It should be noted that the major difference between the two diets was the presence of diterpenoids which provided a daily intake of these compounds of,40 mg and,70 mg for the lean and obese rats, respectively. We have previously shown that part of the ingested diterpenoids resist gastric and small intestine conditions and reach the large intestine and therefore, it is plausible that these molecules and their derived metabolites may be involved in the observed effects. The evaluation of novel compounds or extracts with potential prebiotic effects should also take into account the effects of such products on digestive enzymes in the upper gut and in the hydrolysis and fermentation by the microbiota in the large bowel. We have previously reported that the intake of RE causes a significant inhibition of a butyrate-esterase activity in the stomach of Zucker rats and it was hypothesized that this may cause a reduction in the digestion and absorption of fat from the diet. Fecal composition analysis in the present work does not support this hypothesis since we did not find significant differences in the % of fat eliminated in the feces between the control and RE supplemented animals. We have now also evaluated the effects of the consumption of the RE on a-amylase activity in the small intestine and on b-glucosidase in the caecum of the rats. However, fermentation in the caecum produces a significant daily quantity of SCFA which are partially assimilated into host carbohydrates and lipids equivalent to 8% to10% of caloric requirements. Inhibition of the enzyme activities involved in this process such as b-glucosidase may contribute to reduce dietary energy extraction and to moderate body weight gain which may be beneficial. b-Glucosidases are also de-glycosylating enzymes that can release a range of aglycones from plant compounds which may exhibit toxic or healthpromoting activities. The inhibition of this activity in the caecum by the RE can thus have an additional impact on health through regulation of bioactive microbial metabolites production. It has been generally recognized that gut microbiota clearly differs between, genetically- or diet-induced, lean and obese phenotypes. Genetic predisposition to obesity associated to leptin or leptin receptor deficiencies appear to shape a microbiota specialized for enhanced dietary energy recovery with elevated abundance of Firmicutes and reduced abundance of Bacteroidetes compared with lean mice. We also examined changes in some of the main caecum microbiota groups following the intake of the RE. The RE caused a moderate but significant reduction of total caecum bacteria. It has been reported that germ-free mice and antibiotictreated mice display enlarged caecum and changes in the proportions of Firmicutes and Bacteroidetes proportions which suggest that the RE might have a strain specific bactericidal and/or bacteriostatic effect in the caecum.