Dietary fructans, but not cellulose, decrease triglyceride accumulation in the liver of obese Zucker fa/fa rats

Effects of inulin-type fructans on lipid metabolism in man and in animal models

Studies in rodents show that inulin and oligofructose can reduce the plasma levels of cholesterol and triacylglycerols (TG). In addition, they can oppose TG accumulation in liver and have favourable effects on hepatic steatosis. The hypotriglyceridaemic effect is due to a reduction in hepatic re-esterification of fatty acids, but mainly in the expression and activity of liver lipogenesis, resulting in lower hepatic secretion rate of TG. This repression of lipogenesis is not observed in adipose tissue. The effect on liver lipogenesis can be explained by reduced insulin/glucose levels or by a selective exposure of the liver to increased amounts of propionic acid produced in the large intestine during fermentation of non-digestible carbohydrates. The decrease in plasma cholesterol could also be due to inhibition of cholesterol synthesis by propionic acid or to modifications in the bile acid metabolism. Studies in man yield more conflicting results with a decrease or no effects on plasma lipid levels, and, when a decrease is observed, more marked effects on TG than on cholesterol and more consistent action of inulin than of oligofructose. Besides the difference in the dose of inulin or oligofructose used, differences in metabolic status could play a role in this discrepancy between man and animals since reduction in plasma TG is observed in man mainly in a situation of increased liver lipogenesis (high-carbohydrate diet, obesity, hypertriglyceridaemia). The effects on plasma cholesterol appear also more marked in hyperlipidaemic subjects than in healthy controls, suggesting that inulin and oligofructose have beneficial effects in these types of subjects.

Inulin-type fructans modulate gastrointestinal peptides involved in appetite regulation (glucagon-like peptide-1 and ghrelin) in rats

The hypothesis tested in the present study is that dietary fructans are able to modulate gastrointestinal peptides involved in the control of food intake, namely glucagon-like peptide (GLP)-1 (7-36) amide and ghrelin. After 3 weeks of treatment with a standard diet (control) or the same diet enriched with 100 g fructans varying in their degrees of polymerization (oligofructose (OFS), Synergy 1 (Syn) or long chain inulin)/kg, male Wistar rats were deprived of food for 8 h before sample collection. Dietary energy intake throughout the experiment was significantly lower (P<0.05) in fructans-fed rats than in control rats, leading to a significant decrease (P<0.01) in epidydimal fat mass at the end of the treatment in OFS- and Syn-treated rats. GLP-1 (7-36) amide concentration in portal vein serum was higher in OFS- and Syn-fed than in control rats. Both GLP-1 (7-36) amide concentration and proglucagon mRNA concentrations were significantly greater (P<0.05) in the proximal colonic mucosa of fructans-fed rats v. controls. Normally active ghrelin concentration in plasma increases during food deprivation and rapidly falls during a meal. In the present study, after 8 h of food deprivation, active ghrelin in the plasma remained significantly lower (P<0.05) in OFS and Syn-fed than in control rats. These results are in accordance with the modifications of dietary intake and fat-mass development in short-chain fructans-treated rats and demonstrate the potential modulation of GLP-1 (7-36) amide and ghrelin by fermentable fibres such as fructans, which are rapidly and extensively fermented in the proximal part of the colon.

Prebiotics promote good health: the basis, the potential, and the emerging evidence

BACKGROUND

The prebiotics concept, which was launched in 1995, concerns nondigested and selectively fermented carbohydrate food ingredients. It was thought that their effect in the colon could reduce risk for disease. The prebiotic concept is revisited and possible mechanisms are proposed. The physiologic consequences of prebiotic consumption are evaluated in terms of potential to reduce risk for disease. This is a compilation of several research papers, each of which complied with the World Medical Association Declaration of Helsinki.

METHODS

For human dietary intervention trials, the aim was to perform double-blind, placebo-controlled, cross-over studies. A parallel design was used only for long-term studies. Most research has been done with beta(2-1) fructans, so they are used as an example of prebiotics here.

RESULTS

The results are relevant to the fields of gut function, lipid metabolism, mineral absorption, bone formation, immunology, and cancer.

CONCLUSION

It is observed that modification of intestinal flora by inherently selectively fermented prebiotics is central in determining their nutritional properties. They interact positively through the large intestinal surface with various physiologic processes and are thought to improve health status by reducing risk for disease (markers).

Oligosaccharides: state of the art

Oligosaccharides, consisting of a mixture of hexose oligomers with a variable extent of polymerisation, are food products with interesting nutritional properties. They may be naturally present in food, mostly in fruits, vegetables or grains, or produced by biosynthesis from natural sugars or polysaccharides and added to food products because of their nutritional properties or organoleptic characteristics. The dietary intake of oligosaccharides is difficult to estimate, but it may reach 3-13 g/d per person (for fructo-oligosaccharides), depending on the population. The extent of resistance to enzymic reactions occurring in the upper part of the gastrointestinal tract allows oligosaccharides to become 'colonic nutrients', as some intestinal bacterial species express specific hydrolases and are able to convert oligosaccharides into short-chain fatty acids (acetate, lactate, propionate, butyrate) and/or gases by fermentation. Oligosaccharides that selectively promote some interesting bacterial species (e.g. lactobacilli, bifidobacteria), and thus equilibrate intestinal microflora, are now termed prebiotics. The pattern of short-chain fatty acid production in the caeco-colon, as well as the prebiotic effect, if demonstrated, are dynamic processes that vary with the type of oligosaccharide (e.g. extent of polymerisation, nature of hexose moieties), the duration of the treatment, the initial composition of flora or the diet in which they are incorporated. Experimental data obtained in vitro and in vivo in animals, and also recent data obtained in human subjects, support the involvement of dietary oligosaccharides in physiological processes in the different intestinal cell types (e.g. mucins production, cell division, immune cells function, ionic transport) and also outside the gastrointestinal tract (e.g. hormone production, lipid and carbohydrates metabolism). The present paper gives an overview of the future development of oligosaccharides, newly recognised as dietary fibre.