All fibers are resistant to digestion in the small intestine, meaning they arrive at the colon intact (1). Although most fibers are carbohydrates, one important factor that determines their susceptibility to digestion by human enzymes is the conformation of the chemical bonds between sugar molecules (glycosidic bonds). Humans lack digestive enzymes capable of hydrolyzing (breaking apart) most beta-glycosidic bonds, which explains why amylose, a glucose polymer with alpha-1,4 glycosidic bonds is digestible by human enzymes, while cellulose, a glucose polymer with beta-1,4 glycosidic bonds is indigestible.
Definitions of Fiber
Although nutrition scientists and clinicians generally agree that a healthy diet should include plenty of fiber-rich foods, agreement on the actual definition of fiber has been more difficult to achieve (2-4). In the 1970s, dietary fiber was defined as remnants of plant cells that are resistant to digestion by human enzymes (5). This definition includes a component of some plant cell walls called lignin, as well as indigestible carbohydrates found in plants. However, this definition omits indigestible carbohydrates derived from animal sources (e.g., chitin) or synthetic (e.g., fructooligosaccharides) and digestible carbohydrates that are inaccessible to human digestive enzymes (e.g., resistant starch) (6). These compounds share many of the characteristics of fiber present in plant foods.
* Fiber is a diverse group of compounds, including lignin and complex carbohydrates that cannot be digested by human enzymes in the small intestine.
* Although each class of fiber is chemically unique, scientists have tried to classify fibers on the basis of their solubility, viscosity and fermentability, in order to better understand their physiological effects.
* Viscous fibers, such as those found in oat products and legumes, can lower serum LDL cholesterol levels and normalize blood glucose and insulin responses.
* High fiber intakes promote bowel health by preventing constipation and diverticular disease.
* Large prospective cohort studies provide strong and consistent evidence that diets rich in fiber from whole grains, legumes, fruits and nonstarchy vegetables can reduce the risk of cardiovascular disease and type 2 diabetes.
* Although the results of case-control studies suggested that colorectal cancer was more prevalent in people with low fiber intakes, more recent findings from large prospective cohort studies and three clinical intervention trials do not support an association between fiber intake and the risk of colorectal cancer.
* While the results of case-control studies suggested that breast cancer was more prevalent in women with low fiber intakes, the majority of prospective cohort studies do not support an association between fiber intake and breast cancer risk.
* Numerous controlled clinical trials in people with type 1 and type 2 diabetes have found that increasing fiber intake improves blood glucose (glycemic) control and serum lipid profiles.
* In 2001, the Food and Nutrition Board of the Institute of Medicine established an Adequate Intake (AI) recommendation for total fiber intake. For adults who are 50 years of age and younger, the AI recommendation for total fiber intake is 38 g/day for men and 25 g/day for women. For adults over 50 years of age, the recommendation is 30 g/day for men and 21 g/day for women.
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