EFFECT OF SOLUBLE DIETARY FIBER ON THE VISCOSITY OF GASTROINTESTINAL CONTENTS AND THE ACUTE GLYCEMIC RESPONSE IN THE RAT

The postprandial glycaemic response following a meal is reduced with t he addition of soluble dietary fibre. The reductions in the glycaemia are thought to be due largely to increased viscosity of the gastrointe stinal (GI) contents retarding digestion and absorption. The aims of t he present study were to determine the effect that the GI tract has on the viscosity of meals containing different soluble fibres and to det ermine whether the glycaemic response of a meal (containing the solubl e fibre) was predicted by the viscosity of the digesta in the small in testine. High carbohydrate diets containing 70 g soluble fibre (guar g um, xanthan gum or methylcellulose)/kg or 70 g insoluble fibre (wheat bran)/kg were diluted in water to a final fibre concentration of 18 g/ kg. Following dilution the wheat bran diet had no measurable viscosity ,,while the viscosities of the soluble fibre diets were elevated. When the diets were fed to male Sprague-Dawley rats for 2 weeks the viscos ities of the stomach and small intestinal digesta were not predicted b y the viscosity of the diets measured before ingestion. The action of the GI tract on the viscosity of the soluble fibres was investigated i n vitro by dilution of the diets with acidic and neutralizing solution s, mimicking gastric and duodenal secretions. Dilution of diets with e ither acidic and neutralizing solutions or saline control significantl y lowered the viscosity of all diets, while alterations in the pH of t he diets had little impact on the resultant viscosity. When fasted rat s were orally administered with the differing diets (0.25 g carbohydra te/kg body weight), the postprandial glucose response was reduced foll owing the soluble-fibre-containing meals when compared with the wheat bran-supplemented meal, although the reduction in glycaemia only reach ed statistical significance with xanthan supplementation. These result s indicate that there are large changes in the viscosity of a meal con taining soluble fibre following ingestion, and that dilution of the di et by GI secretions is important in determining the resultant viscosit y in the small intestine. Furthermore, the large differences in viscos ity of the GI contents following consumption of the diets containing t he soluble fibres were not predictive of the postprandial glycaemic re sponse.