Why are disorders of gut–brain interaction (DGBI) often food-related? Duodenal eosinophils and mast cells, small intestinal bacteria, food allergy and altered food intake in functional dyspepsia and the irritable bowel syndrome: a new paradigm

The underlying causes of irritable bowel syndrome (IBS) and functional dyspepsia (FD) have remained largely elusive, but emerging data suggest immune activation and loss of small intestinal homeostasis may explain a major subgroup. FD and IBS symptoms often overlap and may occur early in the post-prandial period, suggesting the origin of symptoms may be much higher in gastrointestinal tract than colon. There is strong evidence low-grade duodenal inflammation, comprising eosinophils and/or mast cells associated with increased permeability, is present at least in a major subset with FD and IBS. This hypothesis is further supported by evidence of circulating increased small intestinal homing T cells and altered duodenal microbiota. We hypothesize a major etiologic pathway whereby interaction of food with intestinal bacteria switches on small intestinal immune activation in FD and IBS leading to presentation of antigens to the mucosa. While the low FODMAP diet provides symptom relief in both IBS and FD, this diet notably also reduces common food protein antigens (e.g., wheat, milk, soy) and urinary histamine levels. The obvious but often overlooked fact that food ingestion usually requires the act of eating adds nuance to determining whether food components or eating itself induces symptoms and that both need to be considered in DGBI in clinical practice. The exciting observations about subtle inflammation in DGBIs offer hope for new diagnostic biomarkers, and if considered in the context of altered dietary patterns and validated against symptom responses, will pave the way for novel DGBI treatment options.