Absorption and metabolism of dietary phenolics

Since the early 1990s there has been growing interest in the protective effects of dietary phenolics and flavonoids. In order to assess the potential health benefits of these compounds information is required on the sites of absorption, the metabolic forms in which they are absorbed and their concentrations in the circulatory systems and body tissues. After consumption of onions, which contain flavonol glucosides, the main components being quercetin-4'-O-glucoside (143 mmoles) and quercetin-3,4'-O-diglucoside (107 mmoles), the flavonols undergo rapid metabolism in the small intestine followed by absorption of glucuronidated, sulfated and methylated quercetin metabolites. These metabolites are detected in the bloodstream reaching a Cmax after 1.0–1.5h. Excretion of metabolites in urine over a 24h period indicates that absorption is ˜4% of intake. In subjects with an ileostomy, the major components in ileal fluid after ingestion of onions are quercetin-3-glucuronide, quercetin-3'-sulfate and quercetin in quantities corresponding to ca. 20% of intake, suggesting that absorption is substantially higher than 4%. A comparative study on the absorption an metabolism of 164 mmoles of quercetin-3-rhamnosylglucoside (rutin) in tomato juice showed trace levels of quercetin and methylquercetin glucuronides, but no sulfated metabolites, in plasma with a Tmax of ca. 5h. There was an 85+% recovery of the ingested ruin in ileal fluid and no quercetin metabolites were detected in plasma collected from ileal volunteers These observations indicate that absorption of rutin is more limited than that of quercetin glucosides and that in healthy subjects it takes place on the large intestine. Other studies have demonstrated that colonic bacteria hydrolyse and breakdown rutin to phenolic acids which are excreted in urine in amounts corresponding to 25% of intake.