The Low-FODMAP Diet Helps IBS Symptoms, but Questions Remain
Anita Slomski, JAMA. Publié en ligne le 26 février 2020
People looking for “low-FODMAP” salsa, granola, or flatbread need only search on Amazon, which offers a few hundred packaged foods promoted to relieve symptoms of irritable bowel syndrome (IBS). Shopping at the grocery store and uncertain whether it’s safe to eat apples, cashews, or spinach on the low-FODMAP diet? There’s a smartphone app that warns against apples and cashews but gives spinach an OK. For those who want information about the diet, YouTube delivers 903 000 results, while Instagram offers 1.5 million.
Gastroenterologists, however, caution against the low-FODMAP diet becoming the latest fad in healthful eating. The diet is restrictive, challenging to follow, and is intended only as a short-term intervention to identify foods that trigger IBS symptoms. And unless it’s administered by a well-versed dietitian, people who try it risk becoming malnourished or wreaking havoc on their intestinal microbiota.
The FODMAP acronym stands for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols—a group of carbohydrates that are poorly absorbed or indigestible in the small intestine and rapidly fermented in the colon.
These food components increase water volume in the small intestine and gas in the colon. For people with the underlying visceral hypersensitivity of IBS, foods high in FODMAPs—wheat, onions, garlic, legumes, dairy products, mushrooms, and cauliflower, to name just a few—can create abdominal pain, bloating, excess gas, diarrhea, and constipation.
The FODMAPs have the same impact on the gut in people with or without IBS, according to Kevin Whelan, PhD, a professor of dietetics who heads the department of nutritional sciences at King’s College, London. But those with IBS feel those effects differently. “The colons of people with IBS are more sensitive to the presence of gas,” Whelan said. “They get a heightened nerve response that healthy people don’t.”
Unsurprisingly then, experts said the low-FODMAP diet has become a first-line therapy for IBS, which affects up to 16% of the US population. “Most patients with IBS would like to avoid drug therapy if possible, and it’s a very rare patient who doesn’t want advice on what they can eat and what they should avoid to prevent symptoms,” said Yuri Saito Loftus, MD, a gastroenterologist at the Mayo Clinic in Rochester, Minnesota, and an author of the American College of Gastroenterology’s IBS management guidelines, which recommend the low-FODMAP diet for patients who want to try dietary therapy.
But as the diet’s marketing flourishes, specialists advise that it shouldn’t be taken lightly or followed haphazardly. What’s more, questions remain about its long-term effects.
Origins Down Under
The exact cause of IBS is unknown, but diet, chronic stress, genetics, inflammation, and the intestinal microbiome all appear to play a role. Altered gut bacteria in patients with IBS may drive their visceral hypersensitivity, but “we don’t yet understand whether the modified microbiome is a cause or a consequence of the disorder,” Whelan said.
Some, but not all, patients with IBS have a less diverse collection of gut microbes than people without the condition, which could make their symptoms worse. “There is probably an inverse correlation between microbiome richness and IBS severity,” said H. Christian Weber, MD, an assistant professor of medicine, pathology, and laboratory medicine at the Boston University School of Medicine. A change in diet, however, can rapidly reconfigure intestinal bacteria and potentially treat IBS.
Starting in the 1960s, studies have implicated various short-chain carbohydrates in IBS gut symptoms. Since then, physicians have advised patients with IBS to eliminate FODMAPs like lactose (a disaccharide) or fructose (a monosaccharide) from their diets in a trial-and-error attempt at symptom relief. But it wasn’t until 2004 that researchers at Monash University in Melbourne, Australia, grouped all the problematic carbohydrate subtypes together under the FODMAP umbrella.
In short order, the Monash team developed a 3-part diet that first eliminates all high-FODMAP foods for 4 to 6 weeks for adults and 2 to 3 weeks for children. In phase 2, patients slowly and systematically reintroduce individual FODMAPs into their diet over 5 to 8 weeks to determine their tolerance threshold for that food. “Maybe you can tolerate eating one piece of bread, but two pieces cause symptoms,” said Kristi King, MPH, RDN, a clinical instructor at Baylor College of Medicine in Houston.
Once patients identify their trigger foods, in phase 3 they devise a long-term maintenance diet that excludes only those that contain the offending FODMAPs. “Patients typically find that they can no longer eat about 10% of the foods they ate before going on the diet,” said King, who devises dietary interventions for adults and children with gastrointestinal tract problems.
Today, Monash University aggressively promotes the diet by certifying packaged foods and recipes as FODMAP friendly and charging a licensing fee for products to carry the Monash University Low-FODMAP Diet trademark. The school also provides low-FODMAP online training for dietitians as well as consumer education explaining why the diet may be right for them.
But Does It Work?
A 2017 review found that at least 10 randomized clinical trials had examined the low-FODMAP diet’s effectiveness during the short-term food—elimination phase, with 50% to 80% of participants reporting symptom improvement.
Some of the trials compared it with the British Dietetic Association—developed NICE (National Institute for Health and Care Excellence) IBS dietary guidelines, with differing results. In the United Kingdom, patients with IBS are first advised to try the easier-to-follow NICE diet, and then to switch to the low-FODMAP diet if their symptoms don’t resolve.
A 2016 US trial compared the low-FODMAP diet and traditional IBS advice based on the NICE diet. The commonsense advice focused on eating regular, small meals and avoiding trigger foods and excess caffeine and alcohol. Both diets adequately relieved overall symptoms among 40% to 50% of patients. But the more restrictive low-FODMAP diet markedly improved abdominal pain and bloating, “the 2 most difficult symptoms to treat in IBS,” according to the study’s lead author, Shanti Eswaran, MD, a gastroenterologist and associate professor at the University of Michigan.
A 2018 systematic review and meta-analysis examined 7 trials comparing a low-FODMAP diet with various control diets among a total of 397 participants. The low-FODMAP diet improved IBS symptoms for only a modest number of patients and the overall quality of the data was very low. Still, the researchers concluded that, of all the dietary interventions, a low-FODMAP diet “currently has the greatest evidence for efficacy in IBS.”
According to Loftus, one of the authors, the less-than-enthusiastic recommendation reflected a range of problems: small studies using different methods and comparison diets, difficulty blinding investigators and participants to the diets, and the challenges of funding dietary trials, which attract no pharmaceutical support. Although the researchers didn’t have enough data for a definitive conclusion, “Patients in these studies consistently reported symptom improvement on a low-FODMAP diet,” Loftus said.
“A Dietitian’s Assistance Is Key”
In a 2018 survey of more than 1500 US gastroenterologists, 77% said they almost always or usually recommend the low-FODMAP diet for IBS. Some gastroenterologists also recommend the diet for patients with inflammatory bowel disease (IBD) that’s in remission, according to Berkeley Limketkai, MD, PhD, director of clinical research at the UCLA Center for Inflammatory Bowel Diseases. “It’s quite common to find IBS symptoms in IBD, and the low-FODMAP diet is a reasonable choice to manage those symptoms,” he said.
But only 21% of gastroenterologists in the survey said they followed their low-FODMAP diet recommendation with a referral to a registered dietitian. Skipping that referral could be a mistake because the treatment’s effectiveness has only been studied in trials involving patients under a dietitian’s supervision.
Too much can go wrong without that guidance, experts said. “Unfortunately, there is a lot of misinformation about what the FODMAP diet entails and how to implement it, even among gastroenterologists,” Limketkai said. “A dietitian’s assistance is key for patients to precisely follow a low-FODMAP diet in its original intent.”
Some patients who adopt a do-it-yourself low-FODMAP diet stay in the elimination phase for too long. Food aversion is common among people with gastrointestinal issues, and restricting all FODMAPs—which are prebiotic and important for gut health—for months can lead to malnourishment and could cause long-term harm to the intestinal microbiome.
Even if patients carefully follow the diet’s instructions, they may not know how to get calcium, vitamin D, and fiber from low-FODMAP sources during its most restrictive phase, which can cause constipation and, in children, interfere with bone growth, King said.
Dietitians may also identify patients who shouldn’t try the approach, such as those with anorexia. All patients should be screened for disordered eating before going on a low-FODMAP diet, according to King. “If a patient is already significantly restricting food, putting even more restrictions on their diet with a low-FODMAP diet can be detrimental,” she said.
Another concern is that the complex IBS treatment could morph into a fad diet for the worried well. Some health food blogs now promote low-FODMAP recipes for the general population. “People who want to optimize their gut health may believe that the low-FODMAP diet is really great for their intestines and will help with nutrient absorption,” Whelan said. But because the diet restricts fiber and prebiotic carbohydrates, the opposite is true for those without IBS.
Long-term Effects Unclear
More research is needed to understand the diet’s long-term safety and effectiveness. Among the limited evidence for the latter is a 2017 study from Whelan’s research group involving 103 patients. Six to 18 months after completing the elimination phase, 82% of them had stayed on the adapted low-FODMAP diet and 57% had satisfactory symptom relief.
Perhaps the biggest concern is whether the diet negatively affects the microbiome long-term, especially if followed incorrectly. During the elimination phase, changes occur in the gut “that run counter to our assumption of what constitutes a healthy microbiome,” said Ashwin Ananthakrishnan, MBBS, MPH, a gastroenterologist and an associate professor of medicine at Massachusetts General Hospital in Boston. The diet reduces beneficial bifidobacteria and anti-inflammatory mediators such as butyrates, he noted, adding that it also increases microbes that could interfere with protection against bacteria–immune system interactions.
“It’s unknown how long the effects on the microbiome last after FODMAPs are reintroduced into the diet after 6 to 8 weeks,” Weber said.
One strategy that could help: taking a multistrain probiotic during the diet’s elimination phase has been shown to increase participants’ bifidobacteria and may help mitigate some of the microbiome effects.
Experts cautioned, however, that patients have to be highly motivated to follow a low-FODMAP diet, and that simpler dietary changes—like cutting out processed carbohydrates—can often help relieve symptoms. “Making alterations to our poor, traditional American diet is all some people with IBS need to feel better,” Limketkai said.
