Structural and physical properties of dietary fibres, and consequences of processing on human physiology

Abstract

Dietary fibre encompasses very diverse macromolecules exhibiting a large variety of physico-chemical properties. They might be naturally present in the food (in cell walls of vegetables and fruits, for instance) or introduced in the food to improve their nutritional properties (e.g. bran in bread products) or their physical characteristics (e.g. gelling agents or improved resistance to storage). The properties that are nutritionally relevant are mainly the particle size and bulk volume, the surface area characteristics, the hydration and rheological properties, and the adsorption or entrapment of minerals and organic molecules. Amongst these properties, the viscosity and ion exchange capacity are the main contributors to metabolic effects (glucose and lipid metabolisms) whereas fermentation pattern, bulking effect and particle size are strongly involved in effects on colonic function. Technological treatments can modify the physico-chemical properties of the fibre. This can be further exploited to optimise both their techno-functional and physiological properties.

Introduction

Almost three decades ago, the dietary fibre hypothesis emerged amongst researchers working on the relationships between the diet and incidence of chronic diseases. Since the concept has stimulated a great deal of research and thrown new insight into the understanding of the cause of some common diseases, and especially those of the large bowel, in western countries. Historically the emphasis of research on fibre has concentrated on the effect of fibre in relation to the amount rather than the type present in the diet. Then, more and more comprehensive studies have been carried out combining in vitro, animal and human experiments, measuring acute and long term effects at the cellular to whole body levels. They have demonstrated that different sources of fibre can have different metabolic and physiological effects. The chemical and physical properties and fate during gut transit and fermentation of dietary fibre have been shown to determine their physiological effects and metabolic impact when they are consumed. In particular, the implication of the physicochemical properties of dietary fibre in their acute effects is now quite well read.

Much less is known about the implication of short chain fatty acids resulting from fibre fermentation and on the mechanisms sustaining the long-term effects of dietary fibre. To progress in this area, the knowledge on the human metabolic regulatory mechanisms must be increased. More attention must be paid to the diet habit as well as to the idiosyncratic physiology of the subject.

We provided here an overview of what was achieved during a concerted research programme funded by the European Commission (DGXII AIRIII-CT94-2203) known as PROFIBRE. The general objectives of the project were to update our knowledge of metabolic and physiological effects of dietary fibre in relation to their chemical and physicochemical properties and to address contemporary topic and developments.

For writing this paper, the authors considered the results of the PROFIBRE members as well as results presented by non-members in symposia organised within the framework of PROFIBRE. The review has been organised according to the working group structure of the project.