Objective: To investigate the in vivo and in vitro effects of black tea on the oxidative modi®cation of low density lipoprotein (LDL). Design: The antioxidant activity of the tea was studied in vitro by measuring the resistance of the LDL to oxidative modi®cation in the presence of copper. The effects of tea consumption in vivo were investigated in two settings. Firstly, to assess the acute effects of tea consumption, ®ve fasting healthy subjects ingested 600 mls (50.7 AE 5.4 mg¯avonoids) of black tea and peripheral venous blood was collected at 0, 30, 60, 90, 120 and 180 min after consumption. Secondly, to assess the effects of chronic tea consumption, a randomised crossover trial of tea (126.8 AE 13.5 mg¯avonoids) and coffee consumption was carried out in ten healthy subjects. Results: Black tea extract increased the resistance of LDL in vitro in a concentration dependent manner. There was no signi®cant change in total plasma antioxidant capacity or susceptibility of the LDL to oxidation over the 3 h period after consumption of black tea. The four-week crossover study in which coffee was used as a control against the black tea showed no signi®cant difference in the total plasma antioxidant capacity or susceptibility of LDL to oxidation between the tea and coffee groups. Serum lipids, including total cholesterol, triglycerides, LDL cholesterol and HDL cholesterol did not change signi®cantly throughout the study. Conclusions: The consumption of moderate quantities of black tea acutely or for one week does not increase plasma total antioxidant capacity or alter the susceptibility of LDL to oxidation. Sponsorship: This work was funded by the Department of Agriculture for Northern Ireland.
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 (28.4AE 1.9 ngaml) to peak after 2 h (248.4 AE 103.9 ngaml), decreasing to baseline again after 24 h (P b 0.05). This was accompanied by an increase in the total antioxidant activity of the plasma from baseline (1.70AE 0.04 mmolal trolox equivalents) to 1.75 AE 0.10 mmolal trolox equivalents after 2 h and 1.76AE 0.08 mmolal trolox equivalents after 4 h. There was no signi®cant 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 ®ndings, 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 af®nity for protein and provides no direct protective effect during LDL oxidation. Sponsorship: This work was funded by the Department of Agriculture for Northern Ireland.
Flavonoid consumption in onions and tea had no significant effect on plasma F2-isoprostane concentrations and MDA-LDL autoantibody titer in this study and thus does not seem to inhibit lipid peroxidation in humans.
The effect of various dietary flavonoids on the susceptibility of low density lipoproteins to oxidation in vitro using both metallic and oon-metallic oxidising agents.
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