Investigating the Antioxidant Properties of Quercetin

Abstract

The antioxidant properties of quercetin stem from its ability to neutralize reactive oxygen species (ROS) and counteract oxidative stress, a key contributor to various chronic diseases. Numerous in vitro studies have demonstrated quercetin’s effectiveness in scavenging free radicals and protecting cellular structures from oxidative damage. Beyond its direct antioxidant effects, quercetin also interacts with cellular signaling pathways, influencing gene expression and modulating enzymatic activities associated with oxidative stress. In vivo studies, both in animals and human trials, have provided insights into the bioavailability and physiological impact of quercetin, yet its significance remains underappreciated. This chapter will focus on the mechanisms by which quercetin enters circulation, its distribution in tissues, and the subsequent effects on markers of oxidative stress. Additionally, we will highlight findings from previous epidemiological studies linking quercetin-rich diets to reduced risk of chronic diseases, emphasizing the potential translational significance of these antioxidant properties in real-world health outcomes. In conclusion, this chapter will provide an overview of quercetin’s antioxidant properties and its potential for therapeutic interventions associated with chronic diseases.

1. Introduction

Quercetin’s chemical structure, characterized by the presence of multiple hydroxyl groups, provides it with potent free radical scavenging abilities, making it a key player in cellular defense against oxidative stress [1]. While there have been various reports indicating the potential therapeutic benefits of quercetin, its mechanism of action remains elusive and there is a dearth of clinical data to support its potential antioxidant properties in humans [2]. Quercetin, belonging to the flavonol subclass is widely distributed in nature and can be found in various food sources, including fruits vegetables and beverages. The availability of quercetin in commonly consumed dietary items has contributed to its accessibility and the growing interest in harnessing its potential health benefits. The structural arrangement of quercetin includes several hydroxyls (∙OH) groups attached to different positions on the benzene rings, specifically at C3, C5, C7, C3′, C4′, and C5′ [3]. These hydroxyl groups are responsible for the antioxidant properties of quercetin, making it a potent scavenger of free radicals in the human body [4]. The conjugation of double bonds in the aromatic rings and the presence of hydroxyl groups further create a complex structure that plays a crucial role in quercetin’s bioactivity and physiological effects. Additionally, the presence of a glycosyl group linked to the quercetin aglycone can affect its absorption, solubility, and in vivo effects [5]. Most studies focused on the biological effects of quercetin are based on in vitro research with quercetin aglycone, which is rarely found in human plasma [6, 7]. The findings from these studies have yielded inconclusive results regarding the exact mechanism of action of quercetin aglycone and its derived metabolites in humans [8]. Given the insufficient data in this research area, several authors have emphasized the importance of studying quercetin and their derived metabolites, considering that these metabolites may be the bioactive forms in the body, requiring further research to understand their mechanism of action and antioxidant activities [9, 10, 11]. In this study, our aim is to conduct a thorough investigation of the antioxidant capabilities of quercetin and its derivatives, and their impact on human health. Specifically, we will focus on their cellular signaling pathways, bioavailability, tissue distribution, and their association with reduced risk of chronic diseases.

2. Antioxidant role of quercetin in scavenging free radicals

Free radicals, including reactive oxygen species and reactive nitrogen species (RNS), are highly reactive molecules produced naturally during metabolic processes or by external factors like UV radiation, pollution, and certain chemicals. When present in excess, these free radicals can induce oxidative stress to cellular components such as proteins and lipids, leading to various chronic health conditions including neurodegenerative and cardiovascular diseases. Quercetin exerts its antioxidative effects through multiple mechanisms. Firstly, it acts as a direct scavenger of free radicals, neutralizing them and preventing them from causing harm to cellular structures as shown in Figure 1. Additionally, quercetin enhances the activity of endogenous antioxidant enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase, which further contribute to the cellular defense against oxidative stress.

Figure 1.

The antioxidant activity of quercetin in scavenging free radicals.

Numerous in vitro and in vivo studies have highlighted the ability of quercetin to neutralize oxidative stress, making it a promising candidate for mitigating various chronic diseases associated with oxidative damage [12, 13, 14, 15]. Various studies have evaluated the antioxidant efficacy of quercetin and its derivatives using different assays, including 2,2-diphenyl-1-picrylhydrazyl (DPPH), Ferric Reducing Antioxidant Power (FRAP), Fe²⁺ chelation, and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays [16, 17]. The antioxidant activity of quercetin derivatives can be influenced by factors such as the acyl group chain length and the molar mass of the polymer. While the acylation of quercetin increases its lipophilicity, this can potentially influence its antioxidant efficacy. According to Oh et al. [18], the esterification of quercetin with various fatty acyl chlorides resulted in the formation of different derivatives, including Q-3′-O-monoester, Q-3-O-monoester, Q-7,3′-O-diester, Q-3′,4’-O- diester, Q-3,3′-O-diester, and Q-3,4′-O diester. The lipophilicity of these derivatives following the esterification process increased as expected. The results showed that quercetin had the highest activity in getting rid of harmful radicals compared to its derivatives. Interestingly, superior antioxidant activity was observed with the short-chain fatty acids derivatives in the ABTS assay, while those with C3:0, C4:0, C6:0, and C8:0 chains showed better performance in the DPPH radical scavenging assay.

In a previous study where the impact of nicotine and quercetin treatments on the viability and antioxidant defense system of HepG2 cell line cultures was investigated, quercetin showed an antioxidant protective effect by restoring the balance between free radical production and antioxidant defense systems [19]. The in vitro experimental results showed that quercetin increased the superoxide dismutase activity in both the control and nicotine treated group. The results were comparable to those from previous studies [20, 21]. Some studies have shown the ability of quercetin to combat oxidative stress by modulating glutathione (GSH) levels. For instance, the supplementation of quercetin in a Western diet increased the GSH/GSSG ratio in mice liver and mitigated obesity and metabolic syndrome [15]. The concentration of quercetin did not significantly influence body weight, fat storage, or blood composition in healthy mice following a standard diet. However, it did decrease oxidative stress levels, specifically in the liver. A higher quercetin concentration was observed to further decrease lipid peroxidation markers in various tissues. Additionally, quercetin’s capacity to neutralize free radicals can be associated with a decrease in ischemia-reperfusion injury, achieved through alterations in the endothelial nitric oxide system [22]. This highlights the potential of quercetin to mitigate oxidative stress and protect against damage caused by ischemia-reperfusion processes. As an extension to this, recent research has emphasized quercetin’s role in enhancing mitochondrial function [23, 24], further contributing to its overall protective effects on chronic and cardiovascular health diseases.

2.1 Absorption, distribution and metabolism of quercetin in tissues

The absorption of quercetin is a complex process influenced by several factors. Dietary intake is the primary source, and its absorption occurs in the small intestine. Quercetin, in its glycosidic form, undergoes hydrolysis by enzymes in the small intestine, releasing aglycones that are absorbed through passive diffusion [25]. Glucose transporters (GLUTs) and multidrug resistance-associated proteins (MRPs) are involved in the active transport of quercetin [26]. The release of aglycones from quercetin glycosides can be mediated by the lactase phlorizin hydrolase (LPH) in the small intestines [25]. Various factors impact the absorption of quercetin, including the food matrix, co-ingestion with other nutrients, and the gut microbiota. The presence of fats and certain nutrients enhances absorption, while high-fiber diets may reduce bioavailability. The bioavailability of quercetin can be improved when consumed alongside fatty acids in higher doses. Humans can absorb significant amounts from food or supplements, with a reported half-life of 11–28 hours. While daily quercetin intakes range from 3 to 40 mg in Western diets, consumers of fruits and vegetables may ingest about 250 mg per day. The recommended daily dose of quercetin supplements in diets should range from 500 to 1000 mg [27]. Additionally, the gut microbiota plays a crucial role in quercetin metabolism, influencing its absorption and bioactivity [28].

Once absorbed, quercetin is distributed to various tissues through the bloodstream. The distribution is influenced by its lipophilic nature, allowing it to penetrate cell membranes. On the other hand, quercetin exhibits a wide tissue distribution, with higher concentrations found in organs like the lungs, kidneys, and liver. The ability of quercetin to cross the blood-brain barrier has also raised interest in its potential neuroprotective effects [29, 30]. In the bloodstream and tissues, it undergoes extensive metabolism and biotransformation with phase II enzymes, such as glucuronosyltransferases and sulfotransferases, facilitating its excretion [31]. Special transporters also play a crucial role in facilitating the absorption and transport of quercetin and its metabolites. The lipid bilayers of cellular membranes, characterized by high polarity, limits the passage of quercetin conjugates where membrane-related transporters come to play to overcome this barrier. In the intestinal epithelial cells, numerous transporters are expressed to aid the passage of substrates from the gastrointestinal tract into the circulatory system [32]. Among these transporters, sodium-dependent glucose co-transporters (SGLTs) located on the apical membrane of intestinal epithelial cells play a significant role in the absorption of quercetin and its glycosides [33]. Specifically, quercetin 4′-β-glucoside and quercetin-3-glucoside are efficiently absorbed through the involvement of SGLT-1. There is evidence that the type of sugar linked to quercetin can influence the absorption ratio of quercetin glycosides in the small intestine [34]. Quercetin circulates in the bloodstream bound to plasma proteins, such as albumin [35]. The presence of binding proteins can influence its bioavailability and may impact its physiological effects. Understanding the binding characteristics of quercetin to plasma proteins is important for optimizing its delivery and enhancing its therapeutic potential. Further research is also needed to elucidate the specific mechanisms by which binding proteins interact with quercetin and how the interaction influences its bioavailability and physiological effects [36]. Furthermore, exploring strategies to enhance the bioavailability of quercetin, such as encapsulation in delivery systems like Quercetin LipoMicel, may improve its absorption and increase its circulating levels in the body [37].

2.2 Cellular signaling pathways of quercetin: modulation of enzymatic activities in oxidative stress

The pathway associated with the nuclear factor erythroid 2-related factor 2 (Nrf2) is vital for protecting cells from oxidative stress. Quercetin has been shown to activate the Nrf2 pathway, leading to the upregulation of antioxidant response element (ARE)-driven genes. This activation enhances the cellular antioxidant defense system, including the expression of enzymes like heme oxygenase-1 (HO-1) and superoxide dismutase. Quercetin and its derivative dihydroquercetin have demonstrated the ability to mitigate cellular damage induced by oxidative stress by activating the Nrf2-ARE pathway, with the involvement of stress-responsive Extracellular Signal-Regulated Kinase (ERK) and c-Jun N-terminal Kinase (JNK) signaling pathways, suggesting a potential neurohormetic role for quercetin as a phytochemical [14, 38, 39].

For instance, Jia et al. [40] examined the antioxidant effects of quercetin on diquat-induced oxidative stress in porcine enterocytes. Pretreatment with quercetin in intestinal porcine epithelial cell line 1 (IPEC-1) cells demonstrated protective effects by mitigating apoptosis through a caspase-3-dependent mechanism, reducing ROS production, maintaining mitochondrial function, and preserving tight junction proteins. The authors proposed that quercetin’s protective role may be linked to the upregulation of Nrf2 protein levels and the modulation of intracellular glutathione content, highlighting its potential in regulating redox homeostasis. In a similar scenario, the activity of quercetin on p38-MAPK and its regulatory effects on the nuclear transcription factor erythroid-2p45-Nrf2 and the GSH antioxidant defense system in HepG2 cells was studied. The study showed a concentration-dependent modulation of p38 and Nrf2 over varying incubation periods, with 50 μM quercetin concentration [14]. In 2014, Saw and colleagues explored the cancer preventive properties of three flavonoids—quercetin, kaempferol, and pterostilbene—found in berries. The research focused on their ability to counteract free radicals using the DPPH and 2′,7′-Dichlorofluorescin diacetate (DCFH-DA) assays. The combination of all three flavonoids showed a synergistic effect in reducing intracellular ROS levels by stimulating the activation of ARE and the levels of mRNA and proteins associated with genes regulated by Nrf2 [38].

There is also evidence that quercetin can modulate enzymatic activities associated with oxidative stress [41, 42, 43]. One example is the inhibition of enzymes involved in ROS generation, such as NADPH oxidase [44, 45, 46]. By downregulating these enzymes, quercetin contributes to the reduction of intracellular ROS levels, thereby mitigating oxidative stress. In addition to that, quercetin interacts with key cellular signaling pathways, including mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) [47, 48, 49]. By inhibiting these pathways, quercetin plays a role in attenuating the expression of pro-inflammatory and pro-oxidative genes, contributing to its overall protective effects against oxidative stress-related damage. Furthermore, it modulates enzymatic activities involved in inflammatory pathways, including the inhibition of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) [50, 51, 52]. Quercetin was shown to markedly reduce the mRNA expression of inflammatory markers such as iNOS, COX-2, and C-reactive protein (CRP) in Chang Liver cells, demonstrating its anti-inflammatory effects [50].

2.3 Protective role of quercetin on chronic disease risk infection

Quercetin exerts its protective effects through diverse mechanisms. Its ability to neutralize ROS and free radicals arises from the presence of hydroxyl (∙OH) groups and a catechol-type B-ring in its molecular structure. Additionally, quercetin activates the Nrf2 pathway, promoting the expression of antioxidant enzymes and cellular defense mechanisms. The interplay of these mechanism positions quercetin as a potent defender against oxidative stress-induced damage. Quercetin, known for its potent antioxidant properties, exhibits cardiovascular protection by reducing oxidative damage to endothelial cells, lowering blood pressure, and improving lipid profiles. Oxidative stress arises from an imbalance between ROS production and the body’s antioxidant defense mechanisms [53, 54]. This imbalance is associated with the onset and progression of several chronic conditions such as myocardial infarction, chronic inflammation, aging, and neurodegenerative disorders [55, 56]. Numerous studies have demonstrated that quercetin exhibits neuroprotective effects and counteracts oxidative stress in vivo. For instance, its neuroprotective effect was shown in a study by Denny et al. [57] who found that the oral administration of quercetin and fish oil supplement enhanced neuroprotection in rats subjected to chronic exposure to the insecticide rotenone, which serves as an animal model for Parkinson’s disease. Since the brain is particularly susceptible to oxidative stress, and a target for the damaging effects of ROS, the antioxidant effects of quercetin against oxidative stress pathways and its potential in preventing brain health diseases such as Alzheimer’s disease (AD) through various pathways, including Nrf2, Paraoxonase-2, JNK, Protein kinase C, MAPK was discussed by Saikia et al. [58]. The study discussed that quercetin activates the Nrf2 pathway by increasing the production of protective proteins in AD brain cells, acting as a natural defense system against oxidative stress. Consequently, the neuroprotective potential of quercetin against oxidative stress responses was demonstrated by its interaction with paraoxonase-2-enzyme, JNK pathways and protein kinase C pathways which plays a role in cell signaling and function. Persistent activation of glutamate receptors, leading to excitotoxicity, contributes to various neurological disorders including Alzheimer’s disease, hypoxic-ischemic brain injury, multiple sclerosis etc. [59]. However, quercetin extracts protect neuronal cells from excitotoxicity-induced damage through mechanisms involving reduced ROS production, preservation of mitochondrial membrane potential, and modulation of multiple biochemical markers associated with cell death and autophagy [60]. Quercetin has emerged as a significant component demonstrating acetylcholinesterase (AChE) inhibitory activity and exerts a positive influence on the expression of nicotinic receptors, thereby augmenting cognitive memory function in Alzheimer’s patients [61, 62, 63].

Chronic inflammation is a prevalent contributing factor in various diseases, and quercetin’s anti-inflammatory properties have been studied in the context of conditions such as rheumatoid arthritis and inflammatory bowel diseases [64]. Previous studies have suggested that quercetin-rich diets may be associated with a decreased risk of developing these inflammatory conditions, providing insights into potential dietary strategies for therapeutic interventions [65, 66, 67]. Its anti-inflammatory properties have been linked to potential cardiovascular benefits. While many studies suggest potential cardiovascular benefits of quercetin, it is important to note that research in this area is ongoing, and not all studies have produced consistent results. Factors such as dosage, bioavailability, and individual variability may influence quercetin’s effects on cardiovascular health.

Excessive generation of reactive oxygen species resulting in oxidative stress has been recognized as a significant factor contributing to endothelial dysfunction-induced hypertension and various cardiovascular diseases [68]. Consequently, mitigating oxidative stress is considered a practical approach for comprehensive management of hypertension and other cardiovascular conditions. Several in vivo studies in animal models that explored the cardioprotective effects of quercetin and demonstrated its ability to lower blood pressure including those with hypertension, high-fat high-sucrose diet-induced conditions, nitric oxide deficiency, angiotensin infusion, and aortic constriction [69, 70, 71, 72]. In a 4-week trial involving eighteen Dahl salt-sensitive rats, the antihypertensive effects of captopril (CAP) and quercetin (QUE) on the renin-angiotensin-aldosterone system were explored with a specific focus on renal effects. While the results from this study indicated that quercetin does not exhibit a significant difference in aldosterone levels, it effectively lowered blood pressure in the hypertensive rat model, suggesting a potential modulation of renal function as the underlying mechanism [72]. Hackl et al. [73] investigated the impact of quercetin on angiotensin-converting enzyme (ACE) activity by assessing cardiovascular responses to bradykinin and angiotensin I. Quercetin pretreatment, administered orally (88.7 umol/kg, 45 min) and intravenously (14.7 umol/kg, 5 min), significantly enhanced the hypotensive effect of bradykinin (10 nmol/kg, i.v.). This study indicated that quercetin possess an ACE inhibitory activity in vitro similar to captopril, demonstrating its potential antihypertensive properties.

In contrast to findings in animal studies, human research trials have not conclusively demonstrated consistent results regarding the antioxidant potential of quercetin, even at elevated dosages. Earlier studies by Egert et al. [8] and others [74, 75, 76] demonstrated no impact on plasma oxidized low-density lipoprotein in healthy individuals or those with pre-hypertension and stage 1 hypertension after quercetin supplementation. The discrepancy between the antioxidant effects observed in hypertensive animal models and the equivocal findings in humans may be attributed to variations in quercetin doses. However, Hertog and his group [77] provided promising results regarding the epidemiological and in vitro/in vivo antioxidant effects of flavonoids, supporting their cardioprotective function. Their study provided evidence supporting the strong cardioprotective effects of various flavonoids, including quercetin. The study showed that men who consumed over 29 mg of flavonols per day experienced a substantial 68% reduction in the risk of coronary death compared to those consuming less than 10 mg per day. Although the study did not specifically explore the link between quercetin intake and blood pressure, the authors noted an inverse relationship between high-quercetin foods and blood pressure, and flavonol intake, including quercetin. While these studies conducted in animal models have indicated a decrease in blood pressure and enhanced antioxidant status with quercetin administration, the evidence from human trials has not been particularly compelling. Human studies examining the effects of quercetin on blood pressure and antioxidant status have not yielded convincing results, indicating the need for additional investigation and exploration of potential factors that influence the outcomes in human subjects. The translation of promising animal findings to human contexts requires careful consideration of various variables that may contribute to the differences observed in the effects of quercetin on blood pressure and antioxidant status.

Quercetin’s ability to scavenge free radicals and modulate inflammatory pathways suggests that it may contribute to reducing oxidative stress and inflammation associated with diabetes, thereby potentially alleviating the risk of diabetic complications. Quercetin’s potential protective effects on pancreatic beta-cells, responsible for insulin production, have been extensively investigated [78]. Various studies have shown that quercetin may protect beta-cells from oxidative stress and apoptosis, thereby preserving their function. Furthermore, quercetin may stimulate insulin secretion, contributing to better glycemic control. Additionally, it may inhibit key enzymes involved in carbohydrate digestion, leading to a more gradual release of glucose into the bloodstream. Various in vitro studies have focused on the antidiabetic effects of quercetin in cellular and animal models. For example, the impact of quercetin on glucose or glibenclamide-induced insulin secretion and its ability to protect against hydrogen peroxide-induced beta-cell dysfunctions, using the INS-1 beta-cell line was examined. The researchers observed that quercetin enhances insulin secretion, protects beta-cells from oxidative damage, and implicates the extracellular signal-regulated kinase (ERK)1/2 pathway [79]. Additionally, quercetin exhibited concentration-dependent inhibition of alpha-amylase and alpha-glucosidase activities. These findings suggest that quercetin’s ability to regulate blood glucose levels may be attributed to its disruption of enzymatic processes and protection of pancreatic tissues from oxidative damage, as evidenced by the inhibition of carbohydrate-metabolizing enzymes and prevention of pancreatic lipid peroxidation [80]. Moreover, potential interactions with medications and variations in individual responses should be considered when assessing the overall safety profile of quercetin for individuals with diabetes.

3. Conclusions

In conclusion, quercetin, with its powerful antioxidant properties, is crucial for defending cells against oxidative stress. While much research has focused on its potential therapeutic benefits, clinical evidence supporting its antioxidant effects in humans is limited. Understanding the mechanisms of action of quercetin and its derivatives is crucial, especially considering that these metabolites may be the active forms in the body. Despite challenges in absorption and bioavailability, quercetin’s ability to mitigate oxidative damage holds promise for combating various chronic diseases. Additionally, its activation of the Nrf2 pathway and interaction with signaling pathways further underscores its potential as a neuroprotective agent. Further research is needed to fully elucidate the mechanisms underlying quercetin’s antioxidant activities including clinical trials to fully elucidate the therapeutic potential and optimal dosage of quercetin for specific health conditions.