Quercetin is a plant flavonol, a type of flavonoid categorized within the larger group of polyphenols. It serves as a pigment, contributing to the colors seen in many fruits, vegetables, seeds, and grains. While present in foods like capers, apples, and tea, onions are widely recognized as one of the most concentrated dietary sources of this compound.
Quercetin Content by Onion Type
The concentration of quercetin differs significantly based on the specific variety of onion, with color acting as a general indicator of content. Darker-colored varieties, particularly red and yellow onions, contain substantially higher levels of the flavonoid than white onions. These varieties should be selected when maximizing quercetin intake is the goal.
Studies show that the total quercetin content in colored onions can vary widely, ranging from approximately 54 to 286 mg per kilogram of fresh weight in different cultivars. White onions, in contrast, often contain only trace amounts of the compound. The pigment concentration is directly linked to the presence of flavonoids; for example, red onions have been reported to contain up to 14 times higher levels than white onions.
Where Quercetin Resides in the Onion
The distribution of quercetin within an individual onion bulb is not uniform; it is highly concentrated in the outer layers. The highest levels are found in the dry, papery outer skin, which is the non-edible part. This dry skin can contain a quercetin concentration up to 48 times greater than the inner flesh.
Moving inward, the fleshy, pigmented outer rings of the bulb contain the next highest concentration. The concentration declines steeply as one moves from the exterior toward the core of the onion. For instance, one study found the concentration of quercetin compounds decreased from 2.67 mg per gram in the outer skin to only 0.35 mg per gram in the inner core. Minimizing the amount of the outer fleshy layers that are peeled away is a practical strategy for maximizing intake.
How Preparation and Storage Impact Quercetin Levels
The way an onion is prepared and stored can significantly influence the final amount of quercetin available for consumption. Since the flavonoid compounds are water-soluble, cooking methods that involve liquid, such as boiling, can lead to substantial losses. When onions are boiled, approximately 30% of their quercetin glycosides can transfer from the vegetable into the cooking water. This loss is minimized if the cooking water, such as in a soup or stew, is also consumed.
Cooking Methods
Methods that use dry heat, like frying or sautéing, are generally more effective at retaining the flavonoid content. Frying, for example, does not cause a significant loss in quercetin intake. Microwaving without added water also helps retain the flavonoid compounds better than boiling. Furthermore, the initial act of chopping or slicing an onion can sometimes temporarily increase the level of certain flavonols.
Storage Conditions
Storage conditions also play a role in retention over time. Storing onions at room temperature (around 24°C) can cause a change in total quercetin content, often showing a rise in concentration followed by a drop over several months. Conversely, refrigeration or controlled atmosphere storage helps maintain the initial quercetin levels more consistently.
Absorption and Bioavailability from Onion Sources
Quercetin in onions is primarily present in a form known as quercetin glycosides, meaning the compound is chemically bound to sugar molecules, such as glucose. The predominant forms in onions are quercetin 3,4′-O-diglucoside and quercetin 4′-O-glucoside. This glycoside form is highly relevant for the body’s ability to absorb the compound, known as bioavailability.
Quercetin from onions is considered more readily absorbed by the body compared to the forms found in other food sources like apples or tea. This higher bioavailability is partly due to the specific type of sugar molecule attached. The absorption of these glycosides can begin quickly in the upper part of the intestine.
Once consumed, the gut microbiota processes the quercetin compounds. Enzymes break the sugar molecule away from the quercetin structure, allowing the resulting compound to be absorbed across the intestinal wall. This mechanism results in a quicker peak plasma concentration of quercetin after consuming onions compared to other sources.