Diet-related gastrointestinal reactions are mainly due to food stimulation of intestinal mechanoreceptors, chemoreceptors (e.g., capsaicin), or alteration of gastrointestinal transport function, osmotic pressure, or secretion function. The specific mechanisms are as follows: i. Food allergy According to statistics, 1% to 4% of the American population has food allergy. The mechanism of food allergy can be divided into IgE-mediated (type 1 hypersensitivity reaction) and non-IgE-mediated. IgE-mediated allergic reactions have a rapid onset and occur most often with peanuts, nuts, eggs, milk, soy, fish, shells, strawberries, and wheat. The most common allergic reaction is acute urticaria, and common gastrointestinal reactions include nausea, dysphagia, abdominal pain, vomiting, and diarrhea. Skin prick tests are more valuable than radioallergen adsorption tests for the diagnosis of IgE hypersensitivity reactions. Non-IgE-mediated hypersensitivity reactions are mediated by Th2 cells and have a slow onset, with symptoms confined to the gastrointestinal tract. II. Food intolerance Food intolerance reflects a non-immune response to a variety of disease processes, including non-celiac gluten sensitivity (NCGS), the effects of chemical components in food (e.g., histamine, glutamates, caffeine), enzyme deficiencies (e.g., lactase), transport disorders (e.g., fructose), and the effects of short-chain carbohydrates in food. In particular, food intolerance is present in 50% to 70% of patients with IBS and seriously affects the quality of life of patients. Food-induced gastrointestinal symptoms are highly prevalent in women. Common intolerant foods include: high carbohydrate diet, coffee, alcohol, milk, chocolate, beans, onions, cabbage, and foods high in fat and spices. Third, lactase deficiency is the enzyme deficiency most closely related to food intolerance. Fructose is a monosaccharide that often causes abdominal discomfort in IBS patients. Fructose absorption in the small intestine is low due to the limited amount of fructose transporter GLUT-5 in the epithelial cells of the small intestine. Unabsorbed fructose accelerates intestinal motility and increases intestinal gas production, with 50% of healthy volunteers exhibiting fructose malabsorption after 25 g of fructose and up to 75% after 50 g of fructose. Short-chain carbohydrates, usually found in grains, fruits, vegetables, legumes, and nuts, include fructans, oligogalactans (galactosomes), and polyols. These substances are difficult to absorb in the small intestine and can increase osmotic pressure in the large intestine and fermentation by colonic bacteria, leading to gastrointestinal discomfort. Colonic bacterial fermentation can in turn produce hydrogen, methane, carbon dioxide and short-chain fatty acids. IV. Altered intestinal osmolality is a potential pathogenic mechanism for diet-related symptoms of IBS and is seen in patients with diarrheal IBS (IBS-D). Some studies have found that microscopic intestinal mucosal epithelial damage, intraepithelial lymphocytic hyperplasia, and widening of the villi gap are more common in patients who consume wheat, soy, milk, and yeast. V. Visceral hypersensitivity Patients with intestinal disease may have lower tolerance to certain foods (e.g., capsaicin) than normal populations compared to healthy populations. The clinical symptoms of SIBO are not specific and are similar to those of food intolerance and intestinal microecological disorders. There is no practical gold standard for testing. VII. Intestinal microecology The alteration of the intestinal microbial environment brought about by an uncontrolled diet can lead to different manifestations of clinical symptoms such as diarrhea, abdominal distension, and constipation.