Dietary fiber is plant-derived material that is resistant to digestion by human alimentary enzymes. Fiber may be divided into two broad chemical classes: 1) non-alpha-glucan polysaccharides (cellulose, hemicelluloses, and pectins) and 2) lignins. Dietary fiber behaves within the gastrointestinal tract as a polymer matrix with variable physicochemical properties including susceptibility to bacterial fermentation, water-holding capacity, cation-exchange, and adsorptive functions. These properties determine physiological actions of fiber and are dependent on the physical and chemical composition of the fiber. Fiber undergoes compositional changes as a consequence of bacterial enzymatic action in the colon. Dietary fiber is of clinical significance in certain disorders of colonic function and in glucose and lipid metabolism. Dietary fiber increases stool bulk by acting as a vehicle for fecal water and by increasing fecal bacterial volume. Use of fiber in the treatment of constipation and uncomplicated diverticular disease is well established. By increasing stool bulk, fiber also reduces the fecal concentration of bile acids and other substances. Certain types of fiber decrease the rate of glucose absorption and attenuate postprandial rises in blood glucose and insulin. Plasma cholesterol levels are reduced by mucilaginous forms of fiber. This effect appears to be mediated in part by an increase in fecal acidic sterol excretion.
膳食纤维是植物来源的物质，能抵抗人类消化酶的消化作用。纤维可分为两大类化学物质：1）非α-葡聚糖多糖（纤维素、半纤维素和果胶）和2）木质素。膳食纤维在胃肠道内表现为具有可变物理化学性质的聚合物基质，包括对细菌发酵的敏感性、持水能力、阳离子交换和吸附功能。这些特性决定了纤维的生理作用，并取决于纤维的物理和化学组成。由于结肠中细菌的酶促作用，纤维会发生成分变化。膳食纤维在某些结肠功能紊乱以及葡萄糖和脂质代谢方面具有临床意义。膳食纤维通过作为粪便水分的载体和增加粪便细菌体积来增加粪便量。纤维在治疗便秘和单纯性憩室病方面的应用已得到充分证实。通过增加粪便量，纤维还能降低粪便中胆汁酸和其他物质的浓度。某些类型的纤维可降低葡萄糖吸收速率，并减轻餐后血糖和胰岛素的升高。粘性纤维可降低血浆胆固醇水平。这种作用似乎部分是由粪便酸性固醇排泄增加介导的。
