Objective: To investigate the in vivo effects of quercetin following the ingestion of fried onions.
Design: Five healthy volunteers, three males and two females aged between 25 and 39 y, ingested 225 g of fried onions after an overnight fast and peripheral venous blood was collected 0, 2, 4, 24 and 48 h after consumption. Quercetin in the plasma, total antioxidant capacity and susceptibility of low 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), decreasing 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 equivalents after 2 h and 1.76 +/- 0.08 mmol/l trolox equivalents after 4 h. There was no significant change in the susceptibility of the plasma or the isolated 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 lipoproteins from plasma at each time point. Quercetin was not detected in either 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 high enough concentrations to increase the overall antioxidant activity of the plasma. Quercetin, however, has a strong affinity for protein and provides no direct protective effect during LDL oxidation.