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 Firmicutes)/(an occupation ratio of bacteria belonging to the phylum Bacteroides)], or may be defined as [(an occupation ratio of all of bacteria belonging to the class Clostridia and bacteria belonging to the class Erysipelotrichia among bacterial belonging to the phylum Firmicutes)/(an occupation ratio of bacteria belonging to the class Bacteroidia among bacterial belonging to the phylum Bacteroides)].本發明提供一種適於飲食品中之利用之具有降低腸內之FB比之作用之劑（腸內FB比降低劑）及含有其之FB比降低用食品組成物。本發明之腸內FB比降低劑係以含有酵母甘露聚糖為特徵之腸內FB比降低劑，本發明之FB比降低用食品組成物係含有該腸內FB比降低劑之FB比降低用食品組成物。此處，FB比可為定義為腸內之[屬於厚壁菌門之細菌之佔有率／屬於擬桿菌門之細菌之佔有率]者，亦可為定義為[屬於厚壁菌門之細菌中屬於梭菌綱之細菌及屬於丹毒絲菌綱之細菌之合計之佔有率／屬於擬桿菌門中擬桿菌綱之細菌之佔有率]者。
