FDNJ

Introduction

Dietary fiber is an essential macronutrient in a healthy diet [15]. A diet low in dietary fiber coincides with the growing global prevalence of obesity, which has tripled over the last half-century [6-9]. Based on data from 2022, obesity and overweight affect an estimated 850 million and 2.5 billion people, respectively [10,11]. Low fiber intake is also a significant contributor to chronic illnesses, including Type 2 Diabetes Mellitus (T2D), dyslipidemia, hypertension, obstructive sleep apnea, and Cardiovascular Disease (CVD) [12]. Reversing these trends requires lifestyle changes, primarily through dietary modification [13,14].

Dietary fiber reduces the risk of developing conditions associated with increased levels of glucose and cholesterol, including obesity, CVD, cancer, and diabetes [15-20]. Digestive enzymes cannot break down dietary fiber and pass it undigested through the upper gastrointestinal (GI) tract. However, dietary fiber can aid in the emptying of the upper GI tract and regulate the absorption of sugar and cholesterol in the intestine, helping to control hunger, blood sugar, and cholesterol levels [2,15,21,22]. Soluble Dietary Fiber (SDF) is fermented and utilized by microbes in the large intestine, affecting microbial growth and the microbiome environment in the large intestine [23-25]. Dietary fiber may benefit the gut microbiome through anti-inflammatory effects that alleviate chronic inflammation and associated conditions, such as chronic constipation [23,26]. Nonfermented dietary fiber’s water absorption and bulking effect in the large intestine may also help with chronic constipation [27,28].

Recommendations for daily dietary fiber intake vary globally. The World Health Organization (WHO) and the European Food Safety Authority (EFSA) recommended 25 g/day of naturally occurring dietary fiber for bowel function, reduced risk of CVD and T2D, and improved weight maintenance [29,30]. The current Dietary Guidelines for Americans recommend 14 g dietary fiber per 1,000 kcal/g based on the observation of CVD risk reduction with increased fiber intake and progression of diet-related chronic diseases associated with fiber-poor diets [31].

Dietary fiber intake in Europe is significantly below recommended levels [32]. Despite observational studies showing an association between dietary fiber and reduced health risks, intake has not changed considerably over the past 20 years [33]. This gap continues despite health promotion organizations encouraging dietary fiber intake for disease prevention and management and efforts to increase the consumption of fiber-rich foods such as whole grains, legumes, nuts, fruits, and vegetables [33]. In the U.S., dietary fiber intake levels are also below recommendations [33]. A typical diet in the U.S. contains an average of 8.1 g of dietary fiber for every 1,000 calories, or 58% of the recommendations, as reported in a 2017 survey by the U.S. Department of Agriculture (USDA) [33]. Inadequate dietary fiber intake is associated with the onset and/or progression of diet-related chronic diseases and is considered a public health concern of the general U.S. population [31].

The nutritional gap in dietary fiber intake, along with the pandemicrelated increases in obesity and associated chronic diseases, has prompted renewed interest in the relationship between dietary fiber intake and human health [34]. The USDA has a recommended diet pattern that provides adequate dietary fiber intake. The dietary fiber intake gap is widened by the Western diet patterns, where fruits, vegetables, and whole grains are under-consumed by more than 85% of adults [31]. Addressing the deficiency of dietary fiber in the diets of industrialized populations requires a multi-faceted approach. Besides lifestyle changes, dietary advice, and education, there is a critical need for easier access to dietary fiber sources in the diet [22,35].

In this review, we focus on the definition, properties, and benefits of dietary fiber, a macronutrient that has not received as much attention as its more prominent counterparts, protein, sugar, oil, and fat. Our objective is to explore the current literature on dietary fiber’s properties and health benefits, particularly emphasizing its metabolic functions in the upper GI tract and large intestine, its impacts on microbiota, and its overall effects on human health. Additionally, we will cover a variety of global regulations on fiber supplement products.

Dietary Fiber Defined

The definition of dietary fiber varies among regulatory agencies and scientific organizations in different countries. Dietary fiber is generally defined by the specific fiber’s enzyme resistance properties and health benefits [36]. The Codex Alimentarius Commission defines dietary fiber as carbohydrate polymers with ten or more monomeric units, which are not hydrolyzed by the endogenous enzymes in the small intestine of humans [36]. The Codex further categorizes dietary fiber into several types: edible carbohydrate polymers naturally occurring in food as it is consumed; carbohydrate polymers obtained from raw food material by physical, enzymatic, or chemical means that have a physiological benefit to health as demonstrated by generally accepted scientific evidence to competent authorities [36]. The Codex also clarifies that dietary fiber from plant origin may include fractions of lignin and other compounds associated with polysaccharides in plant cell walls [36]. Lignin is not a qualified dietary fiber if the compounds are extracted and reintroduced as a food ingredient [36]. The Codex officially accepts degrees of polymerization (DP) of ten or more for carbohydrates to be classified as dietary fiber. Still, it allows for a DP between 3 and 9, depending on individual countries’ regulations [36].

The Codex definition also requires derived or synthetic carbohydrates to show a physiological benefit to human health for classification as a dietary fiber [36]. Still, it does not describe those benefits, and they are demonstrated by generally accepted scientific evidence to competent authorities [36]. Various benefits have been demonstrated with different levels of evidence by academia, institutes, and agencies worldwide [37]. These include reduced blood total and/or Low-Density Lipoprotein (LDL) cholesterol levels, increased fecal bulk/laxation, attenuation of postprandial glycemia/insulinemia, positive modulation of colonic microflora, reduced blood pressure, increased satiety, weight loss or reduction in adiposity, increased colonic fermentation and Short-Chain Fatty Acid (SCFA) production, and decreased transit time.

The U.S. Food and Drug Administration (FDA) defines dietary fiber as two types of non-digestible carbohydrates consisting of three or more sugar molecules (DP ≥ 3): 1) naturally occurring fibers that are intrinsic and intact in plants, including both soluble and insoluble carbohydrates and lignin; and 2) isolated or synthetic non-digestible carbohydrates that have been specifically evaluated and determined by the FDA to have beneficial physiological effects on human health [38]. The list of isolated or synthetic fibers approved by the FDA is growing. Currently, it includes β-glucan soluble fiber, psyllium husk, cellulose, guar gum, pectin, locust bean gum, hydroxypropyl methylcellulose (HPMC), mixed plant cell wall fibers, arabinoxylan, alginate, inulin and fructans, type 2 resistant starch (RS2), cross-linked phosphorylated resistant starch type 4 (RS4), glucomannan, and acacia (gum arabic) [39,40]. Dietary fiber has very different monomer types, glycosidic linkages, and side chains, which makes dietary fiber a complex family of chemicals [41].

Properties of Dietary Fiber

Dietary Fiber: Characterization and Measurement

Human digestive enzymes for carbohydrates include sucrase, glucosidase, lactase, maltase, trehalase, amylase, and chitinase [42,43]. Utilizing the enzyme-resistant properties of dietary fiber, enzymatic gravimetry methods were developed to determine dietary fiber content using the Association of Official Analytical Chemists (AOAC) Official Methods of Analysis (OMA) 991.42, 991.43, 993.19 and American Association of Cereal Chemists International (AACCI) (now Cereals & Grains Association) approved methods of 32-05.01, 32-07.01 [44,45]. Amylase and glucosidase are used to treat fiber samples to break down digestible carbohydrates into simple saccharides and measure long-chain carbohydrates insoluble in an 80% (v/v) water-ethanol solution. These methods are suitable for high DP dietary fibers (DP > 10). For low molecular weight dietary fibers (DP < 10), gel permeation chromatography methods are needed using AOAC OMA 2011.25, 2009.01, 2001.03, and AACC (American Association of Cereal Chemists) approved methods of 32-41.01, 32-45.01, and 32-50.01. The chain length or DP and the amount of side chains determine dietary fiber’s physical and chemical properties, including water solubility, viscosity, and microbial fermentability, which are linked to the physiological effects of dietary fiber.

The viscosity of dietary fiber is proportionally related to its average DP [46]. As the molecular weight and interactions between carbohydrate chains increase, viscosity increases, while the solubility decreases and eventually diminishes [46]. The extended contact between the long molecular chains of viscous dietary fiber with water and digested food leads to enhanced intermolecular attractions that contribute to flow resistance. This results in the viscous gel-forming and bulking properties of dietary fiber in the digestive tract. For a given molecular weight, a branched dietary fiber chain will have a smaller volume and, hence, reduced viscosity than a linear chain fiber. Viscosity is commonly measured in two ways: absolute and kinematic viscosity. Absolute viscosity is typically expressed in millipascal-seconds (mPa s) or centipoise (cP), with 1,000 mPa s being equal to 1,000 cP or one pascal-second (Pa s). Kinematic viscosity, on the other hand, is calculated by dividing the absolute viscosity by the fluid’s density, and it is measured in square meters per second (m²/s). Food mixture exhibits a complex non-Newtonian fluid behavior, and its apparent viscosity depends on shear rate and concentration. No standardization exists regarding viscosity measurement in nutritional sciences, making inferences and comparisons among studies difficult. Due to the confusion surrounding the various methodologies for measuring viscosity, establishing standardized guidelines for conducting proper measurements is critical to ensure continued sound scientific research on how viscosity affects dietary fiber and nutrition [47].

SDF is more likely to be utilized by microbes in the large intestine through fermentation compared with insoluble dietary fiber [48]. When SDF enters the colon through the small intestine, gut bacteria degrade SDFs into oligosaccharides (DP < 10), monosaccharides, SCFAs, and other chemicals through different degradation systems. These breakdown products can act as an energy source when absorbed by the body, and they influence the biological environment within the large intestine. When fermented, SDF starts to lose its water absorption and viscous properties. The gut microbes contain a diversity of carbohydrate-active enzymes, which provide flexibility to the microbiota to switch between different fiber energy sources [43].

Low molecular weight dietary fiber or oligosaccharides (DP < 10) can be readily fermented and utilized by gut microbes [49]. The gas generation from fermentation and GI tolerance is a concern [50]. Not all fibers have the same GI tolerance; for example, oligosaccharides can cause symptoms with low doses (10 g), while other fibers have been consumed at doses up to 50 g without symptoms [50]. The Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols (FODMAP) diet was developed due to the association between consumption of fermentable short-chain carbohydrates and symptom induction in those with functional bowel disorders (FBDs) [50]. The pooled global prevalence for FBDs was recently shown to be as high as 33.4% [51]. FODMAPs may also worsen visceral hypersensitivity, inducing bloating, pain, and discomfort via various peripheral factors, such as altered microbiota, increased intestinal permeability, and activated immune/inflammatory response. FODMAPs contribute to quick colonic gas production through fast fermentation, which may cause bloating and abdominal pain in those with visceral hypersensitivity [50]. Excessive gas production can also cause faster colonic transit in those with IBS-D due to colonic sensitivity to increased intestinal volume [52]. Intestinal irritation can increase luminal water and may contribute to diarrhea [53]. A low FODMAP diet results in a 50% to 80% reduction of IBS symptoms [54]. It is advised that total FODMAP content should be around 2.5 to 3 g per day, while a typical Western diet ranges from 15 to 30 g per day of total FODMAPs [54].

Dietary Fiber: Types and Sources

In the U.S., the FDA approval process for a dietary fiber to be included in the Nutrition and Supplement Facts label consists of an evaluation to determine if the fiber provides beneficial physiological effects on human health [39]. The process begins with submitting scientific evidence by citizen petitions, demonstrating that a fiber has specific health benefits, such as lowering blood glucose or cholesterol levels, or improving bowel function [40]. The FDA reviews the published scientific evidence to ensure it is robust, reliable, and based on well-conducted research. If the fiber meets these criteria, the FDA will officially recognize it as a dietary fiber for nutrition labeling purposes, allowing it to be listed as a dietary fiber on food product labels. A list of the qualified dietary fiber and their functions was published and updated by the agency, see Table 1 [39,40].

Table 1: U.S. FDA approved dietary fiber types and properties.

Alginate is an unbranched binary copolymer of β-1,4 mannuronic acid and α-1,4 guluronic acid of widely varying composition and structure [55]. Alginates contain homopolymeric blocks of contiguously linked mannuronic acid (M-blocks), alternated by guluronic acid (G-blocks) sequences, and mixed GM-blocks can also occur along the polymer chain [56]. Alginate can be extracted from various species of brown algae, such as Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, Macrocystis, and Sargassum, as the calcium, magnesium, and sodium salts of alginic acid. The uronic acid polymer is not water-soluble in its acidic form but becomes soluble when it forms a salt with sodium or potassium. Cations with multiple charges and polycations will significantly reduce alginate’s solubility, which could contribute to the excretion of heavy metals in feces. Alginates have a wide range of molecular weights, which can be between 200 kDa and 1,000 kDa, and vary with biosynthesis and degree of depolymerization during extraction. Alginate has a beneficial physiological effect on postprandial glucose levels, as reviewed by the FDA [39].

Arabinoxylan is a type of hemicellulose and a key component of the cell walls of cereal grains [57]. It is a diversely composed β-1,4 xylan polymer that contains arabinose and various uronic acid side branches [56]. Psyllium husk, the milled seed coat removed from the psyllium seed, is a concentrated source of the soluble fiber arabinoxylan [58]. Arabinoxylan is found in the alkali-soluble fraction of psyllium husk and is thought to be the active component of psyllium husk. Psyllium gum consists of a highly branched acidic arabinoxylan structure with a very high molecular weight of approximately 1,500 kDa, contributing to its limited solubility in water. When dispersed in water, it swells and forms a mucilaginous dispersion with gel-like properties. Strong evidence indicates that arabinoxylan has a beneficial physiological effect on blood glucose and insulin levels [57]. Psyllium husk qualifies as dietary fiber because research has demonstrated its effectiveness in reducing the risk of coronary heart disease (CHD) [38]. Arabinoxylan is qualified as dietary fiber for its blood glucose reduction and insulin level benefit as reviewed by FDA [39].

The structural intricacies of β-glucan molecules reveal wide variations depending on their origin. β-glucan has garnered recognition as a bioactive food ingredient due to its diverse biological activities [59]. It is found in various sources, including bacteria, algae, barley, yeast, mushrooms, and oats. Primary dietary fiber sources for β-glucan include cereals like oats, barley, sorghum, wheat, and rye. Among these, oats and barley share a distinctive molecular arrangement of β-D-glucose units connected by alternating (1→3) and (1→4) linkages [59]. The β-glucan content can vary from cultivar to cultivar under specific environmental conditions. Typically, oats contain 6% to 8% and barley 4% to 10% (w/w) β-glucan [59]. Studies have shown that the molecular weight of oat and barley β-glucans ranges from 130 kDa to 390 kDa and 190 kDa to 410 kDa, respectively.

Some mushroom varieties, such as oyster and shiitake, are good β-glucan sources. Mushroom cell walls are rich in long- or shortchain polymers of glucose subunits with β-1,3 and β-1,6 linkages responsible for β-glucan’s linear and branching structures [59]. For instance, mushroom-derived β-glucans typically feature short β-1,6 linked branches, while yeast-derived counterparts exhibit β-1,6 side branches and additional β-1,3 regions. The molecular weight of β-glucan directly influences its viscosity through its water retention properties and solubility. This property has significant implications for human health, as the viscosity of β-glucan affects its ability to lower cholesterol, regulate blood glucose levels, and support immune system function in the digestive tract. The FDA has approved β-glucan from oat and barley as a dietary fiber based on its role in reducing the risk of CHD [38].

Cellulose is the major structural polysaccharide in plant cell walls and is widely present in vegetables, fruits, and cereals [60]. It is a high molecular weight linear homopolymer of β-1,4 glucose monomers [56]. Every second residue is rotated 180^°, which enables the formation of intermolecular hydrogen bonds between parallel chains. The numerous intermolecular H-bonds and van der Waals forces produce a three-dimensional fibrous crystalline bundle. Cellulose is highly insoluble and impermeable to water and is, therefore, not fermentable by colonic microbes [61]. The FDA has approved cellulose as a dietary fiber on the nutrition label based on its effectiveness in improving laxation [38].

Fructans, oligosaccharide compounds with demonstrated physiological benefits, occur naturally in select monocotyledonous and dicotyledonous plant taxa, including asparagus, garlic, leek, onion, Jerusalem artichoke, and chicory root [62]. Fructan is a naturally occurring polysaccharide that contains mainly β-2,1 fructose linkage with possible terminal glucose linked through α-1,2 glycosidic bond. Inulin is among the most extensively studied fructans and is a mixture of fructan polymers with a DP ranging from 3 to 60 [63]. Oligofructose is a shorter-chain fructan that can be extracted from plants or enzymatically synthesized, referring to fructans with a DP < 10, and accounts for approximately 30% of the total inulin present in chicory root extract [64]. These linkages can be fermented by microorganisms in the colon. While inulin is a soluble fiber, it lacks the typical properties of other soluble long-chain dietary fibers, such as viscosity [65]. Evidence supports that inulin-type fructans extracted from chicory root, as well as those from different sources and synthetic inulin-type fructans, have a beneficial physiological effect on bone mineral density and calcium absorption; and inulin and inulin-type fructans were added to the FDA’s list of approved dietary fibers [39].

Galactomannan is the second-largest storage polysaccharide group in many plant seeds’ endosperm or cell walls [56]. Galactomannan is a linear polymer of β-1,4 linked mannose monomers with galactose residues linked through β-1,6 bonds, forming short side branches. Guar gum, obtained from guar plant seeds, and locust bean gum, from locust bean seeds, are two sources of galactomannan fiber [66]. Locust bean gum generally has an average mannoseto-galactose ratio of about 3.5, the highest among commercially available galactomannans, such as guar gum (1.8) and tara gum (3.0). Locust beans, guar, tara, and cassia gums are manufactured commercially. Isolated galactomannan is hydrophilic, viscous, and has gel-forming properties [66]. The FDA approves this fiber for attenuating blood cholesterol levels and is therefore considered a dietary fiber for nutrition labelling [39].

Glucomannan is a soluble, highly viscous fiber commonly derived from the root of amorphophallus konjac [67]. Glucomannans are linear polymers of both β-1,4 mannose and β-1,4 glucose residues. The basic structure of konjac glucomannan (KGM) consists of glucose and mannose residues in a molar ratio of 1:1.6 to 1.7, polymerized by β-1,4-pyranoside bonds and has a DP up to 6,000 [68]. The glucose and mannose units are linked by β-1,4 glycosidic bonds with occasional acetyl groups at the C-6 position of the sugar units. Acetyl groups scattered randomly along the glucomannan backbone contribute to its water solubility. Konjac glucomannan is a high molecular weight polymer (>300 kDa) that can form viscous pseudoplastic solutions. Glucomannan can lower cholesterol, triglyceride, and glucose levels and may promote weight loss [68]. It demonstrates a beneficial effect on attenuating blood cholesterol levels when taken in doses of 2 to 13 g/day and is considered a dietary fiber for nutrition labeling [39].

Gum acacia, or gum arabic, is a biopolymer consisting of arabinose and galactose monosaccharides, a natural exudate harvested from the acacia tree [69]. The gum is an arabinogalactan protein complex, primarily composed of calcium, magnesium, and potassium salts of arabic acid [69]. The main chain of this polysaccharide is built from β-1,3 and 1,6 galactopyranosyl units, along with β-1,6 glucopyranosyl uronic acid units [56]. Its side branches may contain α-rhamnopyranose, β-glucuronic acid, β-galactopyranose, and α-arabinofuranosyl units with 1,3, 1,4, and 1,6 glycosidic linkages. Arabic acid mainly consists of β-1,3 galactose with 1,6 glycoside-linked side chains of two to five β-1,3 galactose. The fundamental chain and the branches contain arabinose, rhamnose, glucose, and methyl glucose residues. This polymer’s highly branched molecular structure and relatively low molecular weight are responsible for these properties. Available scientific evidence supports the beneficial physiological effects of gum acacia on postprandial blood glucose and insulin levels, and it is considered a dietary fiber for nutritional labelling [39].

HPMC is a synthetic propylene glycol and methyl ester of cellulose, containing methyl groups and hydroxypropyl groups [70,71]. HPMC contains both hydrophilic and hydrophobic groups, behaves as an emulsifier, and is commonly utilized as a film former, stabilizer, and thickener in the food and supplement industries. It meets the definition of dietary fiber for food labeling due to its effect on attenuating blood cholesterol levels [39].

Pectin is present in the cell walls and intracellular tissues of fruits and vegetables [56]. While pectin is abundant in fruits, it is also found in vegetables, legumes, and nuts. Pectin polysaccharides are copolymers of α-1,4 galacturonic acid units with α-1,2 rhamnose residues in adjacent or alternate positions. Pectin is highly heterogeneous in its molecular weight and chemical structure. Pectin is present in the cell walls and intracellular tissues of fruits and vegetables [56]. While pectin is abundant in fruits, it is also found in vegetables, legumes, and nuts. For isolated and purified pectin, the molecular weight is determined by the extraction modes and conditions used, and the average molecular weight is typically between 10 and 100 kDa. Pectin meets the definition of dietary fiber for nutrition labeling based on its effect of attenuating blood cholesterol levels [39].

Starch resistant to amylase hydrolysis is termed resistant starch (RS) [72]. RS is classified into five types based on its structure and formation: Type 1 starch granules are embedded in indigestible plant material is physically trapped within indigestible matrices; Type 2 maintains its natural crystalline structure; Type 3 crystallized starch forms during heating and cooling cycles; Type 4 results from chemical modification such as phosphorylation, esterification, cross-linking, or transglycosylation; and Type 5 develops when starch binds with lipids during cooking [72]. Type 2 RS is found in raw green bananas, raw potatoes, uncooked high-amylose maize/ corn, and potato starch [72]. This type consists of amorphous layers preferentially degraded by α-amylase and a crystalline skeleton resistant to hydrolysis. Type 2 RS has been shown to help reduce insulin levels following a meal containing carbohydrates that raise blood glucose levels, qualifying it as a dietary fiber for nutritional labelling [39]. The FDA has determined that the scientific evidence suggests that cross-linked phosphorylated RS4 can help reduce insulin levels following a meal containing a carbohydrate that raises blood glucose levels [73].

Benefits of Dietary Fiber

Native (Physical) Benefits of Dietary Fiber in the GI Tract

The effects of fiber in the upper GI tract can depend on its physicochemical characteristics, including solubility and viscosity. When distributed in food chyme, dietary fiber changes its physical attributes and the interactions of food components and nutrients with the digestive tract wall. Soluble fiber can help increase food chyme viscosity and lower blood cholesterol, sugar absorption, and blood levels. Dietary fiber helps increase stool bulk and water content and decrease transit time, which could help reduce interaction between carcinogens and the gut wall, reducing bowel cancer risk [74]. A high-fiber diet can also help promote satiation, increase satiety, and reduce energy intake.

Dietary Fiber Fermentation and Benefits

Dietary fiber, resisting breakdown in the upper digestive tract, enters the large intestine, where soluble fibers are fermented by fiber-degrading bacteria, providing an essential energy and carbon source for the intestinal microbiota [49]. The effects of microbial fiber fermentation and metabolism extend beyond the GI tract, influencing various body systems through the gut-brain, gut-liver, gut-kidney, and gut-heart axes [75-77].

The large intestine hosts around 1,000 bacterial species, with microbial populations reaching approximately 10^¹¹-10^¹² CFU/g of colon content [78,79]. The colonic environment, with its slow transit time (2-60 hours) and favorable pH (6-8), is ideal for bacterial growth [80]. Intestinal epithelial cells (IECs) protect underlying body tissues from commensal microbes and invading pathogens by secreting a mucus layer of glycoprotein with 80% carbohydrate content [81]. This mucus acts as a physical barrier and a habitat for beneficial bacteria, providing binding sites and energy sources for microbes. Dietary fiber is crucial for maintaining this mucus barrier homeostasis and enhancing immune tolerance in the gut. When fiber intake is insufficient, the gut microbiota may degrade the colonic mucus barrier more rapidly than it can be regenerated, increasing the risk of pathogen invasion [82].

Gut microbes possess a diversity of carbohydrate-degrading enzymes, with over 130 Glycoside Hydrolase (GH) families, 22 Polysaccharide Lyase (PL) families, and 16 carbohydrate esterase families, allowing them to degrade various fiber sources and switch among different available fiber sources [43]. These microbes can work synergically to break down complex dietary fibers. PLproducing bacteria act as primary degraders that break down long-chain carbohydrates into oligosaccharides, which secondary degraders then ferment [72]. Gut microbes can quickly utilize these generated monosaccharides via glycolytic pathways for pyruvate production and subsequent ATP generation [83]. Cross-feeders, while not necessarily degrading carbohydrates themselves, use the by-products produced by the primary and secondary degraders, further facilitating the breakdown of complex fibers [72]. Around 30 to 50 g of bacterial biomass is produced for every 100 g of carbohydrate fermented, contributing to increased fecal mass [84].

Once pyruvate is produced primarily from carbohydrates and other substrates, the human gut microbiota employs various fermentation strategies to generate energy. The fermentation process produces a variety of metabolites, including gases; such as hydrogen, methane, carbon dioxide; and SCFAs like acetate, propionate, and butyrate, and other metabolites such as formate, succinate, lactate, and alcohols like ethanol and propanol [23]. While fermentation is essential for healthy gut function, low molecular weight fibers can ferment quickly in the proximal colon, produce excessive gas, and cause bloating and discomfort. This fermentation process contributes to the most common side effects of fiber consumption, such as diarrhea and bloating [50]. In contrast, slow-fermenting fibers are more beneficial, distributing SCFA production along the entire colon and enhancing their health effects. The molecular structure of fibers, such as their molecular weight, chain length, and glycosidic bonds, affects the fermentation rate.

SCFAs are key to colonic health, supporting cellular integrity and metabolism while regulating pH and inhibiting pathogen growth. SCFAs account for up to 60% of dietary fiber fermentation end products, and 95% are absorbed by colonocytes [84]. Once absorbed into the bloodstream, SCFAs serve as an energy source for peripheral tissues, with acetate metabolized systemically in the organs, while propionate is processed mainly in the liver. It is estimated that SCFAs contribute about 10% of daily energy requirements [85]. SCFAs also possess anti-inflammatory properties, stimulating IECs and immune cells to produce antiinflammatory cytokines and regulate immune responses. Excess SCFAs are transported to the liver for incorporation into metabolic processes such as gluconeogenesis, lipogenesis, and cholesterol synthesis [23]. Butyrate is the preferred energy source for colonic epithelial cells, promoting their proliferation, enhancing mucin production by goblet cells, and supporting tight junction integrity [86]. Different fibers produce varying amounts and ratios of SCFAs, and their production rate also varies. A diverse intake of dietary fibers supports a richly diverse and stable gut bacterial population, promoting the production of SCFAs and enhancing overall health [28]. SCFA production can help inhibit the growth of pathogenic organisms by lowering luminal and fecal pH, while a low pH environment can help promote beneficial bacteria. A reduced pH environment decreases the degradation of peptides and the formation of toxic compounds like ammonia, amines, and phenolic substances, while inhibiting harmful bacterial enzyme activity [21].

In addition to SCFA generation, pyruvate fermentation produces small amounts of alcohol, including ethanol, propanol, and butanediol, which may contribute to non-alcoholic fatty liver disease in elevated amounts [87,88]. The fermentation and subsequent absorption of SCFAs and alcohols highlight the complex metabolic interactions between gut microbiota and host health.

Dietary Fiber Reduces Blood Cholesterol and Cardiovascular Disease Risk

Cholesterol has vital biological functions, including stabilizing cell membranes and serving as a precursor for steroid hormones, bile acids, and vitamin D. However, cholesterol accumulation in the bloodstream (hypercholesterolemia) can cause atherosclerotic plaques within artery walls, leading to heart attacks and strokes. High levels of fasting total cholesterol, high-density lipoprotein (HDL), and LDL are well-known risk factors for CVD [89]. Cholesterol absorption in the small intestine is vital to human

health and clinical studies have linked cholesterol absorption with plasma concentration of total and LDL cholesterol [90]. Many epidemiologic and clinical studies, summarized in Table 2, suggest that adequate fiber intake consistently lowers blood cholesterol and LDL cholesterol levels, which help reduce the risk of CVD.

Table 2: Perspective, clinical studies, and meta-analyses assessing cholesterol reduction effects of dietary fiber or by year.

In a prospective cohort study of 2,532 men and 3,429 women, the relationship between dietary fiber intake and cardiovascular disease risk factors was examined [100]. In the study, total dietary and non-soluble fiber intakes were inversely associated with hypercholesterolemia. An inverse association was observed of total dietary fiber, soluble fiber, and insoluble fiber intakes with elevated apolipoprotein B, a key component of LDL concentrations, and elevated apolipoprotein B to apolipoprotein A (HDL key component) ratios, a relevant marker of cardiovascular disease risk. In a prospective randomized atherosclerosis prevention trial with 543 children between the ages of 8 months and 9 years, nutrient fiber intakes, weight, height, and serum total, HDL, and LDL cholesterol and triglyceride concentrations were analysed [101]. It was observed that serum cholesterol values correlated inversely with fiber intake; serum cholesterol concentrations decreased with increasing fiber intake.

Cholesterol is used in the liver synthesis of bile acids, which, when secreted in the duodenum, forms an oil fatty emulsion in the digestion tract, aiding further enzymatic fat digestion. Bile acids can be absorbed in the small intestine and recycled through the liver [102]. SDF can increase the viscosity of the food chyme, bind bile acid, reduce its reabsorption in the small intestine, and help the body excrete the bile acids into the large intestine, thus reducing bile acid recycling. Because of this reduced amount of recycled bile acids, the liver will utilize more cholesterol from the blood to make new bile acids, thereby lowering blood cholesterol [102]. Soluble fiber from oats, plant gums, and psyllium has been shown to reduce total and LDL cholesterol without affecting HDL concentrations [15,47,103,104].

Fiber viscosity, rather than the quantity consumed, is essential in reducing total cholesterol and LDL cholesterol [105]. Researchers conducted a three-arm experiment to compare the lipid-lowering effects of different dietary fibers: wheat bran with low-viscosity, psyllium with medium-viscosity, and a high-viscosity fiber blend [105]. Reduction in LDL cholesterol was greater with the highviscosity fiber blend compared with the medium-viscosity psyllium and low-viscosity wheat bran, despite participants consuming a lower quantity of the high-viscosity fiber blend [105]. Confirming these findings, other low-viscosity fibers (acacia gum or fruit juice) did not change plasma lipids or lipoprotein cholesterol concentrations [98,106,107]. A randomized, single-blind study of 57 healthy participants found that consuming 30 g daily of RS (from either raw or retrograded high-amylose cornstarch) for three weeks did not significantly affect serum cholesterol or triacylglycerol levels compared with a glucose supplement [108].

Dietary Fiber Reduces Insulin Resistance and Glycemic Blood Sugar

Blood glucose, and insulin levels and sensitivity are associated with diabetes and numerous diseases such as nerve damage,

kidney disease, eye problems, and cardiovascular issues [109.110]. Research has demonstrated that consuming foods with a highglycemic index is correlated with increased mortality and cardiovascular disease risk compared with diets with a lower glycemic index [110]. High-glycemic index foods interact with insulin resistance in individuals with higher BMI to amplify postprandial blood sugar responses, potentially exacerbating adverse health outcomes through this synergistic effect [110]. Insulin resistance can also alter systemic lipid metabolism, which then leads to the development of dyslipidemia due to high levels of plasma triglycerides, low levels of HDL, and high levels of LDL, leading to plaque formation [111].

Large cohort studies have found that high-fiber whole grains (brown rice, rye, oats, wheat bran) are strongly associated with lower diabetes risk [19,112]. However, fiber from fruits and vegetables does not appear to have as strong an association [19]. Several studies supported the finding that a high-fiber diet helps control blood glucose and insulin resistance, which is vital because these are risk factors for diabetes and cardiovascular disease, see Table 3.

Table 3: Perspectives, clinical studies, and meta-analysis assessing blood glucose reduction and insulin resistance reduction effects of dietary fiber.