Quercetin-glycosides from onions

The bioavailibility of quercetin-glycosides from onions, containing mainly quercetin-p-glucosides, was superior to that of various quercetin glycosides from apples (containing a mixture of quercetin-(3-galactosides and P-xylosides) and of pure quercetin-3-mtinoside (major species in tea). The possible matrix effect of the foods remains unclear. 

The absorption of intact quercetin glycosides has been demonstrated by some authors [64,65,76]. Hollmann demonstrated in ileostomy subjects (who lack colon with the bacterial flora, thus circumventing the problem of microbial degradation), that the quercetin glycosides from regular foods (onions, tea) were far better absorbed than pure aglycone (52% vs 24%). 

Glycosides of flavonols from onions, such as quercetin-4 -0-glucoside and quercetin-3 -0-methyl-4 -0-glucoside, have been found in the plasma of volunteers with a peak of absorption of 0.5 - 4 h [64]. 

Das demonstrated that oral administration of labeled 3-0-methylcatechin to three volunteers was followed by plasma peak levels within 2 hours after administration [69]. Hollman et al. studied the absorption of different glycosides of quercetin and the quercetin aglycone in nine healthy ileostomy subjects to avoid losses caused by colonic bacteria [30-31]. The absorption after oral administration decreased in the following order 52% for quercetin glucosides from fried onions > 24% for quercetin aglycone > 17% for quercetin nitinoside [31]. Consequently, humans absorb considerable amounts of quercetin, but the absorption depends on the glycosidic nature of the flavonoid [31]. 

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