Modern dietary guidelines and nutrition experts agree that a healthy diet should generally include a wide variety of fruits, vegetables, whole grains, and legumes because of the essential nutrients they provide. For some people, though, certain carbohydrates found in these otherwise healthy foods can be difficult to digest and may cause a variety of unexpected digestive symptoms. This group of carbohydrates, known as FODMAPs, is a common trigger for people with IBS and the basis for what’s called the low-FODMAP diet.

What Are FODMAPs?

The acronym FODMAP is a helpful shortening of a more complex term: fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. What this group of carbohydrate molecules has in common is that they tend to be poorly absorbed in the small intestine. This means that instead of being absorbed through the wall of the small intestine, a significant portion of these carbohydrates continues into the colon. Once there, they become a food source for the bacteria that naturally live in the gut.

These bacteria, collectively known as the gut microbiome, play an important role in digestion by helping break down substances the body cannot process on its own. When they consume FODMAP carbohydrates, they do so through a process called fermentation. Fermentation is a normal part of digestion, but it also produces gas as a natural byproduct while drawing additional water into the intestine. Together, these changes increase the volume moving through the digestive tract, causing the intestinal wall to stretch.

For most people, that stretching passes as mild fullness or a bit of gas that is quickly resolved and quickly forgotten. People with IBS often experience the same physical process differently because a nervous system with heightened intestinal sensitivity can register ordinary digestive activity as pain. What is background noise to one person’s gut may manifest for others as bloating, cramping, or sudden changes in bowel habits.1

Why Do FODMAPs Affect People Differently?

Why one person develops real symptoms from a food that someone else digests without any trouble isn’t explained by any single factor. For people with IBS in particular, genetics and a history of gastrointestinal infections both appear to influence which foods end up causing trouble, though no one cause accounts for the full picture. What that variability does make clear is that each person’s set of trigger foods is genuinely their own, not something that could be predicted ahead of time from someone else’s experience.

That variability also means there is no laboratory test that can determine which FODMAP-containing foods will trigger symptoms for a particular person. Two patients with IBS may react very differently to the same meal, and even within a single category of foods, one item may be well tolerated while another consistently causes discomfort. Because these triggers are so individualized, identifying them usually requires a more systematic approach than simply avoiding foods that seem suspicious.

FODMAPs are present in dozens of individual fruits, vegetables, grains, legumes, and dairy products, and it is difficult for anyone to commit that list to memory. The research group that originally developed the low-FODMAP diet is from Monash University, a research institution in Australia. The group maintains a comprehensive, regularly updated reference list that remains one of the most widely used resources for people looking to identify problematic FODMAP foods.

How Does the Low-FODMAP Diet Work?

Because trigger foods vary so much from one person to the next, managing symptoms isn’t as simple as finding a single list of foods to avoid. Instead, the goal is to determine which specific FODMAPs are responsible for an individual’s symptoms while preserving as varied and nutritious a diet as possible. That challenge is what the low-FODMAP diet was designed to address. Rather than serving as a permanent eating plan, it is a structured process that systematically identifies problem foods before gradually reintroducing those that can be tolerated. This process is divided into three phases:

Elimination

The first several weeks are considered the elimination phase, where every major category of FODMAP-containing food gets set aside at once. This initial phase is intentionally broad because testing only specific categories won’t provide enough information. For instance, if fructans stay in the diet while lactose comes out and nothing changes, there’s no way to know whether FODMAPs were ever driving the symptoms in the first place.

Improvement during this window doesn’t point to any one specific food but rather only confirms that FODMAPs as a category are worth pursuing further. Staying in the elimination phase longer than necessary doesn’t add any clarity to the identity of the trigger foods, and research has shown that extended restriction can even negatively impact the beneficial gut bacteria the whole process is attempting to help.

Reintroduction

Once elimination confirms that FODMAPs are contributing to a patient’s symptoms, the next step is determining which specific categories are responsible. Rather than adding everything back at once, each FODMAP group is tested individually using a representative food, such as wheat for fructans or milk for lactose. The amount consumed is gradually increased over several days while symptoms are carefully monitored.

After each test, the diet returns briefly to the elimination baseline before the next FODMAP group is introduced. This helps ensure that any reaction can be attributed to the food being tested rather than to lingering effects from the previous test.

There is no required order for working through the different FODMAP groups, and not everyone reacts to the same foods. Fructans, found in foods like wheat, onions, and garlic, are among the more common triggers, but the only reliable way to identify an individual’s trigger foods is through systematic testing. The purpose of this phase is to replace the broad restrictions used during elimination with a clear understanding of which foods actually trigger symptoms and which can safely remain part of the diet.

Personalization

Once the reintroduction phase is complete, the information gathered can be used to build a long-term eating plan tailored to the individual. Foods that were well tolerated return to the diet without restriction, while foods that consistently triggered symptoms may still be limited. In many cases, tolerance depends on the amount eaten rather than the food itself. A small serving of a high-FODMAP food may cause no symptoms, while a larger portion of the same food may exceed a person’s threshold.

This personalized approach is important because many high-FODMAP foods are also valuable sources of fiber, calcium, and other nutrients. They can also support a healthy gut microbiome, which is one reason the elimination phase is intended to be temporary. Rather than continuing broad dietary restrictions indefinitely, the goal is to include as many nutritious foods as possible while avoiding only those that have been shown to trigger symptoms.

A person’s tolerance to individual foods can also change over time. Stress, illness, medications, and other factors may influence how the digestive tract responds, so foods that once caused symptoms are sometimes worth trying again under appropriate guidance.2

Ask a Gastroenterologist

The low-FODMAP diet works best when implemented as part of a broader evaluation with clinical guidance than as a self-diagnosis tool. Digestive symptoms can have more than one underlying cause, and IBS is only one possible explanation for persistent bloating, abdominal pain, or changes in bowel habits. The gastroenterology team at Cary Gastro can help determine what’s actually behind those symptoms and connect patients with the kind of dietitian-led support that makes this process reliable. If symptoms like these are affecting your daily life, contact our team to request an appointment.




1https://cdhf.ca/en/understanding-the-fodmap-diet/
2https://www.ncbi.nlm.nih.gov/books/NBK562224/