How the Low FODMAP Diet Works - The Elimination Phase

One of the most important concepts to understand about the Low FODMAP Diet is that it is not a long-term diet but a learning process that allows people suffering from Irritable Bowel Syndrome (IBS) to find which foods trigger their symptoms. The “problem” foods vary considerably among individuals. One may be sensitive to wheat oligosaccharides and lactose but tolerate fructose and polyols. Or, one may tolerate onions in small amounts, but, when the portions get bigger, experience symptoms.

There is a lot of detective work to do and, through a systematic process of elimination and re-challenge, ideally, under the guidance of an experienced dietitian, IBS patients can identify the FODMAP subgroups, and specific foods, they don’t tolerate. They become knowledgeable about which foods to avoid or limit, and which ones to enjoy liberally.

The ultimate goal of this journey is to liberalize the diet to include as many foods as possible while maintaining good symptom control and improving quality of life.

This process is comprised of three steps:

  • An elimination phase – all high FODMAP foods are excluded
  • A re-challenge phase – high FODMAP foods are tested systematically
  • A re-introduction phase – a personalized FODMAP diet is identified

In this article, I will focus on the first step, the Elimination Phase.

Low FODMAP Diet Elimination Phase.jpg

5 Things to Know About the Elimination Phase

1. What foods need to be eliminated?

FODMAP stands for Fermentable Oligo- Di- Mono-saccharides And Polyols. These are certain carbohydrates that are poorly absorbed or not absorbed at all in people with IBS and may lead to symptoms of bloating, excessive flatulence, abdominal pain, and either constipation or diarrhea, or a mixture of both. These carbohydrates fall into four main sub-groups:

  • Lactose: milk, yogurt, ice cream;
  • Excess Fructose: certain fruits, high-fructose corn syrup, honey, agave;
  • Polyols/sugar alcohols: certain fruits and vegetables, and some sugar-free foods (candy, gums);
  • Oligosaccharides: types of fiber in wheat, onions, garlic, beans, soy milk. 

2. The goal is to achieve adequate symptom relief

The goal of this phase is to eliminate all high FODMAP foods until symptoms have been reduced considerably, if not entirely eliminated. Research has shown that it takes 2-4 weeks for people experience adequate symptom relief.

Most people see improvement in their symptoms quickly, usually, within 2 weeks, others need more time. Once symptoms have improved, one should move on to the next phase. There is no need to stay in the elimination phase longer.

3. This phase is temporary

It is important to remember that this is only a temporary phase, not a long-term diet, as this phase is very restrictive and eliminates or limits many nutritious foods. If not well planned, a low FODMAP diet can be low in certain key nutrients, mostly calcium, iron and dietary fiber.  

If symptoms do not improve after 4 weeks, there is no reason to continue. It is recommended to go back to one’s usual diet and seek the advice of a medical professional. Other non-diet therapies may help.

4. Some symptoms are normal

Some degree of symptoms, such as gas, are a normal consequence of a varied diet.  For example, oligosaccharides are universally poorly absorbed, as humans lack the enzyme that breaks these sugar chains down into individual sugar units. Most healthy people experience some flatulence after eating foods like beans or cruciferous vegetables. Even though the elimination phase excludes foods high in these carbohydrates, they won’t be completely eliminated.

5. This phase shouldnt be more restrictive than it needs to

There is a tendency to be wary of all fiber-rich foods and eliminate them altogether. It is important to include low FODMAP sources of fiber, such as whole grains (brown rice, quinoa, millet), low FODMAP vegetables, fruits, and legumes in appropriate amounts to obtain essential nutrients (iron is of particular concern) and adequate dietary fiber.


While eliminating most dairy products from the diet (due to the elimination of lactose-rich foods), it is critical to incorporate alternative sources calcium (such as fortified almond milk, canned salmon, and kale), and some lactose-free sources of dairy, such as lactose-free milk or yogurt and some hard cheeses.