An explanation for repeated bouts of abdominal pain, bloating, diarrhea, or constipation may lie in the small intestine

IBS is the most frequently diagnosed gastrointestinal disorder. Patients with this vexing condition often report abdominal pain, discomfort, fullness, and bloating without an apparent explanation in the medical work-up. IBS has long been called a functional disorder, meaning it is a condition with no identifiable cause and no visible signs of damage in the digestive tract. However, the Rome Foundation recently renamed IBS as a disorder of gut-brain interaction after recognizing the interplay between central (e.g., psychological conditions such as stress and anxiety coinciding) and peripheral digestive alterations (e.g., an increased pain perception in the bowel) in most patients with IBS1.

Beyond the involvement of colonic dysbiosis in IBS, the fact that symptoms appear mainly within 1 hour of meal ingestion suggests the small intestine is also involved in its onset and development2.

The small intestine has been overlooked mainly due to technical difficulties in accessing and studying it. Its main functions are the digestion of food and the absorption of nutrients into the bloodstream. It is worth noting that about 70% of body immune cells are found in the small intestine in structures collectively called gut-associated lymphoid tissue (GALT). The small intestine also has its own microbiota but is found at a lower diversity and density compared to the colon3,4.

 

How atypical food allergies in the gut mimic the symptoms of IBS

A subset of patients with IBS often report that their symptoms associated with abdominal pain start at the time of a gastrointestinal infection, which can trigger a localized allergic reaction in the gut. This is one of the first observations that involved the gut immune system and surrounding microbial communities with IBS5.

It has been discovered that an infection in the digestive tract can temporarily disrupt the gut barrier, allowing harmless foods to penetrate and activate immune cells under the layer of cells that line the bowel. Findings in 12 patients with IBS showed that when common food allergens like gluten, wheat, soy, or milk were injected into the rectum, every patient had a localized reaction to one or more allergens6. Similarly, 61% of patients with IBS who responded negatively to food antigens on classical food allergy testing reacted to wheat when their duodenum was monitored in real-time via advanced endoscopic imaging technology. These patients also had a 4-fold higher rate of atopic conditions and altered intestinal permeability. However, IBS symptoms were reduced when wheat products were excluded from the diet7.

Contrary to the common belief that IBS is only “functional,” these findings suggest patients with IBS could have changes at the microscopic level in the gut involving a high number of mast cells that may also be more reactive and which are responsible for initiating an allergic response in the gut and driving abdominal pain. This gut allergy differs from classic food allergies, which can produce hives and other body-wide immune reactions.

 

Which foods should be avoided, and which are the most helpful calming therapies?

Diets for improving meal-related symptoms should be as less restrictive as possible while offering symptom management. Patients with IBS can minimize their symptoms by avoiding the foods that seem to trigger them. While there is no one-size-fits-all diet, the most researched dietary therapies, which should be tailored to individual needs with the supervision of a gastroenterology dietitian are summarized below8-15.

Remember that personalized elimination diets based on food sensitivity blood tests that measure leukocyte activation or IgG/IgG4 are not useful for deciding which foods to include or exclude in IBS.

Eating regularly and taking time to eat is your best plan for better digestion. These are some of the strategies8,9:

  • Consume regular meals (three meals per day coupled with a snack or two, based on personal hunger level)
  • Sit down to eat
  • Chew foods well
  • Take time to eat
  • Avoid skipping meals
  • Keep hydrated by drinking water and noncaffeinated drinks, preferably warm or even hot10 (drink at least eight 8-ounce glasses per day, especially if you are prone to diarrhea)
  • Soluble fiber (e.g., oats) may calm IBS symptoms, whereas insoluble fiber (e.g., wheat bran) worsens abdominal pain and bloating

Reduce the following foods if believed to be triggers8,9:

  • Fatty foods
  • Spicy foods
  • Alcohol
  • Caffeine

Low FODMAP diet11: This consists of eliminating all high FODMAP foods and then reintroducing them slowly, adding one food at a time to determine personal tolerance (it is not the first go-to diet for all people with IBS).

  • FODMAPs are poorly absorbed short-chain carbohydrates that contribute to gut distention through increased gut water content and microbiota fermentation
  • FODMAPs may also lead to gut dysbiosis, increase mucosal inflammation, and activate pain-related brain areas

Gluten vs fructan intolerance9,12,13:

  • Gluten may induce abdominal pain in some patients with IBS through immune system and gut microbiome changes
  • Recent findings suggest fermentable fructans (a type of FODMAP) found in wheat, rather than gluten, are more likely to be involved in IBS symptoms
  • Other components in wheat, such as wheat-germ agglutinin and amylase trypsin inhibitors, seem to play a role in both intestinal and extra-intestinal symptoms

Diet low in food antigens (e.g., milk or wheat)1:

  • Foods rich in FODMAPs also contain substances acting as food antigens, which activate immune responses in the gut in patients with IBS (gluten, amylase trypsin inhibitors, cow’s milk protein)
  • IgE and non-IgE-mediated food allergies may explain IBS-like symptoms in a subset of patients, wheat being one of the most studied food antigens

Role of histamine intolerance1,9:

  • Almost 60% of patients with IBS identify histamine-containing foods (e.g., some fruits and vegetables, oily fish, aged cheeses, alcohol, nuts, eggs, cured meats, and chocolate) as a trigger to digestive symptoms
  • The role of a low-histamine diet has not been sufficiently studied in patients with IBS

Low-salicylate diet1,9:

  • The ingestion of food chemicals beyond the individual threshold for symptoms is a potential trigger of IBS symptoms
  • While chemical-sensitive individuals may benefit from a diet low in chemical-containing foods, decreasing naturally occurring dietary salicylates did not improve digestive symptoms compared to a balanced diet high in salicylates

Probiotics and prebiotics13,14:

  • Some (combinations of) probiotics may improve IBS symptoms, but the findings are limited and evidence for efficacy was low to very low

Beyond diet, cognitive behavioral therapies, including relaxation techniques and yoga16, and vigorous physical activity three to five days a week17 may also reduce physical and psychological symptoms. Another soothing technique for improving overall digestive symptoms and well-being that now starts to be investigated is diaphragmatic breathing. As you slowly breathe in through the nose, the diaphragm contracts and causes the belly to rise. Then, when you slowly exhale through the mouth, the diaphragm retracts and returns to its natural dome-like shape.

 

 

References:

  1. Ford AC, Staudacher HM, Talley NJ. Postprandial symptoms in disorders of gut-brain interaction and their potential as a treatment target. Gut. 2024; 73(7):1199-1211. doi: 10.1136/gutjnl-2023-331833.
  2. Vanuytsel T, Bercik P, Boeckxstaens G. Understanding neuroimmune interactions in disorders of gut-brain interaction: from functional to immune-mediated disorders. Gut. 2023; 72(4):787-798. doi: 10.1136/gutjnl-2020-320633.
  3. Calder PC. Nutrition and immunity: lessons from coronavirus disease-2019. Proc Nutr Soc. 2023; 1-16. doi: 10.1017/S0029665123004792.
  4. Kastl Jr AJ, Terry NA, Wu GD, et al. The structure and function of the human small intestinal microbiota: current understanding and future directions. Cell Mol Gastroenterol Hepatol. 2020; 9(1):33-45. doi: 10.1016/j.jcmgh.2019.07.006.
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  6. Aguilera-Lizarraga J, Florens MV, Francesca Viola M, et al. Local immune response to food antigens drives meal-induced abdominal pain. Nature. 2021; 590(7844):151-156. doi: 10.1038/s41586-020-03118-2.
  7. Fritscher-Ravens A, Pflaum T, Mösinger M, et al. Many patients with irritable bowel syndrome have atypical food allergies not associated with immunoglobulin E. Gastroenterology. 2019; 157(1):109-118.e5. doi: 10.1053/j.gastro.2019.03.046.
  8. Singh P, Tuck C, Gibson PR, et al. The role of food in the treatment of bowel disorders: focus on irritable bowel syndrome and functional constipation. Am J Gastroenterol. 2022; 117(6):947-957. doi: 10.14309/ajg.0000000000001767.
  9. Haller E, Scarlata K. Diet interventions for irritable bowel syndrome: separating the wheat from the chafe. Gastroenterol Clin North Am. 2021; 50(3):565-579. doi: 10.1016/j.gtc.2021.03.005.
  10. Fujihira K, Hamada Y, Yanaoka T, et al. The effects of water temperature on gastric motility and energy intake in healthy young men. Eur J Nutr. 2020; 59(1):103-109. doi: 10.1007/s00394-018-1888-6.
  11. Dean G, Chey SW, Singh P, et al. A diet low in fermentable oligo-, di-, monosaccharides and polyols improves abdominal and overall symptoms in persons with all subtypes of irritable bowel syndrome. Neurogastroenterol Motil. 2024; 36(8):e14845. doi: 10.1111/nmo.14845.
  12. De Palma G, Reed DE, Bercik P. Diet-microbial cross-talk underlying increased visceral perception. Gut Microbes. 2023; 15(1):2166780. doi: 10.1080/19490976.2023.2166780.
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  14. Goodoory VC, Khasawneh M, Black CJ, et al. Efficacy of probiotics in irritable bowel syndrome: systematic review and meta-analysis. Gastroenterology. 2023; 165(5):1206-1218. doi: 10.1053/j.gastro.2023.07.018.
  15. Wu Y, Li Y, Zheng Q, et al. The efficacy of probiotics, prebiotics, synbiotics, and fecal microbiota transplantation in irritable bowel syndrome: a systematic review and network meta-analysis. Nutrients. 2024; 16(13):2114. doi: 10.3390/nu16132114.
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