

Step 2 is the easy part to skip. You stay on the Step 1 diet and run a series of what Monash calls FODMAP challenges: a food rich in only one FODMAP group, eaten daily for 3 days, while you track symptoms. Fructose one round, lactose another, and so on through the groups.

Monash's page gives each challenge 3 days. It doesn't put a total length on the reintroduction phase, so we won't either.

Step 3: Personalization

The aim of Step 3, in Monash's description, is to relax the restrictions as far as possible and settle on a personalized FODMAP diet for the long term. Foods you tolerated in Step 2 come back. The ones that reliably set you off stay limited.

One instruction sits over all three steps. Monash says a FODMAP diet should be followed under the guidance of a dietitian with specialty skills in IBS. We'd repeat that: this is a diet to do with a clinician or dietitian in the loop, and an app plus willpower is a poor substitute.

Where the Diet Sits in IBS Care

The low FODMAP diet has made it into mainstream guidelines. The American College of Gastroenterology's 2021 clinical guideline on IBS recommends a limited trial of a low FODMAP diet to improve global symptoms.

The word that matters there is limited.

NIDDK says much the same in patient terms. A doctor may suggest trying the diet for a few weeks to see if it helps, and if symptoms improve, may recommend slowly adding FODMAP foods back. It adds that you may be able to eat some foods with FODMAPs without having IBS symptoms.

How good is the evidence? A 2022 network meta-analysis in the journal Gut pooled 13 randomized trials covering 944 patients. The low FODMAP diet ranked first among the approaches compared for improving global IBS symptoms. Monash's own figure is that IBS symptoms improve in 3 out of 4 people who follow the diet.

The Gut authors also flagged a gap. Most of the trials were run in specialist care, and most didn't study what happens during reintroduction and personalization. The strict phase is the well-studied one. The long-term phases have much less trial evidence behind them.

Before You Cut Anything: What Should Be Ruled Out First

Monash's definition carries a qualifier that's easy to read past: the diet is for medically diagnosed IBS. Bloating and loose stools have other causes, and a food diary can't tell them apart.

The ACG guideline names two checks for people with IBS and diarrhea symptoms. It suggests blood testing to rule out celiac disease, and a stool marker called fecal calprotectin to rule out inflammatory bowel disease. Those are the guideline's suggestions for your doctor or gastroenterologist, and they're worth asking about before wheat comes off the plate.

If nobody has ever examined you for these symptoms, that visit comes first. The diet can wait a few weeks.

Why the Low FODMAP Diet Is Not Working (and What to Check)

If the low FODMAP diet is not working for you, you're in a known group. By Monash's count, 1 out of 4 people with IBS find their symptoms don't improve on it.

Monash published a short guide for exactly this situation, "What to do if the low FODMAP diet doesn't work for you?" It dates from August 2015, so treat it as a starting list. Its first question is whether Step 1 was followed strictly for at least 2 and up to 6 weeks, ideally with a dietitian who knows the diet. After that, it suggests:

• Going back to your gastroenterologist for further investigation.
• Seeing a dietitian about other food triggers. Monash names natural food chemicals (salicylates, amines and glutamates), plus caffeine, alcohol and spicy foods.
• Gut-directed hypnotherapy. The ACG guideline separately suggests gut-directed psychotherapy for global IBS symptoms.
• Peppermint oil supplements, which Monash describes as a short-term option.
• A strain-specific probiotic.
• Lifestyle approaches to stress: meditation, counselling, exercise.

That list is Monash's, and each item on it is a conversation to have with whoever manages your care.

You May Be Stuck in Step 1

Some people feel better on the strict phase and stay on it. The diet wasn't designed to be used that way. Step 2 and Step 3 are where the list gets wider, and as the Gut review pointed out, they're also the phases most trials never tested. If you've been on Step 1 well past six weeks, the next move is FODMAP reintroduction with guidance.

FODMAP Stacking

Portion size is another place this goes sideways. Monash describes FODMAP stacking as what happens when several low FODMAP servings are eaten in one sitting and the FODMAPs add up. Its serving cut-offs are set conservatively, so a meal can hold more than one low FODMAP serving. Its practical tip is to leave 2 to 3 hours between meals and snacks.

When Relief Is Only Partial

Some people get halfway, with some symptoms easing and others staying. Monash's guide above is the starting list for that conversation with your doctor. Small intestinal bacterial overgrowth (SIBO) is a separate condition that also involves bloating. Our post on how SIBO is identified and treated covers it, including how it differs from IBS.

What Testing Can and Cannot Tell You About Food Triggers

We'll be blunt about this one. Monash's way of finding out which FODMAP groups you tolerate is the Step 2 challenges. The blood panel and stool testing we describe below aren't a substitute for those challenges.

Testing earns its place on a different question: what else is going on alongside the gut symptoms. That's the kind of question functional medicine is built around. Every new patient here starts with an 80+ biomarker panel, drawn in our office, or at a partner lab near you. It includes ferritin, B12, folate, vitamin D and a full CBC. When we retest, we compare against your own earlier numbers.

For some patients a provider will also order a GI360 stool panel, one of the advanced tests we use. It's ordered when a provider thinks your history calls for it.

On diet itself, our claim is a narrow one. For suspected food sensitivities, our providers guide a structured elimination and reintroduction. That's a general method, and it's a different thing from the Monash diet. For the low FODMAP diet specifically, Monash's advice to work with an IBS-trained dietitian stands.

If the bigger picture of gut symptoms is what you're after, our post on gut health in Maine is the wider read.

Frequently Asked Questions

How long should you stay on the low FODMAP diet?

Monash University sets the strict phase, Step 1, at 2 to 6 weeks. NIDDK describes trying the diet "for a few weeks." After that the plan moves to reintroduction, then to a personalized long-term diet with as few restrictions as you can manage. The elimination phase is short term by design and is best done with a clinician or dietitian.

What are the three phases of the low FODMAP diet?

Monash University describes three steps. Step 1 swaps high FODMAP foods for low FODMAP alternatives for 2 to 6 weeks. Step 2 reintroduces one FODMAP group at a time through 3-day challenges while you track symptoms. Step 3 relaxes restrictions as far as possible and sets a personalized diet for the long term.

Can a low FODMAP diet help with IBS?

For many people, yes. The American College of Gastroenterology recommends a limited trial of the diet to improve global IBS symptoms. Monash research found symptoms improved in 3 out of 4 people who followed it, and a 2022 analysis of 13 trials in Gut ranked it first among the approaches compared.

What if the low FODMAP diet is not working?

Monash reports that 1 in 4 people with IBS don't improve on the diet. Its guidance is to confirm Step 1 was followed strictly for 2 to 6 weeks, then go back to your gastroenterologist or see a dietitian about other food triggers. It also lists gut-directed hypnotherapy, peppermint oil, a strain-specific probiotic and stress management.

Is the low FODMAP diet the same as gluten-free?

No. Wheat and rye are limited on the low FODMAP diet because of fructans, a type of carbohydrate. Gluten is a protein. A gluten-free food can still be high in other FODMAPs such as fructose or lactose. NIDDK treats gluten sensitivity in IBS as its own question.

Stop Guessing at What Sets Off Your Symptoms

If you've been through the cut-this, cut-that cycle and want a wider look at what's going on, start with a Free Discovery Call. From there it's the 80+ Biomarker Test & Full Body Scan, the medical team reviews everything, and a 60-Minute Provider Consultation goes over every result. We see patients in Maine and New Hampshire.

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Med Matrix is a functional medicine clinic in South Portland, Maine, serving patients across Greater Portland and southern New Hampshire.