Beyond dietary fiber.

The complexity of plant foods high in dietary fiber poses new challenges to clinical investigators and leads to many study-design dilemmas. There are basic differences in studying purified polymers, highly concentrated but not purified fibers, and diets high in high-fiber whole foods. The fibrils of the plant cell wall are most likely altered when prepared as a pure chemical entity, and when fiber concentrates (eg, wheat bran) are used, the method of preparation may alter the composition of the final product. Whole-plant, high-fiber foods are complex storehouses of a diversity of polymers, including resistant starch, and of bioactive compounds. Furthermore, the addition of a reasonable amount of high-fiber food to the diet not only adds dietary fiber but many digestible, caloric macronutrients that alter the entire diet composition. These problems and dilemmas are reviewed.

Adsorption of a hydrophobic mutagen to dietary fibre from the skin and flesh of potato tubers.

One of the theories to explain the protective action of some dietary fibres against colon cancer is that certain mutagens and/or cancer promoters are adsorbed to these dietary fibres making the mutagens and/or cancer promoters less available to gut mucosal cells. The abilities of 2 contrasting cell wall preparations (dietary fibre preparations) from potato tubers to adsorb in vitro the hydrophobic mutagen, 1,8-dinitropyrene (DNP), were studied using an incubation mixture containing DNP in phosphate-buffered saline (PBS). Walls from potato skins strongly adsorbed DNP and, at the highest wall concentration tested, only a small porportion of the DNP remained in solution. In marked contrast to the skin walls, potato flesh walls adsorbed only a small proportion of the DNP. Unexpectedly, the flesh walls also caused a large increase in the proportion of DNP found in solution. When flesh walls were pre-extracted with PBS, the ability of the extracted walls to bind DNP increased. The material extracted from the flesh walls was able to maintain DNP in solution, when added to the incubation medium in the absence of cell walls. Pectic polysaccharides appear to be the soluble component responsible for maintaining the DNP in solution. Competition between soluble and insoluble fibre components may have major implications for the availability and distribution of hydrophobic mutagens in the alimentary tract.

Effects of solvent extraction on the hypocholesterolaemic action of oat bran in the rat.

In adult male rats fed on a cholesterol-free synthetic diet, plasma cholesterol concentrations were lowest with oat bran, intermediate with cellulose and highest with wheat bran. Plasma triacylglycerols (TAG) were similar with wheat bran and cellulose but higher with oat bran. The concentrations and pools of caecal volatile fatty acids (VFA) were lowest with cellulose and equally higher with oat bran and wheat bran. Plasma VFA concentrations in the hepatic portal vein reflected those in caecal digesta and were unrelated to plasma cholesterol. Feeding oat bran after extraction with n-pentane gave plasma cholesterol concentrations similar to that found with wheat bran. Reconstitution of oat bran with extracted lipids did not restore the cholesterol-lowering effect. Addition of the extracted material to a wheat-bran diet had no effect on plasma cholesterol. Plasma TAG were higher with the oat bran and reconstituted-oat-bran diets than with wheat-bran or cellulose diets. However, extracted oat bran + safflower oil gave similar TAG concentrations to that with wheat bran. These extractions and additions did not change caecal bile acid or neutral sterol concentrations. Effects of these diets on plasma cholesterol were unrelated to their tocotrienol or tocopherol content. Addition of n-pentane to oat bran followed by evaporation of solvent gave plasma cholesterol concentrations that were significantly higher than untreated oat bran but lower than similarly treated wheat bran. It is concluded that oat bran affects cholesterol metabolism through a pentane-soluble component as well as non-starch polysaccharides. It appears that the activity of this lipid is not transferable by simple addition of the solvent extract to the whole diet.

In vitro binding of estrogens by dietary fiber and the in vivo apparent digestibility tested in pigs.

Within the framework of experiments related to the association between dietary fiber and breast cancer an in vitro test system was used to study the binding of estrogens to various fibers (e.g. cholestyramin, lignin and cellulose) and fiber sources (e.g. wheat bran, cereals, seeds and legumes). Furthermore, the in vivo apparent digestibility of the different fiber sources was tested using a mobile nylon bag technique in intestine-cannulated pigs. Estradiol-17 beta (E2) bound more strongly to the various fibers than did estrone (E1), estriol or estrone-3-glucuronide. At increasing pH (greater than 7) binding of both E1 and E2 to wheat bran decreased significantly. Cholestyramine and lignin bound almost all estrogens present in the medium. Linseed (91%), oats (83%), barley chaff (88%) and wheat bran (82%) are other excellent binders of E2. Corn, rye and white wheat flour showed lower binding capacity with a relatively low affinity. Cereals with the highest percentage of lignin in the fiber (greater than 3%) were also the fiber sources with the lowest apparent digestibility. Estrogens bound with the highest affinity (relative to bovine serum albumin) to these fiber sources. Together with wheat bran and lignin, oats, linseed and soybean seem to be products with good perspectives for in vivo evaluation of the lowering effect of dietary fiber on estrogen exposure of estrogen-sensitive tissues.

Adsorption of a hydrophobic mutagen to five contrasting dietary fiber preparations.

The ability of five plant cell wall (dietary fiber) preparations with contrasting compositions to adsorb in vitro the hydrophobic, environmental mutagen, 1,8-dinitropyrene (DNP), was investigated. Many of the fruits and vegetables in Western diets are from dicotyledonous (broad leaved) plants and the dietary fiber from these consists mainly of unlignified cell walls. A representative of this wall type, prepared from immature cabbage leaves, showed little ability to adsorb DNP. Two other cell-wall preparations, representing lignified walls of dicotyledons and unlignified walls of vegetative parts of grasses and cereals (monocotyledons belonging to the family Poaceae), adsorbed DNP much more effectively. However, two further preparations, representing suberized walls of cork cells and lignified walls of vegetative parts of grasses and cereals, were the most effective in adsorbing DNP. Extrapolation of these data to the in vivo situation would indicate that increased consumption of the vegetative parts of grasses or cereals and plant material containing cork cells, for example potato skins, could be effective in removing hydrophobic mutagens from potential contact with colonic mucosal cells.

Physicochemical properties and physiological effects of the (1----3)(1----4)-beta-D-glucan from oats.

In summary, despite the many uncertainties regarding mode of action, these results have established that the soluble dietary fibre from oats, mainly (1----3)(1----4)-beta-D-glucan, is physiologically active in a fashion similar to guar gum. Extraction processes reduce the molecular size of the beta-glucan relative to the native cell wall polymer, and the product was of lower viscosity than a guar gum used for comparison, but this did not appear to affect either in vivo or in vitro activity under the conditions used. There remains a need to develop a clearer picture of the relationship, if any, of viscosity to activity, through dose response studies with products of different viscosity.

Utilization of purified cellulose in fiber studies.

Purified cellulose-type fiber products are widely used in experimental nutrition. Their use in a broad spectrum of studies may potentially lead to the acceptance of the misconception that the various commercially available cellulose products are equivalent. In this paper we have attempted to show that this is not the case. The comparative structural data of Table 2 and the compositional data of Olsen et al provide examples which indicate that purified cellulose preparations should not necessarily be considered equivalent. Unfortunately, our current lack of understanding of how fibers are metabolized and how they may affect specific physiological parameters makes it difficult to determine which, if any, of the measurable structural and chemical properties will be of relevance for a given in vivo study. At present, it appears that researchers utilizing/evaluating the consequences of consuming a purified cellulose-type fiber would be prudent to provide at least a limited amount of data on the properties of the cellulose preparation used in their studies. The characterization of the cellulose product may be done by a variety of methods depending on the expertise of the laboratory. The methods and results discussed in this paper provide an example of the type of information which may be obtained from an in vitro characterization of cellulose products.

Improved methods for analysis and biological characterization of fiber.

Dietary fibers are not uniform, chemically or in their nutritive and biological properties, the only common ground being their resistance to mammalian digestive enzymes. The AOAC method for total fiber is subject to inferences from ash, protein, tannins and resistant starches. These interferences can be reduced by urea enzymatic dialysis. The measurement of soluble and insoluble fiber is nutritionally relevant, since physical properties greatly modify dietary effects of fiber. Insoluble fiber is conveniently measured as neutral-detergent fiber. This procedure has been improved by reducing the starch interference and the time of analysis. Physical and biological properties of dietary fiber can be measured by using relevant procedures for hydration capacity, metal ion exchange capacity and rate of fermentation. The lignin and tannin content modify the characteristics of dietary fiber.