IBS flare-ups happen because the gut in IBS is unusually sensitive and closely wired to the brain. Stress alone can change how sensitive the bowel is and how it moves,[1] and food, hormones and infections act on the same sensitive system. So a flare is often the result of several things adding up rather than one "bad" food, and some flares happen for no obvious reason at all.[2]
What foods trigger IBS?
Most people with IBS link at least some symptoms to food. In a Swedish study, 84% reported symptoms after at least one food item, most often dairy, beans and lentils, apples, flour, and fried or fatty food.[3] The best-studied food triggers are FODMAPs (fermentable oligo-, di-, monosaccharides and polyols), poorly absorbed carbohydrates found in foods such as wheat, rye, onion, garlic, beans and lentils.[4] In a blinded trial, fructose and fructans brought back IBS symptoms, and bigger doses caused more symptoms.[5] Alcohol, caffeine and spicy or fatty food are other common culprits.[2] Our guide to IBS trigger foods covers each one in detail.
Can a big meal or "FODMAP stacking" trigger a flare?
Yes. How much you eat at once matters as well as what you eat. FODMAP amounts add up within a meal, and eating several FODMAPs together causes more symptoms than one alone. This is the idea behind "FODMAP stacking." Monash University, where the low-FODMAP diet was developed, says stacking mainly applies to a single sitting, as long as meals and snacks are spaced at least 2–3 hours apart. Its low-FODMAP serving sizes are set conservatively, so most people can combine several in one meal, but some sensitive people still react when servings add up.[6] A large plate of wheat pasta with garlic bread, or grazing all afternoon, can push you past your personal threshold.
Fat matters too. A high-fat meal made the rectum progressively more sensitive in people with IBS, but not in healthy volunteers,[7] and Monash notes that fats, although low in FODMAPs, can trigger gut symptoms when eaten in excess.[4] That's why traditional IBS advice emphasizes regular meals, no very large meals, and less fat.[8]
Stacking is hard to spot from memory, because the culprit may be a combination rather than any single food. IBS Clue's trigger analysis, for example, counts FODMAPs from meals eaten in the previous four hours when it estimates which FODMAP groups are most likely to be your triggers.
What else causes IBS flare-ups besides food?
| Trigger | What the research shows |
|---|---|
| Stress | Stress affects gut sensitivity, movement and secretion through the gut-brain axis and can set off flare-ups.[1] The link may run both ways: stress can worsen symptoms, and symptoms add stress.[9] |
| Poor sleep | In women with IBS, a worse night's sleep predicted more abdominal pain, anxiety and fatigue the next day.[10] |
| Menstrual cycle | During menstruation, pain and bloating got worse, bowel movements became more frequent, and the rectum was more sensitive than at other phases of the cycle.[11] |
| Gut infections | More than 1 in 10 people develop IBS after infectious gastroenteritis, and the risk is about four times higher in the year after the infection.[12] |
| Antibiotics | In a London general-practice study, people given a course of antibiotics were more than three times as likely to report extra bowel symptoms four months later.[13] Antibiotic use during gastroenteritis also raised the risk of later IBS.[12] |
| Some painkillers | Common painkillers such as acetaminophen (paracetamol), NSAIDs and opioids are unlikely to relieve IBS pain, and some can make gut symptoms worse.[9] |
Don't stop a medicine you need. If you think one is affecting your gut, ask your doctor or pharmacist whether there's an alternative.
What triggers IBS-D and IBS-C?
The same triggers can affect any type of IBS, but some patterns stand out:
- IBS with diarrhea (IBS-D): the link between binge drinking and next-day symptoms was strongest in women with IBS-D,[14] chili caused abdominal pain and burning in people with IBS-D,[15] and sorbitol, a sweetener in sugar-free sweets and gum, is best avoided if you have diarrhea.[16] IBS that starts after a gut infection is usually the diarrhea type.[17] See what to eat with IBS-D.
- IBS with constipation (IBS-C): eating too little fiber may make constipation and abdominal pain worse,[18] but the type matters. Soluble fiber such as psyllium helps IBS, while insoluble fiber such as wheat bran can aggravate it.[19] See what to eat with IBS-C.
How do I find out what triggers my IBS?
- Keep a diary for two to four weeks. Record what and how much you eat, when symptoms start, your stool type, sleep, stress and, if relevant, your cycle. Our IBS food diary guide explains how.
- Make the first-line changes. Have regular meals, cut back on alcohol and fizzy drinks, keep tea and coffee to 3 cups a day, and choose soluble rather than insoluble fiber.[16]
- Try a structured elimination. If symptoms persist, the usual next step is a low-FODMAP diet, followed strictly for 2–6 weeks.[4] Guidelines advise doing it with a dietitian or another professional with dietary expertise.[19][16]
- Reintroduce one FODMAP group at a time. This is the step that confirms which foods, and how much of them, actually trigger your symptoms. If you're still having symptoms on a low-FODMAP diet, Monash suggests considering stacking and non-food factors such as stress.[6]
When should I see a doctor?
See a doctor if your flares become more frequent or your usual pattern changes, and promptly for warning signs such as blood in your stool, unexplained weight loss, fever, anemia or symptoms that wake you at night.[17][20] IBS itself doesn't explain these, so they need checking. See IBS red flags.