Low-FODMAP Diet

FODMAPs include:

• fructose, a monosaccharide found in foods such as fruits and table sugar

• lactose, a disaccharide in many dairy products

• polyols, including sorbitol, mannitol, and other sugar alcohols, which occur naturally in fruits and vegetables and can also be manufactured

• fructans and oligofructans (also called fructooligosaccharides, or FOS), which are chains of mostly fructose, found in foods such as onions, garlic, and wheat

• galactans and galactooligosaccharides (GOS), which are chains of mostly galactose, found in foods such as beans

Their chemical structure makes them osmotically active, directing water into the intestines, where the sugars can be fermented by the gut microbiota. Some FODMAPs are also considered prebiotic because they provide energy to the microbiota, and some of the short-chain fatty acids (SCFA) produced by fermentation could have positive effects on intestinal and cardiometabolic health. However, these characteristics can also lead to diarrhea, constipation, bloating, or flatulence in healthy people and individuals with IBD or IBS.^[2]

Fructans happen to be both a FODMAP and a prebiotic, but this isn't the case for all FODMAPs or all fiber types.

Although the terms FODMAP, fiber, and prebiotic are often used interchangeably, they’re not exactly the same thing. Some FODMAPs are also fiber, but most fibers aren’t FODMAPs. All prebiotics are fiber, but not all fibers are prebiotic. It can be pretty confusing!

Dietary fibers are plant polysaccharides (a type of carbohydrate) that are indigestible and broadly classified as soluble or insoluble in water. Soluble fibers attract fluids in the intestines and can soften stool, while insoluble fibers create bulk. They can also be classified by their ability to ferment. Not all types of dietary fiber are very fermentable, which eliminates them from the FODMAP list. For example, cellulose—a component of the thick strings you see in celery—isn’t considered a FODMAP, because it isn’t water soluble or a preferred energy source for our gut microbes. It creates bulk but doesn’t contribute much to fluid retention or gas production in the intestines.

Prebiotics are fibers preferred by beneficial gut microbes, and fructans are a prime example. Fructans are a type of dietary fiber that happen to be both a FODMAP and a prebiotic. Most FODMAPS, however, don’t fit the definition of fiber, nor are they all considered prebiotic. The anti-prebiotic effect of a low-FODMAP diet is likely due in large part to the reduction in fructans.^[12][13]

Most foods contain a combination of FODMAPs, with the exception of meat and oil. Fruits, vegetables, grains, beans, and nuts contain variable amounts of FODMAPs, and some can be eaten during the restriction phase of the diet.

The FODMAP content of foods is measured in a laboratory setting, where foods are blended and freeze-dried before the FODMAPs are extracted and then analyzed. The FODMAP content is standardized to a 100-gram (g) serving size of the food, and each FODMAP group has a different cutoff for a low FODMAP serving.^[14] The FODMAP content has not yet been calculated on all foods, and food lists may vary based on the author’s interpretation, regional specificity of processed foods, and other factors.

Ripeness, serving size, and cooking or preparation method can influence content and the classification of a food as high FODMAP. For example, fruits that are riper tend to contain more fructose. The amount of food eaten in one sitting can also change the FODMAP content of the meal. Soaking, boiling, and canning can slightly reduce the FODMAP content of some foods, such as almonds, vegetables, and beans.

Meats, oils, and lactose-free dairy are considered low in FODMAPs. Some vegetables and fruits, such as russet potatoes, leafy greens, zucchini, and certain citrus, are also low in FODMAPs and serve as important sources of fiber during the initial phase of the diet. Rice and quinoa are low-FODMAP grains, and many gluten-free bread products can replace wheat products during the restriction phase of the diet. Like fruits and vegetables, nuts vary in FODMAP content; walnuts, sunflower seeds, and pecans are some low-FODMAP options.

Many foods contain moderate to high concentrations of FODMAPs, with serving sizes dictating the overall FODMAP load. Some of the most common include milk, honey, garlic, onions, avocados, apples, sweet potatoes, mushrooms, asparagus, cauliflower, cashews, and wheat products.

FODMAPs can also be found in a variety of protein bars, protein powders, and foods sweetened with sugar alcohols. Ingredients like whey protein concentrate (a source of lactose), chicory root extract (a type of inulin), and sorbitol (a polyol) are used to increase the food’s protein, fiber, and sweetness, respectively.

The restriction phase of a low-FODMAP diet is temporary, followed by food challenge testing and reintroduction of well-tolerated FODMAPs.

Common symptoms of IBS, such as bloating, abdominal pain, diarrhea, and excessive gas, are shared with other diseases, such as IBD, celiac disease, and colon cancer, and can also occur in healthy individuals after the consumption of FODMAPs. Before implementing a low-FODMAP diet, it is recommended that individuals visit a gastroenterologist to rule out the presence of a disease or to be officially diagnosed with IBS. The individual should then seek guidance from a dietitian or health-care provider trained in the implementation of the low-FODMAP diet and management of GI disorders.^[8]

The restriction phase of the diet is individualized based on habitual FODMAP intake, access to low-FODMAP alternatives, presence or history of eating disorders, and other medically necessary dietary restrictions already in place. During the restriction phase, which lasts 4 to 6 weeks, high-FODMAP foods, such as wheat, beans, onions, garlic, and milk, are reduced or replaced with low-FODMAP alternatives, such as rice, potatoes, zucchini, and lactose-free dairy products.

It is prudent to track dietary habits and GI symptoms, such as bloating and stool quality, during the restriction phase. If there is no change in symptoms during the restriction period, it could indicate unintentional intake of FODMAPs, another type of food intolerance, or a non-diet mechanism. If symptoms do not improve after the restriction phase, high-FODMAP foods should be reintroduced in amounts previously consumed to confirm the lack of response.

Reintroduction is the goal of a low-FODMAP diet, so after the initial restriction period, the process of systematic reintroduction of moderate- and high-FODMAP foods should begin. Individual tolerance to the FODMAP groups can be assessed through food-challenge testing. This is the process of ingesting progressively larger servings of specific foods over the course of 3 days and monitoring symptoms, with a 2- to 3-day rest period between tests.

The process of reintroduction will depend, in part, on the style of the restriction phase. Individuals may require representative test foods for entire FODMAP groups if their regular diet contained a wide variety of high-FODMAP foods, whereas a simplified process of testing specific foods would be appropriate for individuals whose diets contained just a few high-FODMAP foods. Symptom response to food-challenge tests guides reintroduction, as foods that share the same FODMAP groups can be reintroduced together once tolerance levels have been established. Improvements in symptoms have been maintained in studies following up 6 months after reintroduction of FODMAPs to tolerance levels. Individuals with IBS who experience a symptom flare after completing the low-FODMAP diet can safely return to a low-FODMAP diet short term until symptoms resolve.^[9]

Short-term low-FODMAP diets lasting just 24 hours have been implemented in endurance athletes to reduce exercise-induced gastrointestinal syndrome, or acute GI distress (commonly cramping and diarrhea) associated with intense endurance exercise.^[4][15]

Long-term adherence to a low-FODMAP diet is not recommended, because the lack of prebiotics may negatively affect the gut microbiome, plus the diet may not be nutritionally adequate. ^[8][9]

Gluten, a protein found in wheat products, is not a FODMAP, but gluten intake might be reduced during the restriction period due to temporarily reducing or eliminating wheat products, which are high in fructans.

A low-FODMAP diet reduces IBS symptom severity, and may also reduce abdominal pain and bloating associated with other GI diseases. However, it isn't always superior to traditional dietary advice for IBS.

Because people with IBS may experience altered intestinal motility and visceral hypersensitivity on a traditional diet, intestinal distention due to high luminal water content or gas production could exacerbate symptoms. ^[2] Early research revealed that temporary restriction of FODMAPs reduced GI symptoms in most individuals with IBS. The low-FODMAP diet was developed in the mid-2010s and today is considered both a first- and second-line treatment for IBS. It improved symptoms in 75% of patients within 6 weeks, according to some studies. ^[3][8]

The low-FODMAP diet has been shown to improve quality of life ^[3] as well as general IBS symptoms, including abdominal pain, bloating, diarrhea, and flatulence. ^[2] However, these results are not consistently superior to the traditional IBS diet following the modified National Institute for Health and Care Excellence (mNICE) guidelines. The low-FODMAP diet may be more useful for people with diarrhea-predominant or mixed-type IBS compared with those who have constipation-predominant IBS or other forms of constipation.^[16]

Some individuals with celiac disease continue to experience GI symptoms even on a gluten-free diet. Two randomized controlled trials (RCT) illustrated that a 21-day low-FODMAP, gluten-free diet improved subjective psychological scores (such as well-being and health perception), physical functioning, abdominal pain, and stool consistency to a greater extent than a traditional gluten-free diet alone, with no reduction in nutritional adequacy. ^[6][17]

Enteral nutrition is often associated with diarrhea, which can lead to nutrient deficiencies and delayed recovery time. One RCT using enteral formulas with low-, moderate-, or high-FODMAP content found more rapid improvement in markers of nutritional status and a greater reduction in diarrhea in the low-FODMAP group. Patients who experienced less diarrhea also exhibited elevations in BMI, which indicates beneficial weight gain, potentially as a result of better nutrient absorption. ^[5]

A small number of studies have shown that short-term low-FODMAP diets (lasting 1-7 days) could reduce exercise-induced gastrointestinal syndrome, but only when compared to a high-FODMAP group.

A non-placebo controlled observational study showed improvements in the crying duration of colicky infants who were breastfed from mothers on a low-FODMAP diet. Later, a follow-up double-blind RCT also found that, compared with a standard Australian diet, eating a low-FODMAP diet while breastfeeding led to shorter crying times in colicky infants, but only in mothers who were assigned to this intervention after the placebo. This indicates that time, rather than diet, led to the reduction in colic. It should be noted that maternal stress levels fell over time in the placebo group but remained stable during the low-FODMAP diet. ^[18]

Due to short study durations, differences in control diets, variability in the tools used to measure symptom improvements, and a lack of blinding, the evidence from RCTs in favor of low-FODMAP diets is at high risk of bias. ^[19] In most studies, the researchers knew which diet the participants were given, and in a few studies, the participants were also able to identify the diet. Though the reintroduction phase is an essential part of the process, the majority of studies haven’t included it, so the results are only indicative of the short-term restriction phase. The control diets used in many studies included a typical Australian or American diet or a diet high in FODMAPs, but some authors suggest that the typical diets would also be considered high in FODMAPs. In a few studies, the control diet was actually higher in FODMAP content than the participants’ baseline diet, and dietary adherence to the low-FODMAP diet was not assessed in many cases. These results indicate that a low-FODMAP diet is effective compared with higher-FODMAP diets, but it may not be superior to traditional dietary guidance for IBS. ^{[3][20][21][10]}

The low-FODMAP diet is not currently recommended for individuals with IBD. Although evidence suggests that it may improve quality of life in IBD, it does not appear to reduce disease severity or markers of inflammation, and long-term use could lead to nutrient deficiencies in this population already at increased risk of malnutrition. ^[10] The low-FODMAP diet may also pose a risk to people with eating pathologies, as adherence to any restrictive dietary intervention could exacerbate an eating disorder. ^[11]

Fiber consumption can be more difficult on a low-FODMAP diet, but there are still many plant foods that are low in FODMAPs and high in fiber, such as several types of berries. The tolerance of fibrous foods without FODMAPs isn’t something that has been well studied. Part of the reason may be that people can react quite differently to both FODMAPs and different fibers and foods.

Rather than simply eliminating foods, replace them with low-FODMAP options. A variety of fruits, vegetables, grains, and nuts are low in FODMAPs, making it possible to meet fiber recommendations. Aim for at least two servings of fruit, five servings of vegetables, and four servings of grains per day. Fermentable fiber supplements, such as wheat dextrin and guar gum, are high in FODMAPs, so they wouldn’t be recommended during the initial phase, but methylcellulose is an unfermentable fiber option. Fiber supplementation is not necessary, though.

Although weight may decrease — or even increase — due to a change in dietary variety, the low-FODMAP diet is not intended for weight loss or long-term health. Serving sizes are only monitored for FODMAP content, not energy intake.

People with IBS may experience visceral hypersensitivity, causing an enhanced perception of pain in response to distension of the intestines, which stretch to accommodate food and gas. However, there is no evidence that FODMAPs damage the intestinal tract, and the presence or absence of FODMAPS in the diet does not appear to influence markers of intestinal inflammation.^[10]

One RCT found that a multistrain probiotic containing bifidobacteria prevented the reduction of this bacteria (in fecal samples) normally observed during a low-FODMAP diet.^[12] However, participants on the low-FODMAP diet experienced symptom improvement regardless of probiotic supplementation.

Lactase, a digestive enzyme taken in pill form or added during manufacturing, can reduce the symptoms of lactose intolerance. Alpha-galactosidase supplementation may reduce GI distress associated with the ingestion of oligosaccharides. ^[22]

A food intolerance is the result of inadequate or absent digestive enzymes, whereas a food allergy is an immune response.

FODMAPs cause digestive symptoms because either they’re poorly absorbed by the intestinal cells (the case with fructose) or they can’t be broken down to absorbable monosaccharides due to the lack of digestive enzymes (the case with GOS, fructans, and sometimes lactose). Their presence in the intestines attracts water, which can increase the water content of stool and lead to diarrhea. Their rapid fermentation by the microbiota leads to gas production. Though these symptoms are uncomfortable, they aren’t life threatening.

A food allergy occurs when the body’s immune system mistakenly identifies certain proteins in food as a potential disease-causing agent and mounts an immune response. This can result in anaphylaxis, which is life threatening.

The term food sensitivity is sometimes used to refer to an allergy or an intolerance. Despite the popularity of IgG food sensitivity tests, an IgG response to a food does not indicate an immune response to a food and is not a valid clinical tool.^[23][24]

Lactose tolerance can fluctuate over time, and emerging evidence suggests that long-term management of IBS symptoms is possible after reintroduction of FODMAPs. ^[9] Daily fiber intake should be increased gradually, at a rate of 3–4 g per week.

Multiple studies have shown reductions in total microbial count and abundances of some bacterial species that produce the beneficial short-chain fatty acid butyrate. Changes have also been observed in fecal pH and numbers of bacteria thought to regulate the immune response, but the effects of these changes are unknown. One RCT explored the potential benefits of supplementing with a prebiotic fiber during a low-FODMAP diet, but it did not prevent the loss of bifidobacteria species or fecal butyrate. The “anti-prebiotic” effect of a low-FODMAP diet is one reason to avoid long-term adherence. ^{[25][12][26][27][28]}

The low-FODMAP diet is not studied as commonly in IBS with constipation as it is in IBS with diarrhea, but the existing evidence suggests that it would be more effective for individuals with diarrhea-predominant or mixed-type IBS. Its effects on other forms of constipation are unknown.^[29]