Modulation of the gut microbiota through the diet to enhance host health and to reduce the incidence of obesity and associated disorders is an important line of research. According to FAO, prebiotics are ‘non-viable food components that confer a health benefit on the host associated with modulation of the microbiota. This concept has been thoroughly revised and since most studied prebiotics are fibers, the latest definition recognizes that prebiotics are ‘selectively fermented ingredients that promote the selective stimulation of growth and/or activities of one or a limited number of microbial genus/species in the gut microbiota that confer health benefits to the host’. Plant extracts enriched in bioactive compounds are also widely investigated as an additional strategy to combat obesity and metabolic disorders since some of their components and derived metabolites appear to exert a number of metabolic regulatory and anti-inflammatory properties as well as to modify the intestinal environment through modulation of the microbiota. We have previously reported that a rosemary extract enriched in the bioactive compound carnosic acid has body weight, serum lipids and insulin lowering effects in female Zucker rats, more noticeable in the lean animals. These effects were partially attributed to the inhibition of a pre-duodenal butyrate esterase activity and potential reduction of fat absorption. We have also shown that the RE differentially modulates the production of anti- and pro-inflammatory cytokines as well as hepatic metabolic gene expression in the lean and obese animals but, the mechanisms triggering these effects and the differences found between the two genotypes are not yet fully understood. Since most investigated prebiotics are fibers, it is important to characterize and quantify the presence of fiber in bioactive enriched plant extracts. Inulin, fructo-oligosaccharides, cellulose, etc, are all prebiotics known to resist upper gut digestion and to reach the large intestine where they are fermented by the microbiota. The main site of bacterial fermentation in rats, the caecum, is enlarged after the intake of these fibers. The consumption of the RE caused a significant augmentation of the caecum weight suggesting the presence of additional non-digested fibers and carbohydrates in the extract. However, considering that the addition of the RE to the standard feed did not modify quantitatively the composition of the main nutrients in the diet and that the RE did not affect the daily food intake, the contribution of the RE to the daily consumption of fiber and carbohydrates was minor. Even if the fiber and carbohydrates from the RE had not been digested in the small intestine neither fermented in the caecum, they would not explain the increase in the fiber excreted in the feces of the animals that consumed the RE.