Determinants of the absorption of the dietary flavonoid quercetin in man

Oxidation of low density lipoprotein is hypothesised to play an important role in the development of cardiovascular disease. It might be prevented by dietary antioxidants. Quercetin is a dietary flavonoid antioxidant and its intake was inversely associated with cardiovascular disease in some studies. Absorption from the diet is a prerequisite for its potentially beneficial role. This thesis describes studies on the absorption and elimination kinetics of dietary quercetin in humans.To perform these absorption studies, we developed a postcolumn chelation technique for quercetin in HPLC with fluorescence detection using aluminum. Only flavonols that contain a free 3-hydroxyl and 4-keto oxygen binding site formed fluorescent complexes with Al 3+. This method improved detectability of quercetin 300 fold as compared to conventional UV detection.We studied the absorption of quercetin in healthy ileostomy subjects so as to avoid losses caused by colonic bacteria. Absorption of quercetin was 52 + 15% for quercetin glucosides found in onions, 17 + 15% for quercetin rutinoside a major quercetin glycoside of tea, and 24 + 9% for free quercetin aglycone.The time course of the plasma quercetin concentration was studied in normal subjects with an intact colon who ingested major dietary sources of quercetin, viz. fried onions containing glucose conjugates of quercetin, apples containing both glucose- and non-glucose quercetin glycosides, and of quercetin-3-rutinoside. Peak plasma levels of quercetin were reached We confirmed that the sugar moiety of the glycoside is an important determinant of absorption in a study with volunteers who ingested solutions of pure quercetin-4'-glucoside and of pure quercetin-3-rutinoside.In conclusion, absorption of dietary quercetin glycosides can be appreciable and depends on the type of sugar moiety of the glycoside. We propose that the sodium-glucose cotransporter is involved in the absorption of quercetin-4'glucoside. Elimination of quercetin from the blood is slow. Repeated consumption of quercetin- containing foods therefore will lead to accumulation of quercetin in plasma