Submitted to: Mutation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2007
Publication Date: 3/30/2007
Citation: Funk, C., Braune, A., Grabber, J.H., Steinhart, H., Bunzell, M. 2007. Model studies of lignified fiber fermentation by human fecal microbiota and its impact on heterocyclic aromatic amine adsorption. Mutation Research. 624:41–48. Interpretive Summary: Lignin is natural polymer found in many types of dietary fiber and it acts to cement polysaccharides, protein, and other components in fiber together. In a previous work, we found that dietary fiber containing lignin was especially effective for adsorbing some types of carcinogens found in cooked meat or fish. In this laboratory experiment, we examined how shifts in pH (acid vs alkaline conditions) and fermentation of fiber during intestinal digestion influenced its binding of carcinogens. For this experiment, we generated a wide array of corn (Zea mays L.) fibers containing carefully defined quantities and types and of lignin. We found that increasing the lignin content of fiber from 0.4 to 14% increased carcinogen binding by two- to three-fold. Lignin also greatly diminished the fermentation of fiber. In contrast, chemical makeup of lignin had small effects on the fermentation of fiber and its capacity to bind carcinogens. We observed that shifts in pH and fermentation of fiber may somewhat diminish the adsorption of some types of carcinogens in the large intestine. Therefore, future studies should examine the fate of free and fiber-adsorbed carcinogens in the gut to further clarify the role of lignified fiber in preventing cancer.
Technical Abstract: This study examined how shifts in pH and fiber fermentation may alter the adsorption of mutagenic heterocyclic amines (HAAs) 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-9H-pyrido[2,3-b]indole (AaC) to dietary fiber in the human small intestine and colon. Nonlignified and artificially lignified maize cell walls were fermented in vitro with human fecal microbiota for 0, 8, or 24 h. We then assessed the adsorption of HAAs to unfermented fiber at pH 6.5 and to unfermented and fermented fibers at pH 7.4 to mimic conditions in the small intestine and colon, respectively. HAAs were effectively adsorbed to lignified fiber by up to 74 % at pH 6.5 and by up to 68 % at pH 7.4. Increasing the lignin content of unfermented fiber from 0.4% to about 14 % increased HAA adsorption by 2 to 3 fold. This increase in lignification reduced microbial fiber degradation from 51% to a minimum of 8% after 24 h of fermentation, whereas variations in the guaiacyl and syringyl makeup of lignin had smaller but significant impacts on fiber degradation. A 24 h fermentation decreased AaC adsorption to lignified fiber at pH 7.4 by up to one-third, while PhIP adsorption was not affected. Our results indicate that lignification increases the adsorption of hydrophobic HAAs to fiber but shifts in pH and fermentation may somewhat diminish adsorption of some HAAs as fiber passes from the small intestine through the colon.