Dietary Fibre - ILSI Argentina

12 Concise Monograph Series
accepted into this category only on the basis of evidence.
Finally, the Agence Française de Sécurité Sanitaire des
Aliments includes a physiological definition of dietary
fibre – namely that it is neither digested nor absorbed in the
small intestine. Unlike previous definitions, it also
specifies that to be defined as dietary fibre, a substance
should have at least one other physiological property, to
increase stool production, to stimulate colonic
fermentation, to reduce fasting blood cholesterol levels,
or to reduce blood glucose or insulin levels.
The Codex Alimentarius Commission has developed
this definition further in 2006. It specifies that dietary
fibre means carbohydrate polymers with a degree of
polymerisation not lower than 3 which are neither digested
nor absorbed in the small intestine. It includes edible
polymers naturally occurring in food (of either plant or
animal origin) and carbohydrate polymers that are
either extracted from food raw material or are
synthesised. It notes that fibres of plant origin may
include associated materials such as lignin or other
compounds, if these compounds are quantified by the
AOAC method for total dietary fibre.
The proposed Codex definition includes the important
criterion of non-digestibility in the human small
intestine and specifies a range of physiological benefits
of different fibre components. Its main advantage over
earlier analytically based definitions is that it allows for
the fact that there are greater similarities in terms of
physico-chemical or physiological characteristics
between, for example, resistant starch, non-digestible
oligosaccharides and fermentable non-starch polysaccharides,
than between the category of non-starch
polysaccharides as a whole. It is therefore more
meaningful in physiological terms.
The Health Council of The Netherlands (2006) defined
dietary fibre as substances that are not digested or absorbed
in the human small intestine, and which have the chemical
structure of carbohydrates, compounds analogous to
carbohydrates, and lignin and related substances. The debate
continues and some recent opinion has favoured a return
to the original fibre definition along the lines that 'Dietary
fibre consists of intrinsic plant cell wall polysaccharides'.
Soluble and insoluble dietary fibre
Early chemistry of non-starch polysaccharides extracted
different fibre fractions by controlling the pH of solutions;
in this context the terms soluble and insoluble fibre
evolved. They provided a useful simple categorisation of
dietary fibre with different physiological properties, as
understood at the time. On the one hand, there are fibres
that principally affect glucose and fat absorption.
Historically, these were referred to as soluble because
many of them were viscous and formed gels in the small
intestine (e.g. pectins and ß-glucans). In contrast, types of
dietary fibre with a greater influence on bowel function
were referred to as insoluble (including cellulose and
lignin). It is now apparent that this simple physiological
distinction is inappropriate because some insoluble fibre is
rapidly fermented and some soluble fibre does not affect
glucose and fat absorption. As the terms soluble and
insoluble may be misleading, the World Health Organization
and the Food and Agricultural Organisation recommended
already in 1998 that they should no longer be used.