Quercetin and its glycosides (QG), vitally natural flavonoid, have been popular for health benefits. However, the absorption and metabolism affect their bioavailability, and the metabolic transformation alters their biological activities. This review systematically summarizes the bioavailability and pathways for the absorption and metabolism of quercetin/QG in vivo and in vitro, the biological activities and mechanism of quercetin/QG and their metabolites in treating glucolipid metabolism are discussed. After oral administration, quercetin/QG are mainly absorbed by the intestine, undergo phase II metabolism in the small intestine and liver to form conjugates and are metabolized into small phenolic acids by intestinal microbiota. Quercetin/QG and their metabolites exert beneficial effects on regulating glucolipid metabolism disorders, including improving insulin resistance, inhibiting lipogenesis, enhancing thermogenesis, modulating intestinal microbiota, relieving oxidative stress, and attenuating inflammation. This review enhances understanding of the mechanism of quercetin/QG regulate glucolipid metabolism and provides scientific support for the development of functional foods.

Natural compounds such as polyphenols play several positive roles in maintaining the oxidative and inflammatory capacity of cells, which leads to their potential use as anticancer therapeutics. There is promising evidence for the in vitro and in vivo anticancer activity of many polyphenols, including resveratrol and quercetin, specifically in the treatment of colorectal cancer (CRC). There is a clear association between resveratrol and quercetin in interfering with the mechanistic pathways involved in CRC, such as Wnt, P13K/AKT, caspase-3, MAPK, NF-κB, etc. These molecular pathways establish the role of resveratrol and quercetin in controlling cancer cell growth, inducing apoptosis, and inhibiting metastasis. The major bottleneck in the progression of the use of resveratrol and quercetin as anticancer therapeutics is their reduced bioavailability in vivo because of their rapid metabolism in humans. Recent advancements in various nanotechnological formulations are promising for overcoming these bioavailability issues. Various nanoformulations of resveratrol and quercetin have shown an optimistic impact on reducing the solubility and improving the stability of resveratrol and quercetin in vivo. A combinatorial approach using nanoformulations of resveratrol with quercetin could potentially increase the impact of resveratrol in controlling CRC cell proliferation. This review discusses the mechanism of resveratrol and quercetin, the two bioactive polyphenolics, in colon cancer, with an emphasis on various types of nanoformulations of the two molecules targeting colon cancer. It also explores the synergistic effect of combining resveratrol and quercetin in various nanoformulations, targeting colon cancer. This research delves into the enhanced pharmacokinetics and potential chemotherapeutic benefits of these bioactive polyphenolics when used together in innovative ways.

This comprehensive exploration delves into the multifaceted attributes of quercetin, a flavonoid with extensive health-promoting potential. The review navigates through its fundamental properties, encompassing its chemical structure, classification as a flavonoid, and its natural prevalence in various sources. Addressing solubility, stability, and bioavailability challenges, the investigation delves into innovative isolation techniques, including solvent extraction, solid-phase extraction, natural deep eutectic solvents, supercritical fluid extraction, microwave-assisted extraction, column chromatography, and high-performance thin-layer chromatography. Transitioning into pharmacological implications, the study unveils quercetin\'s roles in anti-inflammatory pathways, antioxidant effects, and immune modulation, reflecting its versatile significance in health management. The review highlights its impact on wound healing processes and its potential to mitigate arthritis, elucidating its holistic contributions. Culminating in an exploration of recent studies, the analysis underscores quercetin\'s remarkable anti-inflammatory and anti-arthritis activities, reflecting its substantial potential across various ailments. The review concludes by projecting future trajectories, emphasizing prospects for an advanced understanding of quercetin\'s mechanisms, sustainable extraction techniques, clinical integration, and exploration of synergistic combinations. Collectively, this review investigation underscores quercetin\'s dynamic role at the intersection of natural compounds and medicinal applications, offering profound implications for well- being and health enhancement.

Traumatic brain injury (TBI) is a leading cause of disability worldwide, with an estimated annual incidence of 27-69 million. TBI is a severe condition that can lead to high mortality rates and long-term cognitive, behavioral, and physical impairments in young adults. It is a significant public health concern due to the lack of effective treatments available. Quercetin, a natural flavonoid found in various fruits and vegetables, has demonstrated therapeutic potential with anti-inflammatory, antioxidant, and neuroprotective properties. Recently, some evidence has accentuated the ameliorating effects of quercetin on TBI. This review discusses quercetin\'s ability to reduce TBI-related damage by regulating many cellular and molecular pathways. Quercetin in vitro and in vivo studies exhibit promise in reducing inflammation, oxidative stress, apoptosis, and enhancing cognitive function post-TBI. Further clinical investigation into quercetin\'s therapeutic potential as a readily available adjuvant in the treatment of TBI is warranted in light of these findings. This review adds to our knowledge of quercetin\'s potential in treating TBI by clarifying its mechanisms of action.

Parkinson\'s disease (PD) is a neurodegenerative disease that affects dopaminergic neurons, thus impairing dopaminergic signalling. Quercetin (QUE) has antioxidant and neuroprotective properties that are promising for the treatment of PD. This systematic review aimed to investigate the therapeutic effects of QUE against PD in preclinical models. The systematic search was performed in PubMed, Scopus and Web of Science. At the final screening stage, 26 articles were selected according to pre-established criteria. Selected studies used different methods for PD induction, as well as animal models. Most studies used rats (73.08%) and mice (23.08%), with 6-OHDA as the main strategy for PD induction (38.6%), followed by rotenone (30.8%). QUE was tested immersed in oil, nanosystems or in free formulations, in varied routes of administration and doses, ranging from 10 to 400 mg/kg and from 5 to 200 mg/kg in oral and intraperitoneal administrations, respectively. Overall, evidence from published data suggests a potential use of QUE as a treatment for PD, mainly through the inhibition of oxidative stress, neuroinflammatory response and apoptotic pathways.

Fat synthesis and lipolysis are natural processes in growth and have a close association with health. Fat provides energy, maintains physiological function, and so on, and thus plays a significant role in the body. However, excessive/abnormal fat accumulation leads to obesity and lipid metabolism disorder, which can have a detrimental impact on growth and even harm one\'s health. Aside from genetic effects, there are a range of factors related to obesity, such as excessive nutrient intake, inflammation, glycometabolism disease, and so on. These factors could serve as potential targets for anti-obesity therapy. Quercetin is a flavonol that has received a lot of attention recently because of its role in anti-obesity. It was thought to have the ability to regulate lipid metabolism and have a positive effect on anti-obesity, but the processes are still unknown. Recent studies have shown the role of quercetin in lipid metabolism might be related to its effects on inflammatory responses and glycometabolism. The references were chosen for this review with no date restrictions applied based on the topics they addressed, and the databases PubMed and Web of Sicence was used to conduct the references research, using the following search terms: \"quercetin\", \"obesity\", \"inflammation\", \"glycometabolism\", \"insulin sensitivity\", etc. This review summarizes the potential mechanisms of quercetin in alleviating lipid metabolism through anti-inflammatory and hypoglycemic signaling pathways, and describes the possible signaling pathways in the interaction of inflammation and glycometabolism, with the goal of providing references for future research and application of quercetin in the regulation of lipid metabolism.

Malignant tumors are the second most common cause of death worldwide. More attention is being paid to the link between the body\'s impaired oxidoreductive balance and cancer incidence. Much attention is being paid to polyphenols derived from plants, as one of their properties is an antioxidant character: the ability to eliminate reactive oxygen and nitrogen species, chelate specific metal ions, modulate signaling pathways affecting inflammation, and raise the level and activity of antioxidant enzymes while lowering those with oxidative effects. The following three compounds, resveratrol, quercetin, and curcumin, are polyphenols modulating multiple molecular targets, or increasing pro-apoptotic protein expression levels and decreasing anti-apoptotic protein expression levels. Experiments conducted in vitro and in vivo on animals and humans suggest using them as chemopreventive agents based on antioxidant properties. The advantage of these natural polyphenols is low toxicity and weak adverse effects at higher doses. However, the compounds discussed are characterized by low bioavailability and solubility, which may make achieving the blood concentrations needed for the desired effect challenging. The solution may lie in derivatives of naturally occurring polyphenols subjected to structural modifications that enhance their beneficial effects or work on implementing new ways of delivering antioxidants that improve their solubility and bioavailability.

OBJECTIVE: This review aims to provide a theoretical basis and insights for quercetin\'s clinical application in the prevention and treatment of osteoporosis (OP), analyzing its roles in bone formation promotion, bone resorption inhibition, anti-inflammation, antioxidant effects, and potential mechanisms.
RESULTS: OP, a prevalent bone disorder, is marked by reduced bone mineral density and impaired bone architecture, elevating the risk of fractures in patients. The primary approach to OP management is pharmacotherapy, with quercetin, a phytochemical compound, emerging as a focus of recent interest. This natural flavonoid exerts regulatory effects on bone marrow mesenchymal stem cells, osteoblasts, and osteoclasts and promotes bone health and metabolic equilibrium via anti-inflammatory and antioxidative pathways. Although quercetin has demonstrated significant potential in regulating bone metabolism, there is a need for further high-quality clinical studies focused on medicinal quercetin.

BACKGROUND: Metformin is an insulin sensitizer that is widely used for the treatment of insulin resistance in polycystic ovary syndrome patients. However, metformin can cause gastrointestinal side effects.
OBJECTIVE: This study showed that the effects of quercetin are comparable to those of metformin. Therefore, this study aimed to systematically evaluate the efficacy of quercetin in treating PCOS.
METHODS: The present systematic search of the Chinese National Knowledge Infrastructure (CNKI), Wanfang Data Information Site, Chinese Scientific Journals Database (VIP), SinoMed, Web of Science, and PubMed databases was performed from inception until February 2024. The methodological quality was then assessed by SYRCLE\'s risk of bias tool, and the data were analyzed by RevMan 5.3 software.
RESULTS: Ten studies were included in the meta-analysis. Compared with those in the model group, quercetin in the PCOS group had significant effects on reducing fasting insulin serum (FIS) levels (P = 0.0004), fasting blood glucose (FBG) levels (P = 0.01), HOMA-IR levels (P < 0.00001), cholesterol levels (P < 0.0001), triglyceride levels (P = 0.001), testosterone (T) levels (P < 0.00001), luteinizing hormone (LH) levels (P = 0.0003), the luteinizing hormone/follicle stimulating hormone (LH/FSH) ratio (P = 0.01), vascular endothelial growth factor (VEGF) levels (P < 0.00001), malondialdehyde (MDA) levels (P = 0.03), superoxide dismutase (SOD) levels (P = 0.01) and GLUT4 mRNA expression (P < 0.00001).
CONCLUSIONS: This meta-analysis suggested that quercetin has positive effects on PCOS treatment. Quercetin can systematically reduce insulin, blood glucose, cholesterol, and triglyceride levels in metabolic pathways. In the endocrine pathway, quercetin can regulate the function of the pituitary-ovarian axis, reduce testosterone and luteinizing hormone (LH) levels, and lower the ratio of LH to follicle-stimulating hormone (FSH). Quercetin can regulate the expression of the GLUT4 gene and has antioxidative effects at the molecular level.

OBJECTIVE: To systematically evaluate the safety and efficacy of antioxidant therapy in children and adolescents with attention deficit hyperactivity disorder (ADHD).
METHODS: Randomized controlled trials and prospective studies on antioxidant therapy in children and adolescents with ADHD were searched in PubMed, Embase, and Cochrane Library from the inception of databases to November 12, 2022. Two investigators independently screened the literature, extracted data, and evaluated the quality of the included studies. Network meta-analysis (PROSPERO registration number CRD 42023382824) was carried out by using R Studio 4.2.1.
RESULTS: 48 studies involving 12 antioxidant drugs (resveratrol, pycnogenol, omega-3, omega-6, quercetin, phosphatidylserine, almond, vitamin D, zinc, folic acid, ginkgo biloba, Acetyl-L-carnitine) were finally included, with 3,650 patients. Network meta-analysis showed that omega-6 (0.18), vitamin D (0.19), and quercetin (0.24) were the top three safest drugs according to SUCRA. The omega-3 (SUCRA 0.35), pycnogenol (SUCRA 0.36), and vitamin D (SUCRA 0.27) were the most effective in improving attention, hyperactivity, and total score of Conners\' parent rating scale (CPRS), respectively. In terms of improving attention, hyperactivity, and total score of Conners\' teacher rating scale (CTRS), pycnogenol (SUCRA 0.32), phosphatidylserine+omega-3 (SUCRA 0.26), and zinc (SUCRA 0.34) were the most effective, respectively. In terms of improving attention, hyperactivity and total score of ADHD Rating Scale-Parent, the optimal agents were phosphatidylserine (SUCRA 0.39), resveratrol+MPH (SUCRA 0.24), and phosphatidylserine (SUCRA 0.34), respectively. In terms of improving attention, hyperactivity and total score of ADHD Rating Scale-Teacher, pycnogenol (SUCRA 0.32), vitamin D (SUCRA 0.31) and vitamin D (SUCRA 0.18) were the optimal agents, respectively. The response rate of omega-3+6 was the highest in CGI (SUCRA 0.95) and CPT (SUCRA 0.42).
CONCLUSIONS: The rankings of safety and efficacy of the 12 antioxidants vary. Due to the low methodological quality of the included studies, the probability ranking cannot fully explain the clinical efficacy, and the results need to be interpreted with caution. More high-quality studies are still needed to verify our findings.