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Inflammatory bowel disease
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In one pass Inflammatory bowel disease (IBD) is not a food allergy.
Educational content, not medical advice. See a clinician about your own case.
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Chapter 1
Where each disease inflames the gut
Inflammatory bowel disease () is not a food allergy. It is chronic inflammation of the gut, and it comes in two main forms: Crohn's disease and ulcerative colitis. The two have different maps. The inflammation of Crohn's disease can go through the full thickness of the gut wall and can affect any stretch from the mouth to the anus; ulcerative colitis stays in the inner lining (the mucosa) of the colon.
Which stretch and which layer is inflamed decides which nutrients later run short, and it is what separates the logic of iron, B12 and vitamin D deficiency. This chapter lays out the map first; the immune-suppressing drugs are not covered here.
In Crohn's disease a stretch of bowel can narrow and block: if you get cramping belly pain that comes in waves, a swollen belly, vomiting, and can no longer pass gas or stool, seek medical care immediately.
Which stretch and which layer is inflamed decides which nutrients later run short, and it is what separates the logic of iron, B12 and vitamin D deficiency. This chapter lays out the map first; the immune-suppressing drugs are not covered here.
In Crohn's disease a stretch of bowel can narrow and block: if you get cramping belly pain that comes in waves, a swollen belly, vomiting, and can no longer pass gas or stool, seek medical care immediately.
Mechanism · Which layer each disease damages
Torres 2017, a review, describes Crohn's disease as a chronic inflammation arising from a tug-of-war among susceptibility genes, the environment and the gut microbes, with both innate and adaptive immunity out of balance. The typical picture is abdominal pain, chronic diarrhea, weight loss and fatigue, often in young people.Full thickness or lining only: the inflammation of Crohn's disease can go through the whole gut wall (transmural inflammation), which is why structural complications such as narrowing (strictures) and abnormal tunnels (fistulas) occur; it often skips, with healthy stretches of bowel between the diseased ones. Ulcerative colitis damages only the innermost lining of the colon, usually starting at the rectum and spreading upward without gaps.
Why this map decides nutrition: the last part of the small intestine (the distal ileum) absorbs vitamin B12 and also takes bile acids back into the body; when bile acids run short, fat and fat-soluble vitamins, including vitamin D, are absorbed less well. Iron is absorbed mainly at the start of the small intestine, in the duodenum and upper jejunum. Whichever stretch Crohn's disease damages can drag down the nutrients that stretch handles. Ulcerative colitis stays in the colon, so these small-intestine steps are usually unaffected, but long-term bleeding can still drain iron. For what runs short and how it is replaced, see Location decides what runs low.
Chapter 2
Location decides what runs low
Which nutrients run short depends on which stretch is inflamed. Vitamin B12 is absorbed almost only in the last part of the small intestine (the distal ileum), and it also needs intrinsic factor from the stomach; so B12 deficiency is likely when Crohn's disease has damaged this stretch or surgeons have cut it out, while ulcerative colitis without surgery usually does not pull B12 down. How iron is replaced depends on whether the disease is active: when it is stable, oral iron comes first; when it is active, or oral iron was not tolerated, intravenous iron is used.
One more point that is easy to miss: feeling better does not mean the absorbing surface has recovered, so micronutrients should be checked regularly even in remission.
One more point that is easy to miss: feeling better does not mean the absorbing surface has recovered, so micronutrients should be checked regularly even in remission.
Clinical · Four ESPEN recommendations
The 2023 guideline on nutrition in inflammatory bowel disease from the European Society for Clinical Nutrition and Metabolism (ESPEN; Bischoff) has four recommendations directly about what runs short and how to replace it:Recommendation 64 (B12): routine B12 is given in Crohn's disease when surgery has taken out more than 20 cm of the distal ileum, or when a blood test has confirmed deficiency; ulcerative colitis without surgery does not tend to cause low B12.
Recommendations 11 and 12 (iron): these are two separate recommendations and should be read apart. Recommendation 11 makes oral iron first-line for iron deficiency or mild anemia when the disease is clinically inactive and oral iron was tolerated before; recommendation 12 covers intravenous iron, for clinically active disease, previous intolerance of oral iron, hemoglobin below 100 g/L, or a need for erythropoiesis-stimulating agents.
Recommendation 9 (regular checks): micronutrients should be checked regularly, including in remission; the guideline's commentary says at least once a year. Feeling better does not mean the absorbing surface has come back.
Chapter 3
Fiber is not a blanket ban
Many people switch to a long-term low-fiber diet as soon as they are diagnosed with inflammatory bowel disease. But the 2023 ESPEN guideline (Bischoff) narrows the need to restrict fiber to one situation: Crohn's disease, with a stricture already present, and with symptoms of blockage. For ulcerative colitis, and for Crohn's disease without a stricture, the usual advice on fiber largely still applies.
The signs of blockage are cramping belly pain that comes in waves, a swollen belly, vomiting, and no longer passing gas or stool; if these appear, seek medical care immediately rather than adjusting your diet at home.
The signs of blockage are cramping belly pain that comes in waves, a swollen belly, vomiting, and no longer passing gas or stool; if these appear, seek medical care immediately rather than adjusting your diet at home.
In practice · What to eat when there is a stricture
What Bischoff recommendation 19 means in practice: for people with Crohn's disease who have a stricture and symptoms of blockage, change the texture of food (for example cooking it soft or pureeing it, and eating less of the tough, high-fiber foods that clump together), or use tube feeding that delivers nutrition past the narrowed section, rather than deleting fiber from the diet altogether.Why not go low-fiber long term without a reason? Fiber is the raw material that gut microbes ferment; eat little of it for a long time and the microbes lose their raw material, and the short-chain fatty acids they make fall too. This is not the same prescription as the three-phase low- approach described under Irritable bowel syndrome: low FODMAP is a time-limited exclusion followed by gradual reintroduction, not a long-term low-fiber diet.
How severe a stricture is and how your diet should change are not things you can judge yourself; work them out with a gastroenterologist and a dietitian.
Chapter 4
Not a diet cure
Only two things can be drawn on the map: which layer and which stretch is inflamed decides what runs short; whether to restrict fiber follows a stricture with symptoms of blockage, not the name of the disease. Both come from ESPEN's nutrition guideline. This story lays out the anatomy behind that guideline, but it does not replace a gastroenterologist, and it does not present exclusive enteral nutrition (EEN, a period of taking only a nutritional formula and no ordinary food) or any particular diet as a treatment that makes the disease go away.
References · 2
- Torres, J., Mehandru, S., Colombel, J. F., & Peyrin-Biroulet, L. (2017). Crohn's disease. The Lancet, 389(10080), 1741-1755. 10.1016/S0140-6736(16)31711-1
- Bischoff, S. C., Bager, P., Escher, J., Forbes, A., Hébuterne, X., Hvas, C. L., et al. (2023). ESPEN guideline on Clinical Nutrition in inflammatory bowel disease. Clinical Nutrition, 42(3), 352-379. 10.1016/j.clnu.2022.12.004
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