The bane of Western culture, colorectal cancer is the second most common cause of cancer-related deaths, abdicating only to lung cancer. It is a disease largely modulated by environmental factors, and much current belief centres around the notion that dietary fibre may play a role in the prevention of colorectal cancer. To this end, many years and many studies have been dedicated to the elucidation of mechanisms involved in the commencement and development of colonic tumours, that they might provide valuable insights into putative treatments and preventive measures that can be taken to help reduce the cancer rate. From the time that Burkitt (1970) first posited that dietary fibre possessed cancer protective, cancer-preventive properties, a great number of studies have been conducted, with equivocal results. While the precise mechanisms involved have not yet been elucidated, it is apparent that populations with fibre-rich diets generally exhibit the lowest rates of colorectal cancer.
Alabaster et al . (1996) indicate that colorectal cancer in humans is preceded in 50% of tumours by a point mutation in the gene, K-ras . It is considered to be a genetic hallmark signalling increased risk of developing colorectal cancer (Sills et al ., 1999). K-ras mutations are reliable indicators for cancer development potential in that point mutations at these genes greatly increase the risk of developing cancer in a significant number of cases. Kubrusly et al . (1999), for example, cite a 90% observation rate in the case of pancreatic adenocarcinomas. Sills et al . (1999) illustrate the importance of the K- ras gene in mice using 1,3-butadiene, a potent carcinogen. They remarked an 80% mutation rate in K- ras genes in lung neoplasms compared with a latent 30% background mutation rate.
That colonic cells already experience some of the highest turnover rates in the human body (Wasan and Goodlad, 1996) itself only facilitates cancer development. In fact, Wasan and Goodlad (1996) propose that this hyperproliferation, itself, is an important early stage in the development of cancer–something already manifested in the metabolic processes of colonic cells.
Definition of Dietary Fibre
Long the subject of debate, there has never been a consensus regarding a precise definition of “dietary fibre.” Indeed, the term itself is a misnomer–many components included in this family are fibrous at all. There are, however, certain broad definitions which upon which many classification schemes have been based.
Dietary fibre has been defined to include components of plant cell walls and the components of these cell walls (Harris and Ferguson, 1999), such as cellulose and other structural components. Indeed, however, this classification necessarily includes cell wall components and their derivatives, such as pectins and carboxymethyl cellulose, a chemically-modified cell wall constituent. While one of the basic tenets of the classification schemes used to identify dietary fibres has been that the substance be undigestible by the human alimentary tract, this criterion is in itself a source of great variability and debate, since materials may be undigestible by enzymes produced by the human body, yet digestible by indigenous microorganisms which inhabit the gut. Indeed, in Western diets, plant cell walls contribute roughly 95% of the daily intake of dietary fibres (Harris and Ferguson, 1999). Classically, non-starch polysaccharides from sources other than plants have also been included in this definition, such as those originating from micro-organisms, seaweeds, and exudates. The inclusion of such materials as gums and mucilages has further confounded the nomenclature of such materials, as these substances can no longer be characterized as being fibrous, either structurally or molecularly.
The introduction of non-fibrous members to the “dietary fibre” family naturally led to the subclassification of the family into two still broad species, insoluble fibres and soluble fibres. The determination of whether a compound is considered insoluble or soluble has been determined analytically by in vitro studies of solubility in water or other buffer solutions. This, in itself, lends great variability and debate as to whether a component can be classified as insoluble or soluble, and is highly dependent upon the methodology used.
Cellulose is one of the predominant plant cell wall-derived compounds considered dietary fibre. It is a linear molecule consisting of 1,4-linked beta-glucosyl residues (Harris and Ferguson, 1999). Cellulose microfibrils are roughly 3-10nm in diameter, plant species dependent. While vegetable or fruit cell walls may comprise 30-40% cellulose, cereals, known for their lignin content, may contain only up to 4% cellulose (Harris and Ferguson, 1999). It is not generally fermented by either human endogenous digestive enzymes or intestinal microflora present in the human alimentary tract (Wijnands et al ., 1999).
Pectins represent a complex family of soluble fibres that comprise a diverse array of polysaccharide domains known as “homogalacturonan” domains (Jarvis, 1984). Parenchymatous cells of fruits and vegetables are known to contain high concentrations of pectins, but cereals, as with cellulose, generally contain only very small amounts of pectins. There is evidence that pectins may play a protective role against cardiovascular disease by interfering with cholesterol metabolism (Schneeman, 1999; Brown et al ., 1999).
