Absorption and antioxidant effects of quercetin from onions, in man

Objective: To investigate the in vivo effects of quercetin following the in gestion of fried onions. Design: Five healthy volunteers, three males and two females aged between 2 5 and 39 y, ingested 225 g of fried onions after an overnight fast and peri pheral venous blood was collected 0, 2, 4, 24 and 48 h after consumption. Q uercetin in the plasma, total antioxidant capacity and susceptibility of lo w density lipoproteins (LDL) to oxidation were measured. Results: Following the onion meal? quercetin levels increased from baseline values (28.4 +/- 1.9 ng/ml) to peak after 2 h (248.4 +/- 103.9 ng/ml), dec reasing to baseline again after 24 h (P > 0.05). This was accompanied by an increase in the total antioxidant activity of the plasma from baseline (1. 70 +/- 0.04 mmol/l trolox equivalents) to 1.75 +/- 0.10 mmol/l trolox equiv alents after 2 h and 1.76 +/- 0.08 mmol/l trolox equivalents after 4 h. The re was no significant change in the susceptibility of the plasma or the iso lated LDL to oxidation over the 48 h period after consumption of the fried onions. In view of these negative findings, we isolated LDL and other lipop roteins from plasma at each time point. Quercetin was not detected in eithe r LDL or VLDL, but was present in the HDL fraction, although this fraction also contains other proteins including albumin. Conclusions: Quercetin can be absorbed in humans from dietary sources to hi gh enough concentrations to increase the overall antioxidant activity of th e plasma. Quercetin, however, has a strong affinity for protein and provide s no direct protective effect during LDL oxidation. Sponsorship: This work was funded by the Department of Agriculture for Nort hern Ireland.