Changing concepts of dietary fiber: Implications for carcinogenesis

The dietary fiber (DF) hypothesis suggested that DF, as plant cell walls, p rotected against colorectal cancer. The implicit assumption in much histori c literature was that the readily analyzed and quantified nonstarch polysac charide (NSP) component was critical in cancer protection. However, the pre sence of polymeric phenolic components such as lignin or suberin has profou nd effects on the physicochemical properties of the cell walls and largely determines their physiological properties in humans. In certain groups of f ood plants, degradation of cell walls that contain neither lignin nor suber in releases ferulic acid and other hydroxycinnamic acids. These acids have antioxidant, antimutagenic, and other anticancer effects, including modulat ion of gene expression and immune response. Reexamination of literature on cancer protection suggests that plant cell walls containing significant amo unts of phenolic components may be the most likely to protect against cance r. In the last 30 years, the definition of DF has been incrementally expand ed to incorporate NSP extracted from plant cell walls and from sources othe r than plant cell walls, then resistant starch, and most recently nondigest ible oligosaccharides. On the early definitions, increased consumption of D F could only be achieved by increasing the intake of whole grains, brans, o r other food plant material that would increase the intake of plant cell wa lls containing significant amounts of phenolic components. However, the new definitions make it possible to increase "dietary fiber" without consuming any such materials. We suggest that this could have negative connotations for cancer risk in human populations.