Winter 2000 Issue — Cereal arabinoxylans
A natural fibre that presents many attractive effects for preventive nutrition

Denis Despres

Limagrain, France

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Many common diseases in western countries are considered to be due to a deficiency in dietary fibre (DE). Cereal products are excellent sources of DF: they contain cellulose, hemicelluloses, oligosaccharides and resistant starch. Insoluble DF such as cellulose or a part of hemicellulose are of limited nutritional effects because of their low degradability in most monogastric species. By contrast, soluble DF is fermented by the large intestine microflora, and the resulting production of short-chain fatty acids (SCFA) may be involved in the metabolic effects of fibres, which include a decrease of serum cholesterol (Hunninghake et al 1994; Vidal­Quintanar et al 1997) and an enhancement of mineral absorption (Ohta et al 1995; Lopezx et al 2000). Recently, soluble arabinoxylan, a hemicellulose extracted from corn bran [Unit6 de Laboratoire pour l'Innovation dans los C6rdales, ULICE] presents many attractive effects for preventive nutrition in rat model (Lopez et al 1999). The structure of arabinoxylan is composed of xylose, arabinose, galactose and uronic acid. The molecular weight is 250 QOOg/mol. Arabinoxylans produced by the extraction process are highly soluble in water (20%). Arabinoxylan provides low viscosity solution and are compatible with most food ingredients.

Extracted Corn Arabinoxylans are Fermentable Hemicellulose

In rats, the effects of DF on the hypertrophy of the caecum (the first part of rat colon) tends to be proportional to their fermentability rather than to their accumulation in this organ. The incorporation of 8% corn arabinoxylan causes an enlargement of the caecum and hypertrophy of the cecal wall. Moreover, arabinoxylan ingestion induces active bacterial proliferation and it produces high concentrations of SCFA in the large intestine. One of the most striking effects of soluble arabinoxylan is a very high percentage of propionate (up to 45% in molar percentage), together with a low percentage of butyrate. Extensive conversion of DF into propionate is an interesting feature because propionate or its acylCoA derivatives affect various metabolic pathways (gluconeogenesis, ureagenesis, ketogenesis). For instance. propionate inhibits chloesterologenesis and lipogenesis from acetate (Demigné et al 1995).

Corn Arabinoxylans Enhances Mg Balance

Furthermore, it was shown previously thu various resistant starch or oligosaccharici~ stimulate Mg absorption (Ohta et al 199~ Lopez et al 2000). Thus these fermentab~ carbohydrates are able to modify the bivalent cation utilisation and conversely these cations may play a role in SCFA neutralization. As for resistant starch or frutooligosaccharides, corn arabinoxylans strongly stimulate Mg balance. Fermentabii arabinoxylans favours mineral absorption ar the distal parts of the digestive tract in several ways: hypertrophy of the cecal wa and larger surface exchange area, increas of soluble Mg, and accelerated blood flov It is also possible that SCFA directly influence mineral absorption. Cereal soluble arabinoxylans can improve.

Corn Arabinoxylans are Effective in Lowering Plasma Cholesterol

Fermentable carbohydrates do not only exert a role in maintaining symbiotic fermentation, but they also exert a significant plasma cholesterol, the incorporation of arabinoxylans has a significant lower effect on the liver cholesterol (-40%) compared to rats fed control + cholesterol.

Table 1. Effects of arabinoxylans on plasma and hepatic cholesterol concentrations.

Moreover, analysis of the plasma lipoprotein shows a drastic reduction of cholesterol in the LDL ("bad cholesterol") without modifying HDL ("good cholesterol") in rats fed the DE The hypocholesterolemic effect can be explained by several ways.

When the diet does not contain cholesterol, arabinoxylans clearly increase the elimination of bile acids and neutral sterols. As a result, there is a marked induction of the liver 3-hydroxy-3-methylglutaryl-CoA reductase, the hepatic enzyme controlling the cholesterol synthesis. This induction is the result of the fiber-mediated diversion of the cholesterol body pool toward fecal steroid excretion. However, in spite of this adaptative response, soluble cereal arabinoxylans exect a significant plasma cholesreol lowering effect. When the diets contain cholesterol, the hypocholesterolemic mechanisms of arabinoxylans are different. The major effect of these complex carbohydrates is to reduce dietary cholesterol absorption at a weak level: 13% in rats adapted to arabinoxylan diet versus 50% in control rats.

In conclusion, even if a direct extrapolation to humans is still questionable, soluble corn bran arabinoxylans can be considered as a prebiotic product: they have a great impact on intestinal fermentation, mineral absorption and cholesterol metabolism in rats. As arabinoxylans are entirely soluble, they may be easily incorporated into foodstuffs such as frutooligosaccharides. In contrast to FOS, arabinoxylans have an important molecular weight that delays their degradation rate, leading to physiologic fermentation at moderately acidic pH.

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