Quercetin is one of the most consumed and important bioflavonoid components and is widely found in different varieties of fruits and vegetables. Plant species, growing conditions, harvest conditions, and storage methods can influence the polyphenolic composition of fruits and vegetables. Quercetin is found in abundance in onions, apples, and wine. According to several studies, Quercetin is also found in tea, pepper, coriander, fennel, radish, and dill ^[1][32]. More than 20 plants species produce Quercetin: Foeniculum vulgare, Curcuma domestica valeton, Santalum album, Cuscuta reflexa, Withania somnifera, Emblica officinalis, Mangifera indica, Daucus carota, Momordica charantia, Ocimum sanctum, Psoralea corylifolia, Swertia chirayita, Solanum nigrum, and Glycyrrhiza glabra, Morua alba, Camellia sinensis ^[2][33], Allium fistulosum, A. cepa, Calamus scipionum, Moringa oleifera, Centella asiatica, Hypericum hircinum, H. perforatum, Apium graveolens, Brassica oleracea var. Italica, B. oleracea var. sabellica, Coriandrum sativum, Lactuca sativa, Nasturtium officinale, Asparagus officinalis, Capparis spinosa, Prunus domestica, P. avium, Malus domestica, Vaccinium oxycoccus, and Solanum lycopersicum ^[3][9]. Quercetin is available in capsule and powder form as a dietary supplement. The plasma Quercetin concentration rises when Quercetin is consumed in the form of foods or supplements (Table 1). As a result, everyday consumption of Quercetin-rich foods increases Quercetin bioavailability and contributes to the prevention of lifestyle-related disorders ^[1][32].

Quercetin was isolated from a fractionated extract of Rubus fruticosus by using an optimized column in HPLC and increasing its concentration by using a nanofiltration membrane ^[4][34]. Extraction of Quercetin from different plant sources can be followed by effective sample preparation techniques known as the sea sand disruption method (SSDM). The SSDM is used due to its recovery efficiency ^[5][35]. During the isolation of Quercetin and its derivatives in plants’ source, SSDM is used to eliminate errors in the study ^[6][36]. Flavonoids are isolated from the crude extract of plants by using various organic solutions followed by HPLC analysis, which is further characterized by FTIR, NMR, and mass spectroscopy ^[7][37]. Quercetin-3-O-rhamnoside was isolated from P. thonningii leaves by using different organic solvents ^[8][38]. According to one study, dihydroQuercetin, one of the Quercetin derivates, was isolated from Larix gmelinii using ultrasound-assisted and microwave-assisted alternate digestion methods because they required less extraction time, less energy, and were more cost-effective than conventional solvent extraction methods ^[9][39]. Another derivative known as Isorhamnetin was isolated from the crude extract of Stigma maydis through two-stage high-speed countercurrent chromatography processes, where two-phase solvent systems composed of n-hexane-ethyl acetate-methanol-water are used at volume ratios of 5:5:5:5 and 5:5:6:4 to ensure the purity ^[10][40].

Quercetin is taken as glycosides, with glycosyl groups released during chewing, digestion, and absorption. In humans, only a small percentage of Quercetin is absorbed in the stomach, and the primary site of absorption is the small intestine ^[44][70]. Two methods allow Quercetin glycosides to be absorbed in the intestine. One method is lactose polarizing hydrolase (LPH) in the brush border membrane, and another method is the interaction with the sodium-dependent glucose transporter (SGLT1) ^[2][33]. The gut microbiota plays a crucial role in the absorption of Quercetin by enzymatic hydrolysis. After absorption, the metabolism of Quercetin takes place in various organs, including the small intestine, colon, liver, and kidney. Biotransformation enzymes in the small intestine and liver create methylated, sulfated, and glucuronate forms of Quercetin metabolites due to phase II metabolism ^[1][32]. After that, these are released into the bloodstream via the portal vein of the liver. In the small intestine and colon, Quercetin metabolism leads to the generation of phenolic acids. The metabolites of Quercetin are found in human plasma as methylated glucuronide or unmethylated sulfate. The major metabolite of Quercetin, Quercetin-3-o-b-D-glucuronide, is delivered to target tissues via plasma to exert biological activity ^[1][32]. Quercetin had a short half-life and rapid clearance in the blood, and its metabolites appeared in the plasma 30 min after ingestion; however, considerable amounts were excreted over 24 h ^[45][71]. In comparison to other phytochemicals, Quercetin has a high bioavailability. The bioavailability of Quercetin decreases when consumed as a supplement rather than food. Quercetin is excreted from the human body in the feces and urine, and in high doses, it can be discharged through the lungs. 3-hydroxy phenylacetic acid, hippuric acid, and benzoic acid are the excretory products of Quercetin ^[1][32].

Flavonoids, particularly Quercetin, which has well-known antioxidant effects, are gaining popularity these days. Quercetin has been identified as a potential anticancer drug with activity both in in vitro and in vivo models. Quercetin is used to inhibit the spread of various cancers, such as lung, prostate, liver, breast, colon, and cervical cancers, by modifying oxidative stress factors and antioxidant enzymes ^[46][8]. Because of its chemoprotective action against tumor cell lines through metastasis and apoptosis, Quercetin is thought to be a promising anticancer option ^[47][72]. Furthermore, another study revealed the powerful efficiency of combined Quercetin-doxorubicin treatment in maintaining T-cell tumor-specific responses, resulting in better immune responses against breast tumor growth ^[48][73]. Antioxidants work against asthma pathogenesis by avoiding oxidative damage through a variety of methods. Quercetin plays a role in scavenging free radicals that can lead to cell death by damaging DNA and cell membranes. In addition, it has been noted that Quercetin decreases the production and release of histamine and other mediators involved in the development of allergic reactions in mast cells, suggesting that it could be effective against asthma ^[49][74]. In in vitro and in vivo studies, Quercetin has been shown to protect neurons from oxidative and neurotoxic chemicals, saving the central nervous system from oxidative stress-induced neurodegenerative diseases, especially Alzheimer’s disease (AD) and Parkinson’s disease (PD). Quercetin has been shown as anti-Alzheimer’s because it improves mitochondrial morphology, improves memory impairments, protects cognitive deficits, and reduces neurodegeneration ^[1][32].