The role of dietary fiber in the gastrointestinal tract: when, how and why?

Dietary fiber (DF) used to be seen merely as an indigestible component, but it is now recognized as essential for both gut and overall metabolic health. Historically, humans consumed between 70 and 120 g of fiber per day, far more than the less than 20 g typically eaten today, despite WHO recommendations of 25-35 g daily. The physiological effects of dietary fibers depend on a complex interplay of their physicochemical properties (solubility, viscosity, and fermentability) rather than on a single characteristic. These properties determine how fibers interact with different segments of the gastrointestinal tract, influencing motility, fermentation, and barrier function. This review examines how distinct types and combinations of dietary fibers modulate gastrointestinal physiology and symptoms in various disorders, including irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and diverticular disease. For example, fibers such as psyllium, which combine solubility with moderate viscosity and fermentability, can improve bowel habits and reduce symptom burden in IBS, though responses vary among individuals. In IBD, high-fiber diets, particularly Mediterranean-style ones, appear safe and may even confer protection. Similarly, fiber from fruits and cereals is associated with a reduced risk of diverticular disease. Overall, understanding how multiple fiber properties interact to influence gastrointestinal function may allow more targeted dietary recommendations according to specific clinical contexts. Further research is needed to define the optimal types and amounts of fiber for different conditions and individuals.