Effect of Processed Onions on the Plasma Concentration of Quercetin in Rats and Humans

Onion is a major dietary source of the bioactive flavonoid, quercetin. Quercetin aglycone (QA) is exclusively distributed in the onion peel, although quercetin-4'-β-O-glucoside (Q4'G) is present in both the peel and the bulb, and quercetin-3,4'-β-O-diglucoside (Q3,4'diG) is present only the bulb. The bioavailability of flavonoids from fruits and vegetables is frequently affected by the manufacturing process and related conditions. The present study aimed to estimate the effect of food processing on the bioavailability of onion QA and its glucosides, Q4'G and Q3,4'diG, provided through the consumption of onion products. Rats were fed onion peel and onion bulb products-mixed meal or pure QA/Q4'G+Q3,4'diG-mixed meal at 5 mg QA equivalent/kg body weight. A comparison of the blood plasma concentrations strongly suggested that quercetin glucosides (Q4'G and Q3,4'diG) are superior or at least equal to QA in their bioavailability, when each purified compound is mixed with the meal. The intake of a peel powder-containing meal provided a significantly higher increase of plasma quercetin concentration than the peel extract, bulb powder, bulb extract, and bulb sauté containing meals at each period tested. A human ingestion study confirmed the superiority of onion peel powder to onion peel extract. The difference of log P for QA between peel powder and peel extract indicated that a food matrix improves the bioavailability of QA in onion peel products. These results demonstrated that the bioavailability of quercetin provided by not the onion bulb but the onion peel is significantly affected by food processing.

Onion is a popular source of antioxidative flavonoid quercetin and its vascular function attracts considerable attention in relation to anti-atherosclerotic effect. The present study estimated the effect of food processing on the bioavailability of onion quercetin aglycone and its glucosides provided through the consumption of onion products. The intake of a peel powder-containing meal showed a significantly higher bioavailability than the peel extract, bulb powder, bulb extract, and bulb sauté containing meals. Hence, food processing of onion peel may enhance the health impact of onion quercetin by elevating its bioavailability.