Absorption and antioxidant effects of quercetin from onions, in man

Abstract

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.

Bioavailability and efficiency of rutin as an antioxidant: a human supplementation study

“…In vivo the oxidation of LDL has been shown to be a mediating factor in atherosclerosis development (Aviram, 1983;Palinski et al, 1989;Steinberg et al 1989) and Fuhrman et al (1995) have reported a 20% reduction in the propensity of plasma to undergo oxidation following 2 weeks of supplementation with 400 ml red wineaday. Recently McAnlis et al, (1999) have shown that, although dietarȳ avonoids are extensively absorbed, they do not accumulate within the LDL but are tightly bound to plasma proteins located predominantly within the HDL fraction. In this state, no signi®cant protection can be afforded to LDL but¯avonoids or their metabolites may prevent the oxidation of plasma proteins or lipids located within the HDL fraction.…”

Plant polyphenols in cancer and heart disease: implications as nutritional antioxidants

“…Most measure the inhibition of an arti®cially generated oxidative process in the plasma. Although they differ in choice of oxidation source, target and the type of measurement used to detect the oxidized product (Woodford & Whitehead, 1998), in general such methods have detected transient increases in plasma antioxidant capacity following consumption of polyphenol-rich preparations of green tea (Sera®ni et al 1996;Benzie et al 1999;Nakagawa et al 1999), red wine (Fuhrman et al 1995;Whitehead et al 1995;Duthie et al 1998), alcohol-free red wine (Sera®ni et al 1998), whisky (Duthie et al 1998), grape seeds (Koga et al 1999) and onions (McAnlis et al 1999). However, any changes in plasma antioxidant capacity after consumption of polyphenols does not necessarily imply with certainty that analogous changes in redox status occur within relevant cells and tissue.…”

Fruits and vegetables: measuring intake and encouraging increased consumption

“…Six carotenoids (a-carotene, b-carotene, b-cryptoxanthin, zeaxanthin, lycopene and lutein) are found in appreciable amounts in human serum (65,70) . Other candidate biomarkers of FV intake include antioxidant vitamins such as vitamin C (71,72) and flavonoids (73,74) , including quercetin (75) . Some of these compounds have been reliably associated with a particular fruit or vegetable, or a class of FV in observational studies, but less successfully with total FV consumption (69,70,72,(76)(77)(78)(79)(80) .…”