Variability in Fine Structures of Noncellulosic Cell Wall Polysaccharides from Cereal Grains : Potential Importance in Human Health and Nutrition

Cereal Chem. 87(4):272–282 Noncellulosic polysaccharides from the cell walls of cereal grains are not digested by human small intestinal enzymes and so contribute to total dietary fiber intake. These polysaccharides are becoming recognized increasingly for their potential to lower the risk of serious diet-related conditions such as type II diabetes, cardiovascular disease, colorectal cancer, and diverticular disease. The effectiveness of noncellulosic cell wall polysaccharides in improving health outcomes is related to the fine structure and associated physicochemical properties. The two most nutritionally relevant wall polysaccharides of cereal grains are the arabinoxylans and the (1-3,1-4)-β-D-glucans. These polysaccharides have high molecular mass values but are nevertheless soluble in aqueous media, at least in part, where they adopt highly asymmetrical conformations and consequently form high viscosity solutions. Thus, arabinoxylans and (1-3,1-4)β-D-glucans contribute to the soluble fiber component of human diets. The molecular size, solubility, and viscosity of the polysaccharides vary widely not only between different cereals but also within a single species. The variability in these properties reflects differences in the chemical structure of the polysaccharides, which in turn influences the beneficial effects of arabinoxylans and (1-3,1-4)-β-D-glucans in human diets. Here, we summarize information on the variability of fine structures of the arabinoxylans and (1-3,1-4)-β-D-glucans in common cereals and relate these to solubility, viscosity, and health benefits. The recent identification of genes involved in the biosynthesis of the (1-3,1-4)-β-D-glucans opens the way for the genetic improvement of cereal quality parameters that are important in human health. Cereal grains contain macronutrients such as protein, fat, and carbohydrate that are required by humans for growth and maintenance, but cereals also supply important minerals, vitamins, and other micronutrients that are essential for optimal health. Indeed, a major proportion of these nutrients for humans is obtained from foods prepared from cereal grains including rice (Oryza sativa), wheat (Triticum aestivum), barley (Hordeum vulgare), sorghum (Sorghum bicolor), and the millets (Panicum miliaceum and Pennisetum americanum). In traditional agrarian societies, cereal starches remain the principal sources of energy, where they are largely consumed as whole grains, which includes the pericarp-seed coat and the aleurone layer and so are relatively high in fiber content. This is not the case in industrialized countries where refined, low-fiber foods are the norm (Marquart et al 2007). It is becoming clear that whole grain cereal foods in general have considerable potential to improve human health and to substantially lower the risk of serious, diet-related diseases. One of the most important components of whole grain cereals from this standpoint is a complement of indigestible complex carbohydrates (dietary fiber). Within the grain nonstarch polysaccharides (NSP), resistant starch (RS), and oligosaccharides (OS) are the major contributors to total dietary fiber (TDF). Of these, human intestinal enzymes can digest only starch. All other plant polysaccharides have a monomeric composition or stereochemistry that renders them resistant to human digestive enzymes. Thus, arabinoxylans and (1-3,1-4)-β-D-glucans are NSP that constitute the noncellulosic component of cell walls and cell wall residues in cereal grains, but they are not digested by human enzymes, and make up an important proportion of dietary fiber in many diets. Dietary Fiber and Human Health Dietary fiber reduces the risk of contracting serious human diseases and reduces the adverse social and personal impact of conditions such as colorectal cancer (CRC), cardiovascular disease (CVD), and diabetes. These are global issues and are established as the main causes of morbidity and mortality in affluent, developed economies (Jemal et al 2005). There is strong evidence that they are also emerging as serious problems in developing countries through growing affluence that is flowing from industrialization (Mascie-Taylor and Karim 2003). This time-trend not only supports the role of diet and lifestyle as risk factors for these conditions, but is also informative as to opportunities for prevention. The strongest evidence for the contribution of dietary fiber to disease prevention comes from prospective cohort studies such as the European Prospective Investigation into Cancer and Nutrition (EPIC). Typically, these involve the assessment of the diet and lifestyle characteristics of large groups of people who are monitored for an extended period of time and are also assessed for clinical events. Data from EPIC show very strong, dose-dependent reduction in the risk of CRC with greater dietary fiber consumption (Bingham et al 2003). Other studies have shown similar re1 Australian Centre for Plant Functional Genomics, School of Agriculture, Food and Wine, University of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia. 2 CSIRO Food Futures National Research Flagship, Kintore Avenue, Adelaide, SA 5000, Australia. 3 Australian Centre for Plant Functional Genomics, School of Botany, University of Melbourne, Parkville, VIC 3052, Australia. 4 Corresponding author. Fax +61-8-8303-7102. E-mail: geoff.fincher@adelaide.