Functional bowel disorders are disorders of the gut brain interaction (DGBI) characterized by symptoms related to the middle and lower part of the gastrointestinal tract. The term ‘functional’ reflects the lack of detectable structural or biochemical abnormalities that fully explain the symptoms, despite underlying pathophysiological alterations. Functional bowel disorders are further divided into Irritable Bowel Syndrome (IBS), functional abdominal bloating/distension, functional constipation, functional diarrhea, and unspecified functional bowel disorders. Diagnostic criteria for each functional bowel disorder must be met consistently for a minimum duration of three months, and symptom onset six months prior to diagnosis. According to the global epidemiological survey by the Rome foundation, any bowel disorder is present in 33.4 % of the population [1]. It is more common in female (39.3 %) compared to male participants (27.7 %). Worldwide prevalence rates are as followed: 4.1 % IBS, 11.7 % functional constipation, 4.7 % functional diarrhea, 3.5 % functional bloating/distention, and 8.8 % an unspecified functional bowel disorder. Patients with IBS are divided into four different subtypes, depending on their dominant stool pattern: IBS predominant constipation (IBS-C), IBS predominant diarrhea (IBS-D), combination of both constipation and diarrhea (IBS mixed, IBS-M), and an unspecified subtype (IBS-U).

Although no structural abnormalities account for these symptoms, various pathophysiological mechanisms—including visceral hypersensitivity, altered motility, and immune dysregulation—have been implicated. However, the majority of patients report postprandial worsening after meal intake [2]. According to food questionnaires, the majority of the patients with IBS reported symptom onset associated with at least one of the queried food items [3]. Furthermore, symptom severity appeared to correlate with the number of queried food items. Most common reported food items were carbohydrates, fat, histamine-releasing food items and foods rich in biogenic amines.

Fermentable oligo-, di-, monosaccharides and polyols (FODMAPs) are carbohydrates that lead to gastrointestinal symptoms in patients with functional bowel disorders. It is a collective term that includes fructose in excess of glucose, oligosaccharides including fructans/fructo-oligosaccharides, galacto-oligosaccharides, sugar polyols such as sorbitol and mannitol, and lactose. FODMAPs have been well studied in the context of IBS. They appear to transit to the large intestine, where they undergo fermentation by gut microbiota, leading to gas production and intestinal distention [4]. Among these, lactose and fructose are particularly prone to malabsorption, which has been associated with symptoms such as abdominal pain, bloating, and diarrhea in patients with DGBIs. It can be related to abnormal sensorimotor function in the gastrointestinal tract and abnormal gut-brain interactions [5]. Beyond these well-established intolerances, recent findings suggest a higher prevalence of sucrose-isomaltase gene variants in IBS patients. This may lead to symptom generation following consumption of sucrose and starch [6]. Besides these carbohydrates, dietary fiber is likewise subject to fermentation, yielding short-chain fatty acids and gas that can trigger pain, bloating, and flatulence [7]. However, soluble fibers which are less susceptible to fermentation, can also be used as a treatment for IBS-C patients.

In parallel, food sensitivities driven by allergic pathophysiology—mediated by immunoglobulin E (IgE), non-IgE pathways, or their combination—are recognized contributors to gastrointestinal symptoms, with presentations ranging from mild discomfort to severe reactions [8]. Systemic food allergy has been implicated and clinical clues suggested that food could cause a local immune response in IBS patients [9], although IgE testing is negative in the vast majority of patients. Therefore, while carbohydrates have been recognized for their impact on patients with functional bowel disorders, emerging evidence suggests that proteins, and more specifically, protein-based allergens, may also play a significant role. Additionally, with the rising popularity of gluten-free diets in recent years, the influence of gluten on symptomatology in this patient population has garnered increased scientific attention. In addition to conventional dietary components, other food-related factors, including fat and bioactive compounds, may influence symptoms in patients with functional bowel disorders; however, further research is needed to elucidate their roles.

Due to the heterogeneous pathophysiology of these disorders, it is often difficult to find the optimal treatment option and individualized management strategies are essential. Therapeutic strategies for patients with functional bowel disorders should be guided by the predominant symptom presentation. As the majority of the patients report food-related symptoms, it is not surprising that many patients begin dietary self-modifications based on perceived food triggers. National guidelines, including the British Dietetic Association's (BDA) evidence-based guidelines and U.K's National Institute of Health and Care Excellence (NICE) recommend general dietary and lifestyle advice as a first-line intervention [10]. Targeted dietary strategies, such as the low-FODMAP diet and fiber supplementation, may then be employed as second-line therapies. These approaches are also supported by practice guidance from the American Gastroenterological Association (AGA) [11].

This review aims to explore and synthesize current evidence on dietary interventions in the management of functional bowel disorders.