Quercetin is a naturally occurring compound found in many common foods, from red onions to apples. It belongs to a large family of plant-based substances known as polyphenols and, more specifically, to a group called flavonoids. These compounds are responsible for the vibrant colors in many fruits and vegetables.
The World of Polyphenols and Flavonoids
Polyphenols represent a vast category of compounds synthesized by plants. They are characterized by their chemical structure, which includes multiple phenol units, and are found throughout the plant kingdom in foods like fruits, vegetables, and teas. Within this large family, flavonoids are a prominent and diverse subclass, contributing to the pigmentation of flowers, fruits, and leaves.
The basic chemical framework of all flavonoids is a C6-C3-C6 structure, meaning they share a common core made of two benzene rings connected by a three-carbon chain. These are further categorized into distinct groups based on subtle modifications to this core structure. One of the major groups within the flavonoid family is the flavonols, which sets the stage for understanding specific compounds like quercetin.
Defining Quercetin and Its Natural Sources
Quercetin is one of the most abundant and extensively researched flavonols found in the human diet. Chemically, it is defined as a 3,5,7,3′,4′-pentahydroxyflavone, and its structure gives it properties like a characteristic bitter taste and a yellow crystalline appearance.
This flavonol is widely distributed in plant-based foods, though its concentration can vary significantly based on factors like plant variety, growing conditions, and even which part of the plant is consumed. For instance, in red onions, the highest levels are found in the outermost rings and the part closest to the root. Some of the richest dietary sources of quercetin include:
- Capers
- Onions
- Kale
- Apples
- Berries
- Cherries
- Grapes
- Broccoli
- Green and black tea
Studies have also suggested that farming methods can influence its concentration, with one study finding that organically grown tomatoes contained significantly more quercetin than conventionally grown ones.
Key Biological Roles of Quercetin
Quercetin’s most recognized function is as an antioxidant. The presence of five hydroxyl groups on its flavonoid skeleton allows it to effectively donate electrons and neutralize reactive oxygen species, which are unstable molecules that can cause cellular damage. By scavenging these free radicals, quercetin helps to mitigate oxidative stress in the body.
Beyond its antioxidant capacity, quercetin is also known for its anti-inflammatory properties. Research suggests it can influence various pathways involved in inflammation. For example, it has been shown to inhibit enzymes like cyclooxygenase (COX) and lipoxygenase, which are involved in the production of inflammatory mediators. Other areas of ongoing research explore its potential effects on cardiovascular health by protecting LDL cholesterol from oxidation, as well as its possible antiviral and anti-allergic activities.
How the Body Processes Quercetin
In plants, quercetin is typically found in its glycoside form, meaning it is attached to a sugar. This form is not directly absorbed. In the small intestine, enzymes work to cleave off the sugar molecule, releasing the quercetin aglycone—the sugar-free version—which can then be absorbed by intestinal cells.
The intestinal cells and later the liver chemically alter the quercetin molecule, primarily through processes called glucuronidation, sulfation, and methylation. These changes create various metabolites, which are the forms of quercetin that actually circulate in the bloodstream. Very little, if any, free quercetin aglycone is found in the blood after consumption. The bioavailability of quercetin—the fraction that enters circulation to have an active effect—is generally considered low and can be influenced by the food source and the presence of other dietary components like fat.
Quercetin as a Dietary Supplement
Quercetin is widely available as a dietary supplement. These supplements typically provide the compound in a concentrated form, often as quercetin aglycone or quercetin dihydrate, which are different from the glycoside forms found in most foods. Dosages in supplements can vary, but they have been used in studies in amounts up to 1 gram daily for periods of up to 12 weeks.
Manufacturers sometimes combine quercetin with other substances, such as vitamin C or the enzyme bromelain, with the rationale that these additions may enhance its absorption or effectiveness, though research in this area is ongoing. Before adding quercetin or any new supplement to a routine, it is advisable to consult a healthcare professional. They can provide guidance on whether it is appropriate for an individual’s health status and discuss potential interactions with medications, as quercetin can affect certain drug-metabolizing enzymes.