The Reintroduction Protocol, Step by Step

Reintroduction is the phase that determines what you actually eat for the rest of your life, and it is the phase most people skip. Elimination worked, symptoms are settled, and the prospect of deliberately eating something that might undo that feels perverse.

Skip it anyway and you will spend years avoiding four food groups you were never sensitive to.

Before you start: two prerequisites

Your symptoms must be stable and improved. You are measuring changes against a baseline, so you need a baseline. Challenging from an unsettled starting point produces false positives across every group.

You must keep the background diet strictly low FODMAP throughout. This is the part people get wrong. During the entire reintroduction phase — typically six to ten weeks — everything except the test food stays as it was during elimination. Otherwise a reaction could have come from anywhere.

The structure of a challenge

Each challenge runs over three days, followed by a washout.

Day 1 — small dose. Roughly a quarter to a third of a normal serving. Day 2 — moderate dose. About half to two-thirds of a normal serving. Day 3 — full dose. A normal serving, or a little more.

Eat the test dose once in the day, ideally at the same meal each time, and record symptoms that day and the following morning.

Stop immediately if you get a clear reaction. You have your answer; there is no value in pushing to day three to confirm it.

Then wash out for two to three days, eating strictly low FODMAP, before the next group. Delayed reactions are common, and without a washout you will misattribute a slow response to the following challenge.

Choosing the right test food

This is where challenges most often go wrong. Your test food must contain only the group you are testing. Challenging fructans with an apple tells you nothing, because apple carries both excess fructose and sorbitol.

Group	Test food	Day 1	Day 2	Day 3
Lactose	Regular milk	1/4 cup	1/2 cup	1 cup
Excess fructose	Honey	1 tsp	1 tbsp	2 tbsp
Sorbitol	Blackberries	2 berries	5 berries	10 berries
Mannitol	Button mushrooms	1/4 cup	1/2 cup	1 cup
Fructans (wheat)	White bread	1/2 slice	1 slice	2 slices
Fructans (onion)	Raw onion	1 tsp	1 tbsp	2 tbsp
GOS	Canned lentils	1/4 cup	1/2 cup	3/4 cup

Two notes on that table. Fructans are worth splitting into wheat and onion challenges, because a great many people tolerate wheat fructans at bread-sized portions while reacting sharply to onion — and the difference between those two results is the difference between a workable diet and a miserable one.

Polyols are worth splitting into sorbitol and mannitol as well. They behave independently, and lumping them together loses information.

Suggested order

Start with the group you most want back. Motivation matters over ten weeks, and an early clear result is a strong incentive to continue.

Failing that, this order works well because it moves from most to least commonly tolerated:

1. Lactose
2. Sorbitol
3. Mannitol
4. Excess fructose
5. GOS
6. Fructans (wheat)
7. Fructans (onion and garlic)

Allow roughly six to ten weeks in total. It cannot be compressed — the washout periods are structural, not padding.

Reading your results

Results are rarely a clean pass or fail. Four patterns are common:

No reaction at any dose. That group goes back into your diet without restriction. This is the most common outcome, and people are consistently surprised by how often it happens.

Reaction only at the full dose. You are dose-sensitive rather than intolerant. This group returns in moderate amounts — usually the most practically useful result you can get.

Reaction at the small dose. A genuine trigger. Limit it, and consider retesting in six to twelve months, because tolerance does shift.

Ambiguous. Something else was going on — a stressful week, poor sleep, illness, hormonal cycle. Wash out and repeat the challenge later rather than recording a guess.

Keep in mind that stress and sleep move IBS symptoms independently of anything you eat. A challenge run during a genuinely bad week is not worth interpreting.

After the challenges: building phase three

Add back everything you tolerated. All of it, freely. Then apply limits only where you found genuine reactions, and where you found dose sensitivity, work out roughly where your ceiling sits.

Retest your triggers periodically. Tolerance is not fixed, and people who reacted sharply to a group during a difficult year often tolerate it later.

The goal has never been the safest possible diet. It is the broadest diet your gut allows — and after a proper reintroduction, that is usually much broader than the elimination phase led you to fear.