Far from being just indigestible residue, dietary fiber has long been part of the human diet. Anthropological studies suggest that early humans consumed large amounts of fibrous plants, including wild grains, roots, and seeds, and it is estimated that hunter-gatherers consumed between 70 and 120 g of fiber per day [1]. With the advent of industrial food processing, fiber intake declined significantly. The term “dietary fiber” emerged in the 1950s [2], and its clinical relevance gained attention in the 1970s, when epidemiological studies linked high-fiber diets to reduced rates of chronic and gastrointestinal diseases. In 2009, after more than two decades of debate, the World Health Organization and Codex Alimentarius issued an internationally accepted definition as “all carbohydrates that are neither digested nor absorbed in the small intestine and have a degree of polymerization of ten or more monomeric units” [3]. Subsequently, the European Food Safety Authority (EFSA) and the FDA adopted a wider definition to include “all carbohydrates that are neither digested nor absorbed in the small intestine and have a degree of polymerization of three or more monomeric units must also demonstrate physiological effects to human health” [4].

Studying the health effects of dietary fiber is challenging due to the wide variability in intervention types. These include isolated synthetic fibers (e.g., fructo-oligosaccharides), extracted plant-derived fibers (e.g., alginate, psyllium, inulin), whole foods containing intact intrinsic fibers (e.g., prunes, wholegrain cereals), and complex high-fiber diets comprising multiple sources [5]. This heterogeneity complicates the interpretation of fiber's independent effects. Nevertheless, a substantial body of epidemiological and interventional research consistently associates dietary fiber with disease prevention, improved disease management, and reduced mortality [[6], [7], [8], [9]]. These well-established associations have led most countries to recommend a daily dietary fiber intake of 25–35 g for adults. However, average intake worldwide remains low, often below 20 g per day [10,11].

Beyond its role in disease prevention, dietary fiber also holds therapeutic potential, particularly for gastrointestinal disorders. Different guidelines include fiber-related recommendations for conditions such as irritable bowel syndrome (IBS) [12,13], inflammatory bowel disease (IBD) [14], diverticular disease [15], and for managing symptoms like constipation [16] and diarrhea [17]. The therapeutic effects of dietary fiber in these conditions are attributed to its impact on nutrient digestion and absorption, improvement of glycemic control, modulation of plasma cholesterol through reduced bile acid reabsorption, and its influence on gut transit, microbiota composition, and microbial metabolism [18]. These functions are largely determined by the fiber's physicochemical properties, including solubility, viscosity, and fermentability, which shape its activity along different segments of the gastrointestinal tract [19].

This review examines how the physicochemical and functional properties of dietary fibers influence their effects along different segments of the gastrointestinal tract, highlighting both their therapeutic potential and their adverse impact on symptoms in conditions such as IBS, IBD and diverticular disease among others.