Insights into the intestinal bacterial metabolism of flavonoids and the bioactivities of their microbe-derived ring cleavage metabolites

Flavonoids are a group of phytochemicals widely distributed in plants, fruits, and vegetables that possess numerous bioactivities. After oral administration, flavonoids can be metabolized by the intestinal bacteria into a wide range of low-molecular-weight phenolic acids. In this review, the intestinal bacterial metabolic pathways of different flavonoids (flavones, isoflavones, flavonols, flavanones, and chalcones) and the bioactivities of their microbe-derived ring cleavage metabolites are summarized. Flavonoids undergo different intestinal bacterial metabolic reactions, depending on the characteristics of their structure. Free hydroxyl groups, especially 5 and 4' free hydroxyl groups play significant roles in fission metabolism. Microbe-derived ring cleavage metabolites such as 3,4-dihydroxyphenylacetic acid (3,4-DHPAA) and 3,4-dihydroxytoluene (3,4-DHT) possess various bioactivities including antioxidant, anti-inflammatory, antidiabetic, neuroprotective, and anti-colon cancer effects. Also, the intestinal bacteria associated with the bacterial metabolism of flavonoids are covered in this review.