Quercetin is a naturally occurring plant pigment and a type of flavonoid abundant in many fruits and vegetables. As a potent antioxidant, it helps neutralize unstable molecules called free radicals, which can cause cellular damage. This action is linked to supporting the body’s natural defenses and promoting overall health. Since the human body cannot produce quercetin, it must be obtained entirely through dietary intake.
Primary Dietary Sources
Quercetin is found in a wide variety of plant-based foods, though concentrations vary significantly. The single most concentrated food source is capers. Onions, particularly red and yellow varieties, are a major source in the Western diet, containing high levels of this flavonoid in their outer rings.
Among vegetables, leafy greens like kale and members of the Brassica family, such as broccoli, contribute measurable amounts. Colorful fruits also serve as important sources, including apples, dark-colored grapes, and berries like cranberries and blueberries.
Beverages also contain this compound, with black and green tea being notable contributors. Red wine is another source. The average person consumes an estimated 10 to 100 milligrams of quercetin daily through a diverse diet.
Practical Tips for Maximizing Intake
The highest concentration of quercetin is typically found in the outer layers and skin of fruits and vegetables. For example, the majority of the quercetin in an apple is located in its peel, making it important to consume the whole fruit. Peeling onions also reduces the quercetin content significantly, as approximately 90% is found in the first and second layers.
Cooking methods affect the final retained amount because quercetin is water-soluble. Boiling can cause the compound to leach out into the cooking water, leading to significant losses. Conversely, cooking methods that involve less water, such as light steaming, baking, or microwaving, are better for preserving the quercetin content. When boiling is necessary, using the cooking liquid for soups or sauces can help retain the lost nutrients.
Quercetin in Supplement Form
Quercetin is widely available as a dietary supplement for those seeking higher or more standardized doses. These commercial preparations usually come in the form of capsules, tablets, or powders. Supplements often provide a significantly higher dose than achievable through diet alone, with recommended doses sometimes ranging between 500 to 1,000 milligrams per day.
The quercetin in supplements is often extracted from plant sources like the seed pods of the Dimorphandra mollis tree or Sophora japonica. Choosing a supplement allows for a more consistent intake, though the efficacy is heavily influenced by how the body processes the compound.
Understanding Absorption and Bioavailability
Quercetin has poor bioavailability, meaning only a small fraction of the amount consumed is absorbed and made available for the body to use. When consumed in food, quercetin is usually attached to a sugar molecule, forming a glycoside. This natural structure, particularly the quercetin-3-glucoside found in onions, can be more readily absorbed than the pure, isolated form found in many standard supplements.
Dietary strategies can help improve uptake, such as pairing quercetin-rich foods with healthy fats, since the compound is lipophilic. The gut microbiome also plays a role, helping process the compound into forms the body can utilize. To overcome the poor absorption of traditional supplements, manufacturers use advanced delivery systems, such as phytosomes or micro-encapsulation, which significantly enhance solubility and increase the amount that enters the bloodstream.