Flavonoids occur naturally in nature in their glucoside form, and the attachment of a sugar moiety greatly reduces their hydrophobic properties which may have implications for their absorption. Quercetin is a flavonoid that belongs to the sub-category of flavonols, and is widespread in a number of foods such as wine, onions, broccoli, apples, peaches and tea. Quercetin absorption from onions has been extensively studied but the exact route of absorption is still not fully understood. Upon ingestion, flavonoids enter the small intestine where luminal hydrolysis of the glucoside bond occurs, via the brush border enzyme lactase phlorizin hydrolase (LPH). Following this the flavonoid aglycone is free to diffuse into the enterocytes. However, some evidence suggests that the sodium glucose linked transporter-1 (SGLT1) is involved in the transport of certain glucosides into the intestinal cells where cytosolic β-glucosidase is suspected of cleaving the sugar molecule to produce the aglycone form.
Following cellular accumulation of the aglycone, extensive metabolism occurs within the enterocytes, resulting in the formation of a number of phase-II conjugates, most notably sulfates, glucuronides, and methylated and mixed conjugates. Conjugation of flavonoids has a major impact on the chemical properties of flavonoids (e.g. molecular mass, hydrophobicity), and it is likely that conjugation and the associated huge increase in hydrophilicity requires that subsequent efflux from the enterocytes is achieved by an active transport process. Much of the work on flavonoid absorption has occurred using cell culture models with isolated flavonoids in their aglycone form. Therefore questions still surrounding the role of the food matrix in the absorption of dietary flavonoids. Evidence suggests that the aglycone should be more bioavailable than the glucoside form because its hydrophobicity increase membrane permeability. However, this hydrophobicity may decrease its solubility in the gut.
Researchers1 have investigated the absorption of 1.4 mg/kg of quercetin from shallots in 9 healthy subjects in a cross-over design study. The shallot flesh contained 99.2% of their quercetin as glucosides and 0.8% as aglycone, whereas the dried skin of the shallots contained 83% of their quercetin as aglycone and 16.7% as glucosides. Blood samples were collected before and after the consumption of shallots and revealed that the highest quercetin plasma level (3.95 µmol/L) was produced 2.78 hours after consumption of the dried skin. This is in contrast to the plasma level from the shallots flesh (1.02 µmol/L) which was reached 2.33 hours after consumption. The area under the curve was significantly higher following the consumption of the dried skin, compared to the shallot flesh. These results would tend to suggest that quercetin aglycone is more readily absorbed than the glucoside form when present in the food matrix of the original plant material.
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