ABSORPTION AND EXCRETION OF CONJUGATED FLAVONOLS, INCLUDING QUERCETIN-4'-O-BETA-GLUCOSIDE AND ISORHAMNETIN-4'-O-BETA-GLUCOSIDE BY HUMAN VOLUNTEERS AFTER THE CONSUMPTION OF ONIONS
Aa. Aziz et al., ABSORPTION AND EXCRETION OF CONJUGATED FLAVONOLS, INCLUDING QUERCETIN-4'-O-BETA-GLUCOSIDE AND ISORHAMNETIN-4'-O-BETA-GLUCOSIDE BY HUMAN VOLUNTEERS AFTER THE CONSUMPTION OF ONIONS, Free radical research (Print), 29(3), 1998, pp. 257-269
Flavonols are polyphenols found ubiquitously in plants and plant-produ
cts. Flavonols, particularly quercetin, are potent antioxidants in vit
ro and their intake has been associated inversely with the incidence o
f coronary heart disease. The aim of this study was to investigate the
accumulation in plasma and excretion in urine of flavonol glucosides
following ingestion of lightly fried onions. Five healthy volunteers f
ollowed a low-flavonoid diet for 3 days. On day 4, after an overnight
fast, subjects were given 300 g of lightly fried yellow onions which c
ontain conjugates of quercetin and isorhamnetin, including quercetin-3
,4'-diO-beta-glucoside, isorhamnetin-4'-O-beta-glucoside and quercetin
-4'-O-beta-glucoside. Blood collection was carried out at 0 min, 0.5,
1.0, 1.5, 2, 3, 4, 5 and 24 h after the supplement. In addition, subje
cts collected all their urine for 24 h following the onion supplement.
Isorhamnetin-4'-O-beta-glucoside and quercetin-4'-O-beta-glucoside ac
cumulated in plasma with maximum levels, defined as proportion of inta
ke, of 10.7 +/- 2.6% and 0.13 +/- 0.03% respectively. The time of the
quercetin-4'-glucoside peak plasma concentration was 1.3 +/- 0.2 h aft
er the ingestion of onions while a value of 1.8 +/- 0.7 h was obtained
for isorhamnetin-4'-glucoside. Excretion in urine, as a proportion of
intake, was 17.4 +/- 8.3% for isorhamnetin-4'-O-beta-glucoside and 0.
2 +/- 0.1% for quercetin-4'-O-beta-glucoside. Possible reasons for the
accumulation and excretion of isorhamnetin-4'-glucoside in proportion
ally much higher amounts than quercetin-4'-glucoside are discussed. It
is concluded that flavonols are absorbed into the bloodstream as gluc
osides and minor structural differences affect markedly both the level
of accumulation and the extent to which the conjugates are excreted.