Low-FODMAP diet risks: who should not start it alone

What the risks are

The diet removes whole groups of foods, so the main risks come from restriction itself. A 2024 review in The Lancet Gastroenterology and Hepatology says whole-diet interventions that restrict intake can be challenging to deliver effectively and safely, and that there is concern about a role in eating disorder risk. It adds that inappropriate use or inadequate monitoring can risk poor nutrient intake.

A 2017 review in Gut reports that the diet led to a clinical response in 50% to 80% of patients with IBS in at least 10 randomised trials. The same review notes profound changes in the gut microbiota and metabolome, and says the duration and clinical relevance of those changes are not known yet. The Lancet review adds that cost is a practical risk: in one 4-week trial, 82% of patients said the low-FODMAP diet and a gluten-free diet were more expensive than their usual diet.

Who should not start it alone

The American Gastroenterological Association's 2022 practice update names the people who are poor candidates for restrictive diets: those eating few culprit foods, those at risk of malnutrition, those who are food insecure, and those with an eating disorder or an uncontrolled psychiatric disorder. It calls routine screening for disordered eating, by careful dietary history, critical, because eating disorders are common and often overlooked in gastrointestinal conditions.

The Lancet review puts the estimated prevalence of a possible current eating disorder in adults with IBS between 2% and 29%, and a past diagnosed eating disorder between 9% and 17%. In a London study of 233 people starting a low-FODMAP group programme, 23% screened as at risk for eating disorder behavior, and 57% of them adhered to the diet against 35% of the others.

Children are a separate case. An ESPGHAN position paper from 2022 found scarce evidence for the diet in children with IBS and none for other digestive complaints, and warns that a restrictive diet may impact nutritional adequacy or promote distorted eating in vulnerable children. The Lancet review says restrictive diets in older adults should generally be recommended with caution because of altered nutritional needs and the risk of malnutrition.

If you are pregnant or manage another condition with food, ask your clinician before restricting anything. That is our own advice, not a guideline statement.

Get the diagnosis before the diet

The American College of Gastroenterology guideline suggests a positive diagnostic strategy for IBS instead of diagnosis by exclusion, with celiac serology for people with IBS and diarrhea, and a fecal calprotectin test in people with suspected IBS and diarrhea, to rule out inflammatory bowel disease. It then recommends a limited trial of a low-FODMAP diet to improve global symptoms.

NIDDK lists symptoms that may point to another health problem instead of IBS: anemia, rectal bleeding, bloody or black and tarry stools, and weight loss. Do not use a food diary or a diet to explain those. Tell a doctor.

Keep it short and finish the plan

The AGA says the diet has three phases: restriction lasting no more than 4 to 6 weeks, reintroduction of FODMAP foods, and personalization. It also says any diet intervention should run for a predetermined length of time, and if there is no clinical response, it should be abandoned for another treatment. NIDDK likewise describes trying it for a few weeks and then slowly adding foods back.

Support matters. The Lancet review says that in a Canadian survey of 80 patients, fewer reached the therapeutic FODMAP target when they had no dietitian support, and that many drawbacks of whole-diet interventions can be reduced by advice from a dietitian who knows IBS.

Warning signs during the diet

The Lancet review describes eating disorder symptoms as dietary restriction, limited variety or long periods without eating, preoccupation with rules about what to eat, and considerable fear or guilt about food. Weight loss and nutrient deficiencies can follow, and some effects, such as lower bone density, may not be reversible.

If any of that sounds familiar, or if you are losing weight without trying, stop restricting and call your doctor or dietitian. If logging every bite starts to feel stressful, stop the logging too. This is our own advice.