The blood serum indexes of glucose and lipids, the intestinal microbiota and the production of volatile fatty acids (VFA) were determined in Wistar rats which were fed a standard (AIN-P) diet, a modified standard (AIN-M) diet, and diets containing the following yeast cell wall fractions: insoluble glycan (IG), mannan (M), and a mixture of glycan + mannan (G+M) as the only source of dietary fiber. Fractionation of the cell wall was carried out by means of physicochemical extraction processes, centrifugation, and drying in a "spray dryer". Serum indexes were dosed by means of commercial kits and the influence of the fractions on the intestinal microbiota and the production of short-chain volatile fatty acids (VFA) were carried out by collecting intestinal contents (colon, cecum, and rectum). Considering the cholesterol levels at time zero (T0) and at time 28 (T28), diets AIN-P, AIN-M, and M presented a hypocholesterolemic effect considering the hypercholesterolemic nature of the diets. Regarding serum glucose, at the onset of the experiment (T0), a general increase in glycemia was observed which suggests a hyperglycemic effect of the examined diets. The G+M diet yielded the highest significant values of serum lipids at T14 and the lowest values were found in the M diet and the IG diet at T14 and in the AIN-M and AIN-P diets. The AIN-P diet yielded the highest significant values of triacylglycerols at T14 and T28. The lowest levels at T14 were found in the G+M and IG diets and at T28 for the AIN-M and M diets. In general, there were no significant modifications in the intestinal microbiota of the rats in any diet. Regarding the VFA, the acetic acid was predominant, followed by the propionic and butyric acids in all the tested diets.272338348A.O.A.C. Association of Official Agricultural Chemists, official methods of analysis. HORWITZ, W. 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