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Digestive Health Summit – March 2005: Abstracts
Fiber and Gastrointestinal Health
Daniel D. Gallaher, PhD
University of Minnesota
Dietary fiber, which consists of various indigestible polymeric carbohydrates derived from plant sources, have a number of important physiological effects. These include well-established effects such as improved laxation, reduction in serum cholesterol, and improved glucose tolerance. Other effects, such as reduced risk for colon cancer, increased satiety, and stimulation the immune system remain under investigation.
Two attributes of dietary fiber seem to be responsible for most physiological effects - susceptibility to fermentation and viscosity. Fermentable fibers, such as guar gum, pectins, and ß-glucans, produce within the large intestine increases in short chain fatty acids, acidification of its contents, and changes in bacterial enzyme activities. The physiological significance of the increase in short chain fatty acids (mainly acetate, propionate, and butyrate) remains uncertain, in spite of intensive study. The concept that propionate is responsible for the cholesterol lowering effect has little experimental support. Butyrate has been found to inhibit growth of transformed cell lines and has been promoted as a chemoprotective agent. However, in vivo, little evidence exists to support this. Resistance to fermentation, in contrast, is responsible for the best established of all physiological effects of dietary fiber, improved laxation. Dietary fibers or fiber-rich sources such as cellulose, wheat bran, and some vegetables produce large increases in stool weight.
Viscosity, in contrast to fermentation, is clearly associated with reduction in serum cholesterol. Different dietary fibers exhibiting approximately the same intestinal contents viscosity have approximately the same cholesterol lowering ability, regardless of their fermentability. Further, the cholesterol lowering effect is proportional to the intestinal contents viscosity. Interestingly, the mechanism by which viscosity leads to cholesterol lowering may be different for different viscous fibers. For some, enhanced bile acid excretion seems responsible, for others a reduction in cholesterol absorption. Viscosity also seems responsible for the improvement in glucose tolerance. The evidence best supports an effect on delaying gastric emptying as the mechanism for this effect.
Dietary fiber clearly has a major role as a component of a healthy diet. A mixture of dietary fibers, providing types that are resistance to fermentation to promote laxation, and viscous fibers to reduce blood lipid concentrations and improve glucose tolerance is most desirable.
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