The present invention provides: an agent having an activity to reduce an FB ratio in the intestine (i.e., an intestinal FB ratio reducing agent), which can be used suitably in a food or beverage； and a food composition for reducing an intestinal FB ratio, which contains the agent. The intestinal FB ratio reducing agent according to the present invention is one characterized by containing yeast mannan, and the food composition for reducing an intestinal FB ratio according to the present invention is one containing the intestinal FB ratio reducing agent. The term "FB ratio" as used herein may be defined as [(an occupation ratio of bacteria belonging to the phylum
