Dietary Fiber

Dietary fiber consists of non-digestible forms of carbohydrate, usually polysaccharides, that originate from plant-based foods. ^[1] It escapes digestion because humans lack the digestive enzymes required to break it down, and it is classified by water solubility as soluble or insoluble fiber. ^[2] Soluble fibers (pectin, gums, mucilage, fructans and some resistant starches) are found in some fruits, vegetables, oats and barley, whereas insoluble fibers (mainly lignin, cellulose and hemicellulose) are abundant in whole-grain foods, bran, nuts and seeds. ^[2] More recent definitions also include oligosaccharides such as inulin and fructo-oligosaccharides on the basis of their physiological effects, and these are the best known prebiotics. ^[3] Physicochemical properties such as solubility, viscosity and fermentability vary greatly with the origin and processing of a fiber and are important determinants of its functional characteristics and clinical utility. ^[4] The recommended dietary fiber intake for children and adults is 14 g/1000 kcal. ^[5] The recommended daily allowances for total fiber intake for men and women aged 19-50 are 38 gram/day and 25 gram/day, respectively. ^[2]

Because human alimentary enzymes cannot digest most complex carbohydrates and plant polysaccharides, these are metabolized by gut microbes that generate short-chain fatty acids (SCFAs), including acetate, propionate and butyrate. ^[6] SCFAs can directly activate G-protein-coupled receptors, inhibit histone deacetylases and serve as energy substrates. ^[7] SCFAs are important fuels for intestinal epithelial cells, promote their secretion of glucagon-like peptide-1, and affect intestinal motility, barrier function and host metabolism. ^[8] Soluble fibers lower blood cholesterol by several mechanisms, and beta-glucan, the major active compound of oats, has cholesterol-lowering and antidiabetic effects. ^{[2] [9]} Insoluble fibers may decrease intestinal transit time and increase fecal bulk, promoting digestive regularity. ^[2] Proposed mechanisms for weight management include promoting satiation, decreasing absorption of macronutrients and altering secretion of gut hormones. ^[10]

In a series of meta-analyses covering 185 prospective studies and 58 clinical trials, observational data suggested a 15-30% lower all-cause and cardiovascular mortality and incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer in the highest versus lowest fiber consumers. ^[12] In the same analysis, risk reduction was greatest at daily intakes between 25 g and 29 g, dose-response curves suggested further benefit at higher intakes, and the certainty of evidence for fiber was graded as moderate. ^[12] The clinical trials in that analysis showed significantly lower body weight, systolic blood pressure and total cholesterol with higher compared with lower fiber intakes. ^[12] An umbrella review of meta-analyses of observational studies covering 38 health outcomes and 17,155,277 individuals found convincing evidence for inverse associations of fiber intake with cardiovascular mortality, pancreatic cancer and diverticular disease, and highly suggestive evidence for all-cause mortality, cardiovascular disease and coronary heart disease. ^[13] Meta-analyses of randomized trials and guidelines found that foods high in soluble fiber, such as oats, barley and psyllium, caused at least moderate reductions in

Average intakes in US children and adults are less than half of recommended levels, and intake remains below recommended levels worldwide. ^{[5] [13]} For adults with diabetes, the authors of that meta-analysis proposed increasing daily fiber intake by 15 g or to 35 g as a reasonable target. ^[15] In a meta-analysis of randomized trials in adults with chronic constipation, fiber supplementation was effective, with psyllium, doses >10 g/d and treatment durations of at least 4 weeks appearing optimal, despite considerable heterogeneity. ^[16] Flatulence was more frequent with fiber supplementation than with control treatment. ^[16] The optimal dose, type and source of fiber for managing gastrointestinal symptoms and preventing gastrointestinal disorders remain unclear. ^[4] Different fiber sources and components, such as resistant starch and non-digestible oligosaccharides, should be expected to have characteristically different effects on physiology and disease risk.