Cereal Foods World 42(8):690-694

Full-text article: Publication no. W-1997-0714-03F

Rye Dietary Fiber and Fermentation in the Colon. K. E. Bach Knudsen, H. N. Johansen, and V. Glitsų, Danish Institute of Agricultural Sciences, Tjele, Denmark. Copyright 1998 American Association of Cereal Chemists, Inc.

Rye is an important dietary carbohydrate source for humans in northern and eastern Europe. For instance, the present intake level makes rye breads the foremost important dietary fiber (DF) source in Denmark and Finland. The content of DF is two to three times higher in whole-meal rye bread than in white wheat bread; arabinoxylans being the predominant DF component. Studies with people who have had ileostomies show that most nonstarch polysaccharides in rye escape digestion in the small intestine, along with small amounts of starch and fructans. The ileal effluent also contains significant quantities of nitrogen, fat, and bile acids and minor quantities of plant lignans. In the large intestine, a significant fraction of the carbohydrates and some protein are broken down by microbial fermentation to short-chain fatty acids and a variety of other metabolites. In vitro and in vivo data suggest that the fermentation of rye DF yields molar proportions of acetate, propionate, and butyrate of about 60:17:18, which are not different from those found in wheat bread. The concentration of fecal free bile acids was lower in subjects fed whole-meal rye bread compared with those consuming wheat-based diets. During the fermentation process in the large intestine, the plant lignans matairesinol and secoisolariciresinol are converted to the biologically active mammalian lignans enterolactone and enterodiol. Unfermented DF residues affect the bowel habit, leading to a higher luminal and fecal bulk. It is likely that the higher intake of DF, derived primarily from whole-meal rye, in Denmark and Finland compared to the United Kingdom is responsible for the higher stool weight and faster transit time in the former as compared to the later population.