Isomaltooligosaccharides (IMOs) produced by Leuconostoc mesenteroides ATCC 13146 fermentation with a sucrose:maltose ratio of 2:1 have been discovered to be effective prebiotics in mixed cultures of microbial populations, including cultures from chicken ceca. Surprisingly in mixed microbial cultures this IMO composition proved as effective as FOS. Thus, these IMOs can be used as effective prebiotics for both birds and mammals. Moreover, the IMOs were discovered to be effective non-competitive inhibitors of .alpha.-glucosidase. These IMOs also will be useful, as an .alpha.-glucosidase inhibitor, in a therapeutic application for several diseases, including obesity, diabetes mellitus, pre-diabetes, gastritis, gastric ulcer, duodenal ulcer, caries, cancer, viral disease such as hepatitis B and C, HIV, and AIDS. A diet with 5-20% IMOs was also shown to reduce the abdominal fat tissue in mammals.