The low-FODMAP diet: how it works, who it helps, and how to start

What FODMAPs are

FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols. They are short-chain carbohydrates that the small intestine absorbs poorly. A review in the Journal of Gastroenterology and Hepatology names the groups: fructose, lactose, fructans and galactans, and polyols.

The idea is that these carbohydrates bring extra water and fermentable material into the gut, which can stretch the bowel and set off gas, bloating and pain in people with a sensitive gut. NIDDK lists some examples: apples, pears and mango, beans, cabbage, garlic and onions, and sugar alcohols such as sorbitol and xylitol.

The three phases

The diet is not one list of banned foods. It has three steps, and the middle one is the point.

• Elimination: you cut high-FODMAP foods for a short time to see if symptoms ease. The trial published in Gastroenterology used 21 days, and NIDDK says to try it for a few weeks.
• Reintroduction: if you feel better, you add FODMAP groups back one at a time, in a measured amount, and note your response. NIDDK says you may be able to eat some of these foods without symptoms.
• Maintenance, sometimes called personalization: you eat as widely as you can, avoiding only the groups that reliably cause symptoms and the amounts that do.

What the evidence says

In a randomized, controlled, single-blind cross-over trial in Gastroenterology (Halmos 2014), 30 people with IBS ate a low-FODMAP diet and a typical Australian diet for 21 days each. Overall symptom scores were lower on the low-FODMAP diet than on the typical diet, which scored 44.9 on a 100 mm scale. Bloating, pain and wind also fell. Most food was provided to the participants, which is not how people eat at home.

A 2017 review in Gut counted at least 10 randomized or randomized comparative trials in which 50% to 80% of people with IBS responded to the diet, mostly in bloating, wind, diarrhea and overall symptoms. The same review notes that the diet changes the gut microbiota, and that how long this lasts and whether it matters is unknown.

The 2021 American College of Gastroenterology guideline recommends a limited trial of a low-FODMAP diet in people with IBS to improve global symptoms. Note the word limited.

Who should see a doctor or dietitian first

IBS is diagnosed after other conditions are considered. The ACG guideline suggests blood tests for celiac disease in people with IBS and diarrhea, and a fecal calprotectin test to rule out inflammatory bowel disease. NICE tells clinicians to check for red flag signs, including signs of cancer, before settling on IBS. Do not start the diet to explain symptoms nobody has examined.

Talk to your doctor before starting if you have not been diagnosed, if your symptoms are new or changing, or if you notice anything alarming, and ask what to watch for.

A 2010 review in the Journal of Gastroenterology and Hepatology says the diet requires expert delivery by a dietitian trained in it. NIDDK also suggests talking with a dietitian. Ask for one before you begin, especially if you have other health conditions or are unsure how to read food lists.

A short test, not a way of life

The elimination phase is meant to be brief. Staying on the strict phase without reintroducing anything leaves you on the most restrictive version of the diet, and the Gut review says the long-term effect of the diet on gut bacteria is not yet known.

Success is not an empty food list. It is a list of the few foods and amounts that reliably trouble you, and the rest back on your plate.

How to log it

A diary turns guesses into patterns: what you ate, the amount, the time and any symptoms, including bowel changes. A stool chart, such as the Bristol chart, gives you a shared scale to describe stools.

The step-by-step plan, with a baseline week and the reintroduction challenges, is in our beginner guide.