Quercetin is also a flavonoid and often present as its glycoside, quercetin-3-O-rutinoside (rutin), in the human diet. After being ingested, rutin undergoes hydrolysis by the microflora in the colon before absorption (2). The microflora in the colon has enormous catalytic and hydrolytic potential. They will first cut down the glycogen of rutin to utilize as their carbon source (3) and can further catalyze the breakdown of the C6–C3–C6 flavonoid skeleton to a variety of phenolic acid catabolites (2, 4, 5). Thus, the bioactivity of polyphenol is very much dependent on its catabolism and catabolites that occur before absorption and disposition in target tissues and cells.