Food and the gut
The low FODMAP diet: a step-by-step guide
The low FODMAP diet is a three-phase eating plan that reduces certain fermentable carbohydrates for a few weeks, then brings them back one group at a time to find which ones trigger your symptoms. It is the dietary approach with the most evidence for irritable bowel syndrome. It is a short-term test, not a diet for life, and it should be done with a dietitian.
In short
- FODMAPs are short-chain carbohydrates that are poorly absorbed and quickly fermented by gut bacteria.
- In a sensitive gut, the resulting gas and fluid cause bloating, pain and altered stools.
- The diet has three phases: restrict for two to six weeks, reintroduce, then personalise.
- Staying on strict restriction for months is not advised. It narrows the diet and reduces helpful gut bacteria.
What are FODMAPs?
The name stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols.
| Group | Found mainly in |
|---|---|
| Fructans | Wheat, onion, garlic |
| Galacto-oligosaccharides | Rajma, chana, chole and other pulses |
| Lactose | Milk, ice cream, soft cheese |
| Excess fructose | Honey, apple, mango, pear, watermelon |
| Polyols (sorbitol, mannitol) | Cauliflower, mushrooms, some stone fruits, sugar-free sweets and gums |
These are healthy foods for most people. FODMAPs are not harmful; they simply cause symptoms in a gut that is sensitive to stretching.
How do FODMAPs cause symptoms?
They are small molecules that draw water into the small intestine. When they reach the colon, bacteria ferment them rapidly and produce gas. In most people this passes unnoticed. In IBS, where the gut’s nerves are over-sensitive, the same stretching is felt as bloating, pain or urgency.
What does the evidence show?
In a controlled trial in which all meals were provided, people with IBS had lower symptom scores on a low FODMAP diet than on a typical diet. International guidelines now recommend a limited trial of the diet for IBS. Not everyone responds; if there is no improvement after four to six weeks of careful restriction, the diet should be stopped.
What are the three phases?
Restriction: two to six weeks
High FODMAP foods are swapped for low FODMAP alternatives. Portions matter, as several small amounts add up.
Reintroduction: six to eight weeks
One group is tested at a time, in increasing portions over three days, with a break between groups. You keep a record of symptoms.
Personalisation: long term
Foods that caused no trouble return to your diet. Those that did are eaten in the portion you tolerate. The aim is the widest diet that keeps you comfortable.
What are the Indian food swaps?
| In place of | Try |
|---|---|
| Several wheat rotis, bread, maida | Rice, poha, jowar or ragi roti, one roti with a meal |
| Onion and garlic in cooking | Hing, ginger, cumin, curry leaves, the green part of spring onion |
| Rajma, chana, chole | A small bowl of moong dal, eggs, fish, chicken, firm tofu |
| Milk | Lactose-free milk, small portions of curd or buttermilk |
| Apple, mango, watermelon | Firm banana, papaya, orange, pineapple |
| Cauliflower, mushrooms, peas | Carrot, beans, brinjal, spinach, pumpkin, capsicum |
| Honey, sugar-free sweets | A little sugar or jaggery |
| Cashews, pistachios | Peanuts, walnuts |
See a full Indian IBS diet chart.
Who should not follow this diet?
- People without a confirmed diagnosis. Warning signs need investigation first.
- People with an eating disorder, or a history of one
- People who are underweight or already eating a very limited diet
- Children, unless supervised by a specialist team
What are the drawbacks?
The restriction phase lowers the intake of fibre and of the carbohydrates that feed beneficial gut bacteria, and their numbers fall. It can reduce calcium intake if dairy is cut without a replacement. It takes planning and is difficult when eating out. These are the reasons the first phase is kept short and the later phases matter.
When should I see a doctor?
Before starting, if you have never been assessed. Promptly, at any time, for blood in the stool, weight loss, fever, anaemia or symptoms that wake you at night.
Questions patients ask me
Is the low FODMAP diet a diet for life?
No. Strict restriction lasts two to six weeks. The long-term diet includes every food you tolerate.
Can I follow it as a vegetarian?
Yes, with planning. Protein comes from firm tofu, paneer in moderate amounts, small portions of moong dal, curd, peanuts and seeds. A dietitian’s help is particularly useful here.
Will it treat my IBS completely?
It reduces symptoms in many people. It does not change the underlying sensitivity of the gut, which is why other parts of treatment, including the gut-brain connection, still matter.
Is low FODMAP the same as gluten-free?
No. Wheat is reduced because of its fructans, not its gluten. Small amounts of wheat are often tolerated, and gluten-free foods can still be high in FODMAPs.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- Controlled trial of a low FODMAP diet in irritable bowel syndrome. Halmos EP et al. Gastroenterology. 2014. PubMed 24076059
- American College of Gastroenterology guideline on irritable bowel syndrome. Lacy BE et al. Am J Gastroenterol. 2021. doi:10.14309/ajg.0000000000001036
- British Society of Gastroenterology guideline on irritable bowel syndrome. Vasant DH et al. Gut. 2021. PubMed 33903147
- International consensus statement on the definition of prebiotics. Gibson GR et al. Nat Rev Gastroenterol Hepatol. 2017. PubMed 28611480