The Low-FODMAP Diet, Explained

When Tamara Duker Freuman learned about the low-FODMAP diet just over a decade ago, she started using it with her patients who were suffering from irritable bowel syndrome. It was “a game changer,” the registered dietitian said.

In one review of studies published in 2020, for instance, researchers estimated that around 52 to 86 percent of patients with I.B.S. who followed the diet had significant improvements in symptoms like bloating, pain and diarrhea.

But the diet — which temporarily eliminates foods that are high in certain types of carbohydrates known to cause I.B.S. symptoms — isn’t appropriate for everyone, experts say. Here’s how it works, and how to tell if it’s right for you.

What are FODMAPs?

FODMAPs are certain types of carbohydrates — called fermentable oligosaccharides, disaccharides, monosaccharides and polyols, as the acronym suggests — that often can’t be fully digested or absorbed in the small intestine and are instead fermented by microbes in the colon. This can cause gastrointestinal distress.

The list of foods containing high levels of FODMAPs is long: It includes dairy products like milk, yogurt and ice cream; fruits like apples, mangoes and watermelon; vegetables like onions, garlic and asparagus; wheat-based pastas and breads; legumes like beans and lentils; nuts like cashews and pistachios; and sweeteners like honey, high-fructose corn syrup and low-calorie sweeteners.

If you don’t have digestive issues, FODMAPs typically don’t “cause anything except a bit more flatulence,” said Dr. William Chey, a professor of gastroenterology and nutritional sciences at Michigan Medicine.

But if you have I.B.S. or certain other gastrointestinal conditions, they can cause symptoms like pain, bloating and diarrhea.

Who might — and might not — benefit from the diet?

Because FODMAPs are found in so many foods, the low-FODMAP diet is very restrictive, which is one reason experts advise caution and suggest careful planning before embarking on the diet.

Good candidates include I.B.S. patients who regularly consume high-FODMAP foods and who notice that their symptoms worsen after meals, said Dr. Lin Chang, a gastroenterologist and professor of medicine at the David Geffen School of Medicine at the University of California, Los Angeles. They should also be people who are open to trying a complex, new diet, she said.

The low-FODMAP diet may also help soothe the symptoms for those with other gastrointestinal conditions such as inflammatory bowel disease and functional dyspepsia (chronic stomach discomfort and pain), and those with an overgrowth of bacteria in the small intestine, Dr. Chey said.

The diet is not a good fit, however, for people who have symptoms of disordered eating, which can be common among those with GI conditions, Dr. Chang said.

Because the low-FODMAP diet is so restrictive, it can worsen an existing eating disorder, which may be life threatening in some cases, said Beth Rosen, a registered dietitian in New York.

How does the diet work?

The low-FODMAP diet includes three phases, and it’s best to navigate them with the help of a trained dietitian, Ms. Rosen said.

In the first phase, called the elimination phase, you’ll replace all high-FODMAP foods with low-FODMAP foods for two to six weeks (two weeks is usually long enough to see if the diet is helpful), she said. Popular foods you can eat in this phase include rice, quinoa, eggs, hard cheeses, most types of meat and fish, and many fruits and vegetables such as citrus fruits, strawberries, carrots and broccoli.

If your symptoms don’t noticeably improve during this phase, you should stop the diet; FODMAPs are probably not a problem for you, Dr. Chang said.

If you do start to feel better, it’s time for the reintroduction phase, during which you add back foods high in FODMAPs one at a time, in increasing amounts over several days, Ms. Freuman said. The goal is to identify which FODMAPs, and in what amounts, lead to symptoms.

The final phase is the personalization phase, during which you add back any high-FODMAP foods in amounts you can tolerate and create a balanced diet that is sustainable for the long term.

“It’s a learning diet,” Ms. Freuman said. It’s “about empowering you with the knowledge to really understand your triggers.”

After identifying the most bothersome foods, you can also consider taking an over-the-counter enzyme supplement — such as Lactaid, Beano or Fodzyme, depending on which types of FODMAPs are giving you trouble — to help you tolerate them, Ms. Freuman said.

If the classic low-FODMAP diet is not appropriate or realistic for you, a “gentle” FODMAP diet may be a better fit, Ms. Rosen said, especially if you’re malnourished, pregnant, have other dietary restrictions or a history of disordered eating, or if you are under 18 or an older adult.

For this version of the diet, the first phase involves eliminating a smaller group of foods that are most often problematic, such as wheat, onions, garlic, beans, milk and certain fruits; then you move on to the reintroduction and personalization phases as usual.

What are some other downsides of the diet?

A common pitfall of the diet is that people become stuck in the first phase, Ms. Freuman said. “They become so attached to the strictest form of the diet, and they’re terrified to eat anything off of the diet.”

This can affect quality of life — people are unable to enjoy eating out, for instance — or lead to malnutrition and disordered eating, she said.

A registered dietitian can help you follow the diet correctly and safely, particularly during the tricky reintroduction phase, Dr. Chey said. But there are also many evidence-based resources available to help you navigate the diet on your own, including a smartphone app from Monash University in Australia and books written by registered dietitians, he added.

Keep in mind that the low-FODMAP diet is just one approach to managing I.B.S., Ms. Freuman said. Other people may find relief by adjusting their fiber intake, for example, or eating smaller meals and avoiding fatty or spicy foods.