Quercetin: An Emerging Natural Flavonoid in Modern Therapeutics with Multifaceted Pharmacological Potential

Flavonoids are plant secondary metabolites built around a benzopyrone ring that carries one or more phenolic or polyphenolic groups. They are found throughout the plant kingdom in fruits, herbs, stems, cereals, nuts, vegetables, flowers, and seeds. These bioactive phytochemicals are largely responsible for the medicinal value and diverse biological activities of many plants. So far, more than 10,000 different flavonoids have been isolated and identified, and many of them are now widely recognized as useful therapeutic agents. Flavonoids are produced naturally via the phenylpropanoid pathway, and their actual effects in the body depend strongly on how well they are absorbed and how bioavailable they are. Beyond their roles in natural dyes, cosmetics, skin?care products, and anti?wrinkle preparations, their most important applications lie in medicine. Flavonoids have been studied and used for their anticancer, antimicrobial, antiviral, antiangiogenic, antimalarial, antioxidant, neuroprotective, antitumor, and antiproliferative properties. For example, flavonoid?rich apple peel extracts have been shown to inhibit angiotensin?converting enzyme (ACE) in vitro, act as effective antihypertensive agents, help prevent cardiometabolic disorders, and support better maintenance of cognitive function with advancing age. Large population studies also indicate that higher dietary intake of flavonoids is associated with lower mortality from cardiovascular disease.

In plants, flavonoids perform a wide variety of functions. They are synthesized in specific tissues and are responsible for many of the colors and aromas of flowers and fruits, which attract pollinators and promote fruit and seed dispersal, thereby supporting germination and early seedling growth. They help plants cope with biotic and abiotic stresses, act as natural UV filters, and serve as signal molecules, allelopathic compounds, phytoalexins, detoxifying agents, and antimicrobial defenses. Flavonoids also contribute to frost hardiness and drought resistance, and may be involved in heat acclimatization and freezing tolerance. Early work in floral genetics showed that mutations affecting flavonoid biosynthesis could dramatically alter flower color, and functional gene silencing in plants has been linked to disruptions in flavonoid pathways. In humans and animals, flavonoids are now widely regarded as beneficial for health, particularly in disease prevention and chemoprevention. Around 6000 flavonoids are known to contribute to the vivid pigments of fruits, vegetables, herbs, and medicinal plants. Detailed reviews, such as those by Dixon & Pasinetti and by Kumar & Pandey, have explored their roles in agriculture, neuroscience, and protection against human diseases, while more recent work has focused on their potential in the management and prevention of neurodegenerative disorders, including Alzheimer’s disease. Current research continues to investigate flavonoids as key dietary components with health benefits, to refine their classification, and to define future directions for their use in nutrition, medicine, and drug development.

Classification

 Flavonoids are divided into several subgroups based on how the B ring is attached to the central C ring and on the degree of unsaturation and oxidation of the C ring. When the B ring is attached at position 3 of the C ring, the compounds are known as isoflavones. If the B ring is attached at position 4, they are called neoflavonoids. When the B ring is attached at position 2 of the C ring, the flavonoids form a series of further subgroups defined by specific structural features of the C ring. These include flavones, flavonols, flavanones, flavanonols, flavanols (catechins), anthocyanins, and chalcones.

Fig. 1. Flavonoid classes, subclasses and natural sources

.Table1. Flavonoids, their classes and rich dietary sources

Flavonoids are a large class of plant metabolites that occur naturally in many fruits, vegetables, herbs, and other plant parts, and are widely recognized for their antioxidant properties and their ability to influence cell?signaling pathways. Because of these effects, plant extracts rich in flavonoids and other phytoconstituents often show important biological activities, including antidiabetic, antihyperlipidemic, free?radical scavenging, and anti?inflammatory actions. Free radicals are known to contribute significantly to metabolic disorders and can reduce quality of life, which is why there has been growing interest in plant?based antioxidants in recent decades. Within the flavonoid family, quercetin is a particularly well?studied bioflavonoid. Bioflavonoids were first identified in 1930, and over the past thirty years quercetin has become one of the most intensively researched members of this group. When taken at appropriate doses, quercetin has been shown in numerous studies to exert a broad range of biological activities, including immunomodulatory, antibacterial, antitumor, neuroprotective, antiallergic, antioxidant, and anti?inflammatory effects. Clinical research further suggests that quercetin can help lower blood pressure in hypertensive individuals and reduce their risk of developing cardiovascular disease, highlighting its potential as a valuable plant?derived compound for the prevention and management of modern lifestyle?related diseases.

Free radicals and Health

Producing low to moderate, “physiological” levels of free radicals is actually essential for normal body function and defense. For example, phagocytic cells deliberately release free radicals to kill invading microbes. Molecules such as nitric oxide (NO), superoxide anion, and other reactive oxygen species (ROS) also act as important regulatory mediators in cell?signaling pathways. In higher organisms, NO and ROS help control vascular tone, regulate oxygen tension for breathing (ventilation), and influence erythropoietin production for red blood cell formation.[6] Free radicals are generated through both enzymatic and non?enzymatic processes. Enzymatic sources include the mitochondrial respiratory chain, phagocytosis, prostaglandin synthesis, and the cytochrome P450 system, while oxygen itself can drive non?enzymatic reactions that form free radicals.[7] Reactive oxygen species (ROS) and reactive nitrogen species (RNS) arise from endogenous factors—such as immune cell activation, inflammation, mental stress, intense exercise, ischemia, infection, cancer, and aging—as well as exogenous factors, including air and water pollution, cigarette smoke, alcohol, heavy and transition metals, and various pharmaceutical or pharmacological agents.

Quercetin

Quercetin (3,5,7,3′,4′?pentahydroxyflavone) is a widely distributed flavonoid found in many parts of plants, including flowers, bark, stems, and roots, as well as in common foods and beverages such as wine, tea, and a variety of vegetables and fruits. Rich dietary sources include apples, onions, berries, capers, dill, cilantro, and lovage, among others. Structurally, quercetin consists of three benzene rings and five hydroxyl (–OH) groups, a configuration that contributes to its notable biological and antioxidant properties.

Quercetin is a yellow, crystalline compound with a distinctly bitter taste. It is almost completely insoluble in cold water, only slightly soluble in hot water, but readily soluble in alcohol and lipids. Chemically, quercetin is an aglycone (or aglucone), meaning it does not contain any carbohydrate (sugar) moieties in its structure. This pigment contributes to the vivid coloration of many flowers. At the cellular level, quercetin is a potent antioxidant. Free radicals are highly reactive molecules that quickly attack other substances to gain electrons and become more stable; this electron “theft” turns the attacked molecule into a new free radical and can trigger damaging chain reactions in living cells. Structural studies have shown that specific features of the quercetin molecule are crucial for its antioxidant activity: the hydroxyl groups at positions 3, 5, and 7 on the A ring and at 3′ and 4′ on the B ring, the double bond between the second and third carbons in the central ring, and the carbonyl group at the fourth carbon all play key roles. Because of this structure, quercetin displays a wide range of biologically important effects. It has been reported to exert anticancer, antiallergic, antidiabetic, anti?obesity, and anti?hyperuricemia / anti?gout activities, among others. Notably, one of its most important impacts is its ability to inhibit the growth and spread of several types of cancer, including cancers of the breast, cervix, lung, colon, prostate, and liver.

Fig. 2 illustrates the chemical structure of quercetin and summarizes these key properties

Traditional Uses of Quercetin

Plant sources rich in quercetin have long been used in traditional medicine to manage a wide range of health problems. These uses include lowering blood pressure and blood glucose levels, providing antibacterial, anti?inflammatory, immune?stimulating, cardioprotective, and antioxidant effects, and supporting wound healing. Quercetin?containing remedies have also been used for their neurological benefits, such as reducing the risk of stroke and neuropathy, helping to maintain healthy cholesterol levels, relieving dyspepsia (indigestion), and lowering the risk of cancer according to traditional practices.

The Sources of Quercetin and Its Daily Intake

Quercetin (3,5,7,3′,4′?pentahydroxyflavone) is a bitter?tasting flavonoid found widely in plant?based foods. It occurs in many common vegetables and fruits such as tomato, potato, onion, green pepper, apple, parsley, grapes, broccoli, and blueberry, and is also present in plant species like tea, coriander, pepper, radish, fennel, and dill. More broadly, quercetin is obtained from a variety of vegetables, fruits, berries, nuts, beverages, and other plant?derived products. Among foods, raw capers contain some of the highest levels of quercetin (around 234 mg per 100 g of edible portion), whereas black and green tea contain relatively low amounts (about 2 mg per 100 g). Overall, the estimated daily intake of total dietary flavonoids ranges roughly from 50 to 800 mg, with quercetin often representing a large proportion of this intake. Actual quercetin consumption depends mainly on how frequently fruits, vegetables, and tea are included in the diet. For example, reported average daily intakes of quercetin in some regions of China are around 4–5 mg, with main food contributors including apples, potatoes, celery, lettuce, oranges, eggplant, and kiwifruit (Actinidia). In a larger adult population sample in China, average daily quercetin intake was reported at about 20 mg, based on self?reported food?frequency data. In the United States, average daily flavonoid intake has been estimated at around 13 mg, with roughly one?third of that amount coming from quercetin.

Fig. 3. Chief sources of quercetin.

The Role of Quercetin in Disease Management Through Different Mechanisms:

Anti?inflammatory effects

Inflammation is a protective response that removes damaged cells and pathogens and starts tissue repair, but excessive or chronic inflammation contributes to disease. Quercetin helps control this process by inhibiting key inflammatory enzymes, cyclo?oxygenase (COX) and lipoxygenase, which lowers the production of prostaglandins and leukotrienes. Experimental studies show that quercetin reduces inflammatory mediators (such as NO synthase, COX?2, and CRP), suppresses both acute and chronic inflammation in animals, and can lower CRP in some healthy individuals. It may also inhibit xanthine oxidase and thereby reduce uric acid levels, offering potential benefit in gout.

Table 2. Anti-inflammatory Potential of Quercetin in Disease Management

Cardiovascular protection

Dietary patterns rich in fruits and vegetables are linked to a lower risk of stroke and coronary heart disease, partly due to bioactive compounds like quercetin. Quercetin has antihypertensive, anti?atherosclerotic, and antiplatelet effects, improves endothelial function, and protects LDL cholesterol from oxidation. Human studies report that quercetin?rich extracts can enhance flow?mediated dilation, and doses around 150 mg/day have reduced systolic blood pressure and oxidized LDL in high?risk subjects. Quercetin also influences fat metabolism by limiting fat accumulation, promoting fat?cell death, and altering glucose uptake, supporting its role in overall cardiometabolic health.

Anticancer Potential of Quercetin

Quercetin is a plant?derived flavonoid that shows considerable promise as a natural anticancer agent with relatively low toxicity. Experimental and preclinical studies indicate that it can slow tumour growth by inhibiting cancer cell proliferation, arresting the cell cycle, and triggering apoptosis and autophagy. Quercetin also interferes with key oncogenic signalling pathways (such as PI3K/AKT, MAPKs, Wnt/β?catenin, and NF?κB) and modulates cancer?related microRNAs and epigenetic marks, thereby reducing tumour invasion, metastasis, and resistance to chemotherapy. Because it acts on multiple molecular targets at once, quercetin is being actively investigated both as a chemopreventive nutrient and as an adjuvant to conventional cancer therapies, with the goal of improving efficacy while limiting side effects.

Anti?Ageing Activity of Quercetin

Cellular senescence is an irreversible arrest of the cell cycle triggered by stresses such as telomere damage, DNA injury, and oxidative stress. Accumulation of these senescent cells contributes to age?related tissue degeneration, increased fat deposition, and liver steatosis. Studies in transgenic mice have shown that a combination of quercetin and dasatinib can selectively clear senescent cells and reduce liver fat, suggesting a potential anti?ageing strategy.

In islet transplantation models, quercetin has also shown promising anti?ageing effects. When rat islets were pre?treated with quercetin delivered via polymer microspheres and then cultured long term, the presence of quercetin slowed islet ageing. Transplanting these quercetin?treated islet clusters into diabetic mice resulted in better blood?glucose control than control islets, indicating that local antioxidant delivery (such as quercetin) may improve cell?therapy outcomes.

Quercetin additionally protects the liver in models of bile duct ligation (BDL) and carbon tetrachloride–induced injury. It reduces extracellular matrix deposition and modulates matrix metalloproteinase?9 (MMP?9) and its inhibitor TIMP?1. Mechanistically, quercetin appears to prevent liver failure by inhibiting the TGF?β1/Smads pathway, activating the PI3K/AKT pathway, and suppressing excessive autophagy, all of which contribute to its overall anti?ageing and organ?protective effects.

Neuroprotective Effects of Quercetin

Neurodegeneration is a key pathological feature in many brain disorders. Growing evidence suggests that plant?based medicines and nutraceuticals can help prevent rather than cure these conditions, and numerous phytochemicals have shown modulatory effects on the nervous system in experimental models.

Quercetin has demonstrated notable neuroprotective activity. In animal models, it reduced hallmark features of neurodegeneration, including extracellular β?amyloid accumulation, tau pathology, astrogliosis, and microgliosis in brain regions such as the hippocampus and amygdala. Behaviorally, quercetin treatment improved learning and spatial memory performance and enhanced risk?assessment behavior in maze tests.

Other studies show that quercetin can counteract aluminum?induced oxidative stress by improving mitochondrial antioxidant defenses (e.g., increased superoxide dismutase activity and reduced ROS production). It also modulates apoptosis?related proteins—downregulating pro?apoptotic factors like Bax and p53, upregulating anti?apoptotic Bcl?2, influencing caspase activation, and reducing DNA fragmentation—thereby protecting neurons from cell death. In cell culture models (such as PC12 cells exposed to hydrogen peroxide), pre?treatment with quercetin improved cell viability, further supporting its role as a neuroprotective antioxidant and anti?apoptotic agent.

Antidiabetic Activity of Quercetin

Quercetin has shown notable antidiabetic effects in experimental models of diabetes. It lowers blood glucose, improves lipid profiles (triglycerides and cholesterol), and enhances glucose tolerance, partly by preserving pancreatic β?cell structure and boosting hepatic glucokinase activity. Quercetin also reduces oxidative stress markers (NO, MDA), increases total antioxidant capacity and key antioxidant enzymes, and improves renal function in diabetic nephropathy by downregulating profibrotic factors such as CTGF and TGF?β1. Overall, it helps reduce blood glucose and HbA1c, improves insulin sensitivity, and supports liver and kidney function in diabetes.

Anti?Obesity Effects of Quercetin

Obesity is a major global health problem and a key risk factor for type 2 diabetes. In high?fat?diet (HFD) animal models, quercetin and quercetin?rich red onion extract have been shown to counteract obesity?related changes by limiting body?weight gain, reducing total body fat and liver weight, and lowering serum triglycerides. Histological studies reveal smaller adipocyte size and less lipid accumulation in quercetin?supplemented groups, along with improved glucose tolerance and insulin sensitivity. Quercetin also decreases inflammation in adipose tissue, including reduced macrophage and mast cell infiltration, supporting its potential as an anti?obesity and metabolic?protective agent.

Anti?Arthritis Activity of Quercetin

Rheumatoid arthritis (RA) is a chronic autoimmune disease that damages cartilage and bone, and standard treatments such as NSAIDs and corticosteroids often cause significant side effects. Quercetin has shown promise as a safer adjunct in RA management. In animal models, purified quercetin reduced inflammation and joint damage in experimental arthritis, partly by inhibiting adenosine deaminase activity and lowering inflammatory cytokines and neutrophil infiltration. In clinical studies on women with RA, eight weeks of quercetin supplementation significantly reduced morning pain, early?morning stiffness, and after?activity pain.

At the cellular level, quercetin acts on fibroblast?like synoviocytes (FLSs), key cells in RA joint destruction. It decreases production of inflammatory cytokines, promotes apoptosis of pathogenic FLSs (via upregulation of lncRNA MALAT1 and inhibition of the PI3K/AKT pathway), and suppresses their migration and invasion. Quercetin also modulates non?coding RNAs, increasing miR?146a and reducing GATA6 expression, which further helps limit synovial cell aggressiveness. Together, these findings support quercetin as a potential complementary therapy for RA with both anti?inflammatory and disease?modifying effects.

Effects of Quercetin in the Respiratory System

Respiratory diseases such as asthma, COPD, and lung cancer are major global causes of death, and traditional medicines have long been used against respiratory and infectious conditions. Quercetin has shown protective effects in several experimental respiratory models. In mice exposed to long?term cigarette smoke, quercetin reduced lung damage, decreased inflammatory cell infiltration, improved lung structure, lowered levels of inflammatory proteins, and limited emphysematous lung enlargement.

In COPD?related studies, quercetin restored corticosteroid responsiveness in cigarette smoke extract–exposed cells and in peripheral blood mononuclear cells from COPD patients, partly by activating AMPK and increasing Nrf2 expression. Quercetin also protected against cigarette smoke–induced mucus overproduction: it reduced goblet cell hyperplasia, inflammation in rat lungs, and downregulated Muc5ac expression and NF?κB activation in vitro. In pneumonia models (LPS?stimulated lung cells), quercetin decreased the release of pro?inflammatory mediators (IL?1β, PGE2, IL?6, nitrite), reduced ROS generation and apoptosis, and inhibited NF?κB activation, supporting its potential as a supportive agent in respiratory inflammatory diseases.

Wound?Healing Effects of Quercetin

Effective wound care is vital for health, as delayed healing increases complications and costs. Wound repair is a complex, multi?step process influenced by factors such as nutrition, medications, radiation, smoking, and oxygen levels. Natural compounds with collagen?stimulating, anti?inflammatory, and antibacterial properties are therefore of great interest.

Quercetin has shown clear wound?healing benefits in both in vitro and in vivo models. It promotes fibroblast proliferation and migration, accelerates closure of cutaneous wounds in mice, restores more normal dermal structure, and increases collagen fiber content. In rat wound models, quercetin?containing gels led to faster wound contraction, smaller unhealed areas, and significantly higher numbers of fibroblasts compared with controls, along with progressive reductions in inflammatory cells and marked increases in collagen I synthesis. High?dose quercetin groups showed the quickest wound contraction and the greatest collagen deposition and fibroblast distribution on histological examination, highlighting its potential as a topical wound?healing agent.

Anti?Depression Activity of Quercetin

Depression is a common mental disorder that causes low mood, loss of interest, sleep and appetite changes, poor concentration, and impaired daily functioning. Conventional antidepressants can help, but often produce side effects such as sexual dysfunction, cardiovascular issues, and weight gain. Experimental studies suggest quercetin may offer a gentler, supportive option.

In animal models, quercetin reduced corticosterone?induced depression?like behaviors, while exerting antioxidant and anti?inflammatory effects in the hippocampus and prefrontal cortex. In chronic stress models (CUMS/CUS), quercetin improved depression?related behaviors, lowered markers of oxidative and nitrosative stress (such as MDA and iNOS), and increased protective factors like SOD and Bcl?2. It also helped normalize stress?related hormonal changes (corticosterone) and improved measures such as climbing ability and sucrose preference. In chemotherapy?related models (adriamycin?treated rats), quercetin lessened anxio?depressive?like behavior, reduced brain oxidative stress, and partially corrected immune and neuroendocrine disturbances, supporting its potential as an adjunct in stress? and drug?induced mood disorders.

Immunomodulatory Effects of Quercetin

Quercetin shows important immunomodulatory activity by acting on key inflammatory and immune pathways. It can inhibit activation of the NLRP3 inflammasome, thereby reducing expression of inflammatory proteins and caspase?1. Quercetin also limits foam?cell formation and promotes autophagy, helping delay cellular senescence and potentially slowing atherosclerosis progression. In rheumatoid arthritis models, it lowers circulating inflammatory cytokines and reduces neutrophil infiltration, contributing to less joint inflammation. In airway epithelial cells, quercetin pretreatment decreases Akt phosphorylation, viral endocytosis, and IL?8 production; notably, even when given hours after rhinovirus infection, it can still reduce viral load and dampen IL?8 and interferon responses, highlighting its dual antiviral and immune?regulating potential.

Antiviral Activity of Quercetin

Quercetin has shown broad antiviral effects against multiple human and animal viruses. In cell models, it inhibited dengue virus type 2, human T?lymphotropic virus?1, Japanese encephalitis virus, and hepatitis C virus, often producing large reductions in viral load and infectivity. In EBV?associated gastric carcinoma cells, quercetin both triggered apoptosis and cell?cycle arrest and reduced EBV latency and infection, suggesting potential as a lead compound for EBV?related cancers.

Quercetin and its derivatives have also demonstrated activity against Zika virus, influenza A virus, porcine epidemic diarrhea virus, and herpes simplex viruses (including drug?resistant strains) by reducing viral attachment, entry, and replication. In animal models, quercetin?3?glucoside improved survival and reduced weight loss after Zika infection. Additionally, quercetin can enhance antiviral responses when combined with other agents such as vitamin C, generally with minimal added side effects, highlighting its promise as a supportive antiviral and adjunct to standard therapies.

Clinical Effects of Quercetin

Quercetin is reported to offer multiple health benefits, including protection against conditions such as osteoporosis, lung cancer, and cardiovascular disease. Population studies suggest that people with higher flavonoid intake have a lower risk of cardiovascular events. In chronic obstructive pulmonary disease (COPD)—a leading cause of death—current therapies are only partly effective and can have side effects. Preclinical data indicate that raising plasma quercetin levels can markedly reduce lung inflammation and slow disease progression, supporting its role as a potent in vivo anti?inflammatory agent. Animal studies with quercetin?enriched diets have also shown reduced expression of inflammatory genes. Clinically, a 12?week trial using 1000 mg/day quercetin lowered upper respiratory tract infection rates in middle?aged and older adults.

Quercetin has shown neuroprotective properties, particularly when combined with fish oil in rat studies, and has been linked to beneficial effects in neurodegenerative disease models. Owing to its strong free?radical scavenging capacity, quercetin may help prevent cancers driven by oxidative stress. Several clinical trials have examined its cardiovascular effects: doses around 150–162 mg/day reduced systolic blood pressure and plasma oxidized LDL in overweight or prehypertensive/hypertensive subjects, and 500 mg/day lowered systolic blood pressure in women with type 2 diabetes. Some studies found genotype?specific effects, with quercetin lowering blood pressure but modestly altering HDL and apoA1 in certain apoE genotypes. Beyond cardiometabolic outcomes, small clinical studies combining low?dose quercetin (20 mg) with curcumin (480 mg) have shown benefit in familial adenomatous polyposis and in cadaveric kidney transplant recipients, suggesting potential as an adjunct therapy in diverse clinical settings.

Bioavailability of Quercetin and Nanoformulations

Although many in vitro studies show strong beneficial effects of quercetin, translating these findings into in vivo benefits is challenging because of factors such as bioavailability, metabolism, dose, physiological variability, and long?term safety. Quercetin is rapidly metabolized and excreted, which means only small amounts reach and persist at diseased sites. Human data suggest relatively low absorption (roughly in the single?digit to low?teens percentage range), and animal studies show that most of an oral dose can be metabolized within an hour.

Fig. 4. Recent strategies on the enhancement of quercetin bioavailability.

Improving quercetin’s bioavailability is therefore crucial for fully exploiting its therapeutic potential. Nanoformulation approaches—such as hybrid hydrogels, self?nanoemulsifying delivery systems, zein nanoparticles, LipoMicel systems, and poly(lipoic acid) nanoparticles—have substantially enhanced quercetin’s stability, solubility, absorption, and protection from degradation. Pharmacokinetic studies report dramatic increases in exposure: up to about 18? to 62?fold higher total bioavailability with hybrid?hydrogel quercetin, several?fold higher dissolution and in vitro uptake with self?nanoemulsifying systems, and multiple?fold increases in AUC and Cmax with LipoMicel formulations and polymeric nanoparticles compared with unformulated quercetin. Overall, nano? and advanced delivery systems markedly improve oral absorbability and systemic levels of quercetin, supporting their use in future therapeutic applications.

Table 3. The Impact of a Quercetin-Based Nanoformulation on Different Pathogenic Processes