Characterisation of Dietary Fibre Properties to Optimise the Effects on Human Metabolism and the Transcriptome

Abstract: It is well established that dietary fibre, especially soluble dietary fibre, has beneficial effects and can prevent diseases associated with the modern lifestyle. This has been explained by the viscous effects of soluble fibre, which can reduce or delay the absorption of carbohydrates and fat in the small intestine, resulting in lower blood concentrations of glucose, insulin and cholesterol. Oats, rye, sugar beet fibre and barley, all recognised for their high content of soluble fibre, were investigated in this work.

The ability of fibres to form viscous solutions is determined by the concentration, solubility, the polymer molar mass and aggregate formation. The fibre releasability (solubility) and solution behaviour were analysed under physiological conditions to examine the effects of processing, the surrounding matrix and gastrointestinal conditions. Less than half of the β-glucans was released from oat bran when analysed with an in vitro method imitating gastrointestinal digestion, and the same was found regarding the release of pectin from coarse particles of sugar beet fibre. Reducing the particle size by milling improved the releasability, while mixing the milled fractions with a solid food matrix (protein, fat or starch) decreased or delayed it. The effect of food processing on barley β-glucan aggregation was studied using asymmetrical flow field-flow fractionation. Boiling was shown to disrupt the largest aggregates, while higher temperatures also seemed to degrade the polymer chain. Freeze-thaw cycles resulted in cryogelation, which could lead to reduced viscosity. The low pH in the in vitro gastric digestion phase also disrupted the aggregates, but this was followed by re-aggregation/gelation at the neutral pH mimicking the conditions in the small intestine. This may entrap nutrients in the aggregate or gel matrix, thereby reducing absorption.

A human study was performed with oats, rye and sugar beet fibre, and mixtures of these three. Fibre-rich meals were optimised to promote high releasability of fibre and retention of aggregates. Changes in traditional biomarkers, as well as in the transcriptome, in response to the fibre-rich meals, were examined in healthy men and women. The postprandial glucose levels were lowered by rye bran, while oat bran lowered the insulin concentrations. The lowering effects may be related to fibre source, the amount of soluble and insoluble fibre and pre-processing of the fibre. In contrast, a spray-dried oat drink increased the postprandial levels of insulin, possibly due to its low fibre content in combination with certain amino acids and a high amount of carbohydrates in the liquid matrix. Women showed, on average, a more pronounced glucose lowering response than men, indicating that different amounts of dietary fibre should be recommended for men and women. Changes in gene expression in peripheral blood mononuclear cells after the intake of oat bran were investigated with microarray analysis. Gene sets and pathways related to insulin secretion were significantly suppressed in response to the oat bran meal compared with the control meal.

In conclusion, many factors may influence the properties of dietary fibre. It is therefore valuable to use in vitro methods to simulate gastrointestinal digestion to characterise and optimise fibre functionality prior to testing their effects on human metabolism and the transcriptome.