It was proposed, more than 30 years ago, that DFs act as a physiological obstacle to energy intake by different mechanisms including: (a) displacement of available calories and nutrients from the diet; (b) increasing chewing - which limits intake by promoting saliva and gastric juice resulting in an increased satiety; and (c) decreasing the absorption efficiency of the intestine (Heaton 1973). During ad libitum energy intake, a mean loss of weight of about 1.9 kg was found to have occurred after 14 g per day of additional fibre intake, which may have been due to the 10% lower energy intake observed. This effect was more pronounced in obese subjects (Howarth et al. 2001). Pereira and Ludwig (2001) report that in most studies, DF intake was associated with beneficial effects on energy intake. Some studies report mixed or no effects on satiety (Pereira & Ludwig 2001). The composition of diet, the type of fibre, the dose, the time of administration, the subject characteristics (healthy or obese), as well as the method of satiety assessment, are confounding parameters rendering the analysis of food intake modulation following DF intake difficult.
The mechanisms by which DF intake modulates food intake and body weight are multiple and are interestingly described in a recent review (Slavin 2005). It is proposed that DF may have an effect on satiation (sensation of fullness during an eating period, leading to the cessation of eating) and/or satiety (sensation of fullness between eating episodes that tends to inhibit the resumption of eating) (Gerstein et al. 2004). The effect on satiation may be related to the intrinsic properties of DF-containing foods (such as their lower energy density), prolonged chewing and mastication, or their gelling properties in the stomach. It has been shown that DF forming gels in contact with acids (alginate, guar gum), increases the sensation of fullness, through the distension of the gastric antrum; this may occur without any modification of gastric emptying (Hoad et al. 2004). However, delayed gastric emptying may explain per se the satiating effect of other DFs (Slavin 2005). Satiation and satiety are under neuronal and hormonal control (Ritter 2004). Recently, progress has been made in the comprehension of the relation between events occurring in the gut, and the central effect of gastro-intestinal peptides involved in the control of food intake (Badman & Flier 2005). This will be discussed later in this chapter.
Various types of DF may be of interest. Maeda et al. (2005) demonstrated that the addition of agar in the diet resulted in marked weight loss due to a reduction of food intake, and also improved cholesterol level, glucose and insulin response, and blood pressure. The Framingham Offspring Study reported that the prevalence of metabolic syndrome - defined following the National Cholesterol Education Program criteria - was improved by high cereal fibres intake (contributing to the beneficial effect of whole grain) (McKeown et al. 2004).
Knowledge of the biochemical mechanism allowing DF to modulate satiety, glucose or lipid metabolism, and hypertension is essential when proposing key nutritional advice for specific disorders associated with the metabolic syndrome. In this context, the modulation of gastro-intestinal peptides by NDOs, such as fructans, is an interesting area of research, leading to an understanding of how events occurring in the gut participate in the control of food intake, obesity and associated disorders. We propose, before entering into discussion about the relevance of NDOs in the modulation of food intake, to describe current knowledge related to the nutritional properties of NDOs.
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