Impact of Quercetin on Bone-Related Diseases

Abstract

Quercetin (QE) is a widely distributed dietary flavonol with antioxidant and anti-inflammatory properties that has attracted interest as a modulator of bone remodeling and osteoporosis-related bone loss. In vitro data on osteoblasts, osteoclasts, and mesenchymal stem cells indicate that QE attenuates oxidative stress, suppresses pro-inflammatory signaling, and promotes osteogenic differentiation through modulation of pathways such as Nrf2/ARE, NF-κB, Wnt/β-catenin, and ER stress-related cascades. In vivo findings from animal models of estrogen deficiency, diabetes, and glucocorticoid-induced osteoporosis demonstrate that QE improves bone mineral density, trabecular microarchitecture, and biomechanical strength while reducing osteoclast number and activity, thereby attenuating osteoporotic bone deterioration. Collectively, preclinical evidence positions QE as a pleiotropic agent promoting osteoblastogenesis, inhibiting osteoclastogenesis, and balancing redox/inflammatory homeostasis in bone, despite bioavailability challenges. Future research should prioritize clinical trials with optimized formulations (e.g., nanoparticles) to validate efficacy, safety, and fracture outcomes in humans. The present review critically evaluates the chemical characteristics, pharmacokinetics, safety profile, and bone-targeted biological activity of QE, emphasizing effects on bone cells and skeletal metabolism.

1. Introduction

Quercetin (QE) is a polyphenolic compound identified chemically as 3,3′,4′,5,7-pentahydroxyflavone. It cannot be endogenously synthesized in the human body [1]. The name ‘quercetin’ comes from quercetum (it is a Latin word that refers to a specific type of oak wood or forest) and has been used since 1857 [2]. In nature, it occurs in multiple chemical forms, mostly as glycosides. This flavonoid is predominantly found in fruits, vegetables, nuts and seeds. Red onions are considered the most bioavailable food source of it. Identification and quantification of quercetin typically employ HPLC methods with acetic acid-based mobile phases, validated for precision in nanoparticles and biological samples. After consumption, QE undergoes digestion in the gastrointestinal tract and liver. Its metabolites can be detected in biological fluids such as urine or blood. Dietary supplements usually contain aglycone, the free state of quercetin. QE demonstrates antioxidant, anti-inflammatory, antiviral, anti-obesity, and antidepressant action. Moreover, it prevents cancer, diabetes, asthma, hypertension, and cardiovascular diseases [3].

The use of this substance is particularly noteworthy in the treatment of osteoporosis. Osteoporosis is a disorder of the skeletal system that significantly influences the quality of a patient’s life. This illness affects over 200 million women worldwide, with one-tenth of women aged 60 and one-fifth aged 70 impacted, contributing to a global health crisis where hip fractures are projected to double by 2050. The risk of this condition increases with age, and it is definitely more common among postmenopausal women. It leads to reductions in bone mass and consequently increases bone fragility. Osteoporosis typically affects the hip, spine and wrist regions [4,5].

The subject of QE is a popular and promising topic nowadays. The first studies about the positive impact of QE on bones are from 2008. Over the years, the topic gained more and more attention [6]. However, the majority of research in this field comprises experiments conducted in vitro, in silico, or utilizing animal models. This poses a significant challenge to the implementation of modern therapeutic interventions, as the findings derived from animal studies often do not accurately reflect the unique treatment requirements of human subjects. No human clinical trials specifically on quercetin’s bone-protective effects were identified in the literature; the evidence remains predominantly preclinical, including in vitro studies on osteoblasts/osteoclasts/MSCs and in vivo models like ovariectomized rats, diabetic rats, and glucocorticoid-induced osteoporosis. Therefore, the full validation of all methods used in treatments involving QE is impossible [7]. Nevertheless, the research on the subject under discussion is becoming more and more promising with each passing day.

This review analyzed the biological and pharmacological properties of QE. It is based on the in vitro and in vivo research conducted in recent years. The aim of this article is to understand the mechanisms of QE’s actions on the bones and skeletal system, including its influence on bone cells as well bone tissue metabolism pathways.

2. Chemical Properties, Pharmacokinetics, and Safety of Quercetin in Relation to Bone

QE belongs to the group of secondary plant metabolites. It belongs to the flavonol class of flavonoids [8]. Biosynthesis of QE in plants takes place with the participation of the shikimic acid pathway and the phenylpropanoid pathway, in which anthranilate and phenylalanine, respectively, act as precursor compounds [9]. This compound is present in numerous fruits, vegetables, seeds and berries, including apples, onions, tea leaves, mangoes and other products of plant origin [10,11]. QE is one of the best-known flavonoids, with documented therapeutic properties. Due to its widespread occurrence in the human diet and its wide spectrum of biological activity, this compound is the subject of intensive research on its potential use in the prevention and treatment of many diseases. It is effective in the context of neurodegenerative diseases, cardiovascular diseases, cancer and diabetes. With respect to its chemical structure, it is a pentahydroxyflavone containing five hydroxyl groups located at positions 3, 3′, 4′, 5 and 7, which gives it a characteristic yellow color and provides it solubility in lipids and alcohol. The chemical formula of QE is presented in Figure 1. The QR molecule is composed of two aromatic rings connected by a three-carbon γ-pyran ring, to which hydroxyl groups that can undergo various substituent modifications are attached. Its strong antioxidant properties result from the polyphenolic nature of the structure, in particular from the presence of a catechol moiety in the aromatic ring B and hydroxyl groups located at positions 3 and 5 [12].

QE occurs naturally both as an aglycone and as glycosidic conjugates, with the glycoside forms being predominant in plant-derived foods. The glycosidic form of QE is formed by substituting a hydroxyl group—most often at the 3-molecule position—with a glycosyl residue such as glucose, rhamnose or rutinose. The introduction of a sugar fragment into the structure of QE results in an increase in its solubility in water compared to the aglycone form. The QE aglycone is practically insoluble in cold water and displays only limited solubility in hot water, whereas it is readily soluble in alcohols and lipid media [13].

Among dietary sources, the most prevalent forms include QE-3-O-glucoside (isoquercetin) as well as its glucuronide derivative, quercetin-3-O-glucuronide [14]. After oral administration, QE is mainly absorbed in the small intestine, especially in its upper sections, although its bioavailability is relatively low [15]. This is attributable to both its poor aqueous solubility and limited stability under gastrointestinal conditions, as well as to extensive first-pass metabolism [16].

In plant tissues, QE is primarily present in the form of hydrophilic glycosidic derivatives. These compounds can be hydrolyzed to a free aglycone form under the influence of β-glucosidases located in the small intestine, which promotes its absorption by passive diffusion through the intestinal epithelial barrier [17]. It is also assumed that intestinal sodium/glucose cotransporter-1 is involved in the absorption process, which allows direct transport of glycosidic forms of QE to the systemic circulation. After absorption, QE may undergo oxidation reactions, leading to the formation of reactive metabolites, such as quercetin–quinone and quercetin–quinone methides [18]. Upon passing through the intestinal wall, QE is rapidly biotransformed into enterocytes and hepatocytes. In these processes, phase II enzymes catalyze its coupling to various groups, forming glucuronides, sulfates, and methylated metabolites. Such conjugation significantly changes the chemical characteristics of the compound—metabolites are more polar and occur in systemic circulation in much higher concentrations than the free form (aglycone) [18,19]. As a result of metabolic transformations, conjugated QE derivatives are formed, which, compared to the parent form, are often characterized by reduced activity or a different profile of biological activity. Furthermore, QE is a substrate of efflux transporters, including P-glycoprotein, which actively remove it from enterocytes into the lumen of the intestine, leading to further limitation of its absorption [20]. The fraction of QE that has not been absorbed in the small intestine is broken down by the colon microbiota into simpler phenolic acids, which are then absorbed and transported to the liver, where further conjugation processes take place [21,22].

Early pharmacokinetic studies in humans indicated that after a single oral administration, QE has a very low bioavailability, estimated at approximately 2%. In the case of quercetin glucoside, which is its naturally occurring form, the degree of absorption in healthy volunteers at a dose of 100 mg was assessed at a level of 3% to 17% [23]. In a study by Goldberg et al. [24], oral administration of 10 mg QR (per 70 kg of body weight) in healthy men showed limited absorption, with maximum serum concentrations of total quercetin (free form + metabolites) reaching approximately 30 min after ingestion. Conjugated metabolites (glucuronides and sulfates) dominated the systemic circulation, and free quercetin accounted for only 17.2–26.9% of the maximum concentration. Total urinary excretion over 24 h was 2.9–7.0% of the ingested dose, with peak serum concentrations in the range of 10–40 nmol/L, indicating low systemic exposure [24]. The limited bioavailability of QE is most likely a consequence of its poor absorption in the gastrointestinal tract, intensive metabolic processes, and/or rapid elimination from the body [23]. The absorption efficiency also varies depending on the type of sugar attached to the QE molecule [25], the dietary matrix and other nutrients in the diet. Dietary fibers and antinutritional compounds bind quercetin, reducing its solubility and intestinal absorption; for example, high-fiber matrices slow hydrolysis of glycosides by β-glucosidases. Non-heme iron from plant foods and supplements chelates quercetin via its catechol moiety, inhibiting basolateral iron transport while mutually decreasing quercetin’s uptake. High-fat meals may enhance absorption via micelle formation, but excessive fiber or pH extremes (e.g., low gastric pH degrading to phenolic acids) further limit bioavailability to <10% [26]. In addition, it is suspected that endogenous factors may affect QE bioavailability [27].

3. Mechanisms of Action of Quercetin in the Context of Bones

It appears that QE affects bone health through a combination of pro-osteogenic, anti-resorptive, anti-inflammatory and antioxidant actions, which have been associated with the modulation of several signaling pathways in bone cells. However, as these pathways are highly interconnected and much of the available evidence comes from in vitro and animal studies, the precise contribution of each pathway to the bone-related effects of quercetin remains to be fully established. Below, selected mechanisms are briefly described and then expanded in subsequent sections.

3.1. Promotion of Osteogenesis

Osteogenesis is the process by which bone is formed. It begins with mesenchymal stem cells (MSCs) differentiating into osteoblasts, which synthesize the organic bone matrix (osteoid). This is then progressively mineralized into mature bone tissue [28]. QE can modulate osteogenesis by promoting the commitment and activity of osteoblast lineage cells, while protecting them from inflammatory and oxidative damage. The canonical Wnt/β-catenin pathway is essential for osteoblast differentiation and bone formation [29]. In the absence of Wnt ligands, β-catenin is phosphorylated by a destruction complex containing Axin, APC, CK1 and GSK-3β and subsequently degraded [30], which maintains low expression of osteogenic transcription factors such as Runx-2 and Osterix [31,32]. Although direct causality has not been established in all models, experimental data suggest that QE may influence this process by increasing Wnt3 and β-catenin levels and reducing GSK-3β activity [33]. This may allow β-catenin to accumulate and translocate to the nucleus, where it can activate the transcription of osteogenic genes via TCF/LEF [34].

Estrogen receptor (ER)-dependent signaling pathway is also of great importance. Bone morphogenetic protein 2 (BMP-2) and transforming growth factor beta (TGF-β) are central regulators of osteoblast differentiation and bone formation. They act through both canonical Smad-dependent pathways and non-canonical MAPK signaling [35]. In the canonical arm, BMP-2 or TGF-β binding to their type I and type II serine/threonine kinase receptors leads to the phosphorylation of receptor-regulated Smads (Smad1/5/8 for BMPs and Smad2/3 for TGF-β). These subsequently form complexes with Smad4 and translocate to the nucleus, which then work with osteogenic transcription factors, such as Runx2 and Osterix, to increase the expression of bone-specific genes [36]. These include alkaline phosphatase, type I collagen, osteopontin, and osteocalcin. This process promotes osteoblast maturation, extracellular matrix deposition, and mineralization [37]. Additionally, BMP/TGF-β receptors can activate ERK, p38 and JNK MAPKs [38,39,40], which modulate the phosphorylation and transcriptional activity of Runx2 and other factors, thereby further fine-tuning osteoblast differentiation and function [41]. QE appears to support this osteogenic axis at several levels; for instance, experimental data suggest that quercetin can increase the expression of BMP-2 and TGF-β and enhance Smad signaling downstream in MSCs and pre-osteoblasts. This is accompanied by the upregulation of RUNX2, osterix, osteopontin and osteocalcin, as well as increased ALP activity and matrix mineralization. This is consistent with a pro-osteoblastic effect. Notably, at least some of this response is mediated via the ER. Inhibition of ER signaling reduces quercetin-induced activation of the BMP-2/Smad pathways and decreases the expression of osteogenic markers in bone-marrow mesenchymal stem cells (BMSCs), therefore supporting the idea that QE acts as a phytestrogen, amplifying BMP/TGF-β-driven osteoblastogenesis under estrogen-deficient conditions [42,43].

3.2. Suppression of Osteoclastogenesis

The RANKL/RANK/OPG system is the primary regulatory axis of osteoclast differentiation and bone resorption [44]. RANKL, a protein produced by osteoblasts, osteocytes and immune cells, binds to RANK on osteoclast precursors, recruiting TRAF6 and activating NFκB, MAPKs and AP-1. These, in turn, induce NFATc1 and c-Fos, thereby driving the expression of osteoclast genes such as TRAP, cathepsin K and MMP9, which are necessary for cell fusion and matrix degradation [45,46]. Osteoprotegerin (OPG) functions as a soluble decoy receptor, sequestering RANKL. Consequently, the local RANKL/OPG ratio dictates the extent of osteoclastogenesis, and its imbalance is a hallmark of high turnover osteoporosis and a pivotal therapeutic target of anti-resorptive drugs [47]. QE has been demonstrated to modulate this axis by lowering RANKL and/or increasing OPG expression in osteoblast-lineage cells, thereby reducing the RANKL/OPG ratio [48]. In addition, it has been shown to directly inhibit RANKL-induced NF-κB/MAPK/AP-1 activation in osteoclast precursors, thus attenuating NFATc1 and osteoclast effector genes [5]. In the context of functional assays, these effects are manifested as a reduction in the number of TRAP-positive multinucleated osteoclasts, an impairment in F-actin ring formation, and the presence of smaller resorption pits [49]. In addition, the data demonstrate a decrease in osteoclast numbers and the preservation of trabecular microarchitecture in animal models, thus supporting the hypothesis that RANKL/RANK/OPG signaling is a significant anti-resorptive target of QE.

3.3. Integrated Control of Inflammation and Oxidative Stress via NF-κB and Nrf2

It is evident that chronic, low-grade inflammation and oxidative stress play a pivotal role in the process of bone loss [50]. The underlying mechanism involves the simultaneous promotion of osteoclastogenesis and impairment of osteoblast differentiation and survival by pro-inflammatory cytokines and reactive oxygen species (ROS), respectively [51,52]. NF-κB is a central transcription factor in this context: it is activated downstream of RANKL, TNF-α, IL-1β and IL-6 in osteoclast precursors and osteoblast-lineage cells, controlling genes essential for osteoclast formation and osteoblast function [5,52]. In osteoclast precursors, RANKL–RANK–TRAF6 signaling rapidly induces NF-κB, which cooperates with NFATc1 and c-Fos to drive the expression of osteoclast-specific genes and resorptive activity [5,51]. Conversely, in osteoblasts, NF-κB activation suppresses osteogenic programs and increases RANKL expression, thereby shifting bone remodeling towards resorption [53]. In turn, the Nrf2-Keap1-ARE pathway has been identified as a pivotal endogenous defense mechanism against oxidative stress in bone tissue [54]. In the presence of oxidative or electrophilic stress, Nrf2 dissociates from Keap1, translocates to the nucleus, and induces antioxidant and cytoprotective enzymes, including superoxide dismutase (SOD), catalase, glutathione peroxidase, and heme oxygenase-1 (HO-1). These enzymes function to limit ROS-induced damage in osteoblasts, osteocytes, and osteoclast precursors [55]. The results of experimental studies suggest that activation of Nrf2 provides protection to osteoblasts against apoptosis induced by ROS, whilst also promoting osteogenic differentiation and suppressing the process of osteoclastogenesis [56]. However, it has been observed that Nrf2 deficiency is associated with reduced bone mass and impaired trabecular structure [57]. What is more, Nrf2 and NF-κB are not independent—there is well-described, bidirectional crosstalk in which Nrf2 deficiency can enhance NF-κB-driven inflammatory output, whereas NF-κB activity can also modulate Nrf2 transcriptional function, linking redox balance with inflammatory tone [58]. The available evidence suggests that QE may modulate both NF-κB and Nrf2 signaling. However, given the close interaction between inflammatory and redox-sensitive pathways, these effects should be considered as part of an interconnected regulatory network, rather than as fully independent and directly established mechanisms [1]. It has been demonstrated to reduce IκB phosphorylation and NF-κB nuclear translocation in macrophages, osteoclast precursors and osteoblasts, which in turn lowers the production of TNF-α, IL-1β and IL-6, decreases RANKL expression and attenuates RANKL-induced osteoclastogenesis both in vitro and in vivo [59,60,61]. Concurrently, QE (in the form of glycoside isoquercitrin) activates Nrf2, elevates the expression of downstream antioxidant enzymes, and curtails biomarkers of oxidative damage (e.g., malondialdehyde) in osteoclasts, thereby promoting osteogenic differentiation and safeguarding these cells under conditions of oxidative or inflammatory stress [62]. Although available studies suggest that quercetin may modulate signaling pathways such as Nrf2/ARE, NF-κB, Wnt/β-catenin and ER stress-related cascades, these mechanisms should be interpreted with caution. These pathways are closely interconnected, and most of the current evidence originates from in vitro systems and animal models. An overview of the probable role of QE in inflammatory and oxidative stress reactions in bone tissue is shown in Figure 2.

4. In Vitro Model Data Supporting Quercetin’s Bone-Preserving Effects

In vitro determination of QE’s influence on bone cells is mainly linked with MSCs, murine or human osteoblast cells, which can be used for investigation of its effect on cell viability, proliferation, mineralization and expression of osteogenic genes [5]. MSCs are adult stem cells able to differentiate into osteoblasts, chondrocytes, myocytes, adipocytes, neurocytes and hepatocytes [5,63]. A significant aspect of bone metabolism is estrogen, a hormone with a pivotal impact on bone mineral density. Its level is associated with menopause, when a decrease in estrogen contributes to an increase in osteocyte apoptosis, leading to osteoporosis [64].

The influence of QE on osteocytes, osteoblasts and osteoclasts was presented by Khosla et al., who proposed that bone resorption and maintenance of bone formation are decreased by estrogen [65]. Significant in vitro studies were performed by Pang et al. [42], who decided to investigate the influence of QE on mouse bone-marrow mesenchymal stem cell (BMSC) proliferation and osteogenic differentiation. Scientists revealed that the flavonoid has a positive impact on BMSC proliferation, alkaline phosphatase (ALP) activity, extracellular matrix production and mineralization. The significant enhancement of BMSC proliferation was observed at concentrations ranging from 0.1 to 5 µM. Similar effects of quercetin have been detected previously by Zhou et al. [66], who revealed increased cell proliferation on days 1, 4 and 7 of their experiment, with 2 µM achieving the greatest effect. The evaluation was based on rBMSC isolated from 4-week-old male rats, which were used for future in vitro analyses. To evaluate the hypothesis that quercetin can induce osteogenic differentiation of BMSCs, the flavonoid was applied to rBMSCs at various concentrations. Its influence on proliferation, osteogenic differentiation and expression of angiogenic factors was evaluated. The most significant effect of the compound was observed at the concentration 2 µM. Additionally, extracellular signal-regulated protein kinases (ERK) were observed. An equally important activity of the quercetin hydrate (QH) was observed during in vitro studies towards osteoblast cell proliferation and differentiation using pre-osteoblastic cell lines MC3T3-E1 and MG-63 at two concentrations [67]. Significant increases in cell proliferation and viability at both concentrations, as well as more mineral matrices, were observed. These results indicate that QH can enhance osteoblast differentiation, improve matrix structuring, and development in vitro, as well as support bone health through anabolic effects on the skeleton. The positive influence of QE on osteoblast differentiation and expression of osteogenesis-related markers was observed at concentrations of the flavonoid ranging from 0.01 to 5 µM [68]. Positive effects of QE were also observed in studies based on MC3T3-E1 osteoblasts stimulating lipopolysaccharide (LPS) [69]. The effect suggests that the flavonoid can be an effective compound in case of abnormal human bone loss induced by LPS in chronic inflammatory diseases. Similarly important is evaluation of the compound as a potential agent in the prevention and treatment of osteoporosis. Detailed analyses of QE and rutin, individually and in combination, showed enhanced cell proliferation, osteoblastic differentiation and mineralization in UMR 106 cells, an epithelial-like cell that was isolated from the bone of a rat with osteosarcoma that is widely used as an osteoblast-like model [70]. The results, along with in vivo experiments, suggest that QUE is a promising candidate for treatment of glucocorticoid-induced osteoporosis.

Significant studies have been performed using the human fetal osteoblastic cell line (hFOB) that aimed to evaluate the effect of QE on increases in cell numbers [71]. Various tests, including MTT and trypan blue, confirmed the hypothesis that the flavonoid positively increases cell numbers and viability. Additionally, wound healing and cellular adhesion in osteoblasts were observed for 1 µM QE. It is interesting to note that QE at concentrations of 20–100 µM reduced cell number and viability in tumor osteoblasts (ROS 17/2.8 cells). Interesting results were also presented by Kim et al. [72], who focused on inhibition of cell proliferation and increasing osteogenic differentiation in human adipose stromal cells (hADSC). The findings indicate that the flavonoid is able to increase osteogenic differentiation as well as bone regeneration. The results suggest that quercetin’s effects are independent of estrogen receptor activation, providing insight into its mechanism of action. Interestingly, plants rich in QE have a positive impact on bone cells. An example is that the ethanolic fraction of Cissus quadrangularis can be enriched by rutin and quercetin (1.06 ± 0.12 mg/g). Human osteoblast-like MG-63 cells grown in this medium revealed higher ALP levels [73].

Flavonoids are characterized by high antioxidant and free radical scavenging activity. This feature can be used in bone therapy. It is known that QE provides protection against H₂O₂-, cigarette- and menadione-induced oxidative stress in MC3T3-E1 osteoblast cells and primary human osteoblasts [60,74].

A significant number of studies focus on osteoclastogenesis. Numerous results suggest that QE inhibits the formation of osteoclast-like cells, bone resorption pi and F-actin ring formation in RAW264.7 cells, human peripheral blood mononuclear cells (PBMCs) and bone-marrow macrophages treated with macrophage colony-stimulating factor (M-CSF) [75,76,77,78].

QE has been shown to reduce hydroxylysylpyridinoline levels in human peripheral blood mononuclear cells stimulated with M-CSF and RANKL, as well as in highly purified rabbit osteoclasts [61,79]. In addition, treatment with quercetin at concentrations of 15, 25 or 50 μM inhibited lipopolysaccharide-induced osteoclast differentiation and cell viability in RAW264.7 cells [80]. The effects of QE on bones, along with detailed findings, are presented in

Table 1. Effects of quercetin on bone based on in vitro studies.

Type of Cell	QE Concentration	Study Results	Ref.
Murine osteoblastic MC3T3-E1 cells treated with H2O2 or menadione	1–10 μM	Increase: cell viability	[74]
Rat bone-marrow-derived MSCs treated with TNF-α	1 μM	Increase: cel viability, calcium nodule formation, osterix, Runx-2, β-catenin; Decrease: pNF-κB	[81]
Stem-cell spheroids cultured in osteogenic medium	1 μg/mL	Increase: ALP, Runx-2	[82]
Rat femoral-diaphyseal and metaphyseal tissues	1 or 10 μM	Increase: calcium content	[83]
Mouse bone-marrow-derived MSCs	25–50 μM	Increase: mineralization, ALP, cell proliferation, osteopontin, Runsx-2, osteocalcin, osterix, osteoprotegerin	[84]
Rat bone-marrow-derived MSCs	0.1, 1 or 10 μM	Increase: ALP, COL1, cell differentiation, Cbfα1, TGF-β1, BMP-2, cell differentiation, p-p38, p-JNK	[85]
Rat bone-marrow-derived MSCs	1 μM	Increase: ALP, COL1, Runx-2, osteocalcin, osteopontin, BMP-2, ANG-1, VEGF, osteoprotegerin, bFGF, p-ERK, p-p38, p-AKT; Decrease: RANKL	[48]
Primary human osteoblasts exposed to cigarette-smoke medium	25, 50, 100 μM	Increase: SOD, cell viability, HO-1, p-Nrf2, p-ERK1/2; Decrease: ROS	[60]
Murine osteoblastic MC3T3-E1 cells treated with H2O2	1 μg/mL	Increase: cell growth, collagen, mineralization, ALP; Decrease: RANKL, MDA, protein carbonyl, nitrotyrosine	[86]
Murine osteoblastic MC3T3-E1 cells treated with TNF-α	1–10 μM	Increase: apoptosis, Fas activation, PARP cleavage, degradation of procaspase-8, caspase-8, caspase-3, AP-1 activity, p-JNK Decrease: cell viability, Bcl-2, cytochrome c	[87,88]
Rat calvarial osteoblast-like cells	0.1–10 μM	Decrease: cell proliferation, ALP, osteocalcin, deposition of calcium, mineralised nodules	[89]
MSCs induced to differentiate into osteoblasts	10 μM	Decrease: cell proliferation, mineralization, ALP, COL1, osteocalcin	[90]
RAW264.7 cells treated with RANKL	40–160 μmol/L	Increase: Bcl Decrease: cell apoptosis, osteoclast number, PERK, caspase-12, caspase-3, IRE1, TNF-α, IL-1β, IL-6, GRP78, CHOP, TRAP, RANK	[78]
Mouse bone-marrow cells treated with PTH	0.01–1 μM	Decrease: osteoclast number	[83]
Highly purified rabbit osteoclasts	50 μM	Increase: apoptotic osteoclast Decrease: resorption pit area, ROS, hydroxylysylpyridinoline	[79]
RAW264.7 cells treated with M-CSF and RANKL	2–5 μM	Increase: disruption of actin ring Decrease: osteoclast formation, pit formation, TRAP activity	[91]
Human osteoblast-like MG-63 cells	1–50 μM	Increase: ALP, p-ERK, estrogen receptor signaling	[92]
Human adipose-tissue-derived stromal cells	5 μM	Increase: Runx-2, osteogenic differentiation, ALP, osteopontin, p-ERK, Runx-2, BMP-2	[72]
Human PBMCs treated with M-CSF and RANKL	1–10 μM	Decrease: resorbed area, osteoclast number, hydroxylysylpryridinoline	[75]

Abbreviations: AKT, protein kinase B; ALP, alkaline phosphatase; ANG-1, angiopoietin-1; AP-1, activator protein 1; Bcl, B-cell lymphoma protein family; Bcl-2, B-cell lymphoma 2; BMP-2, bone morphogenetic protein 2; Cbfα1, core-binding factor subunit alpha 1; CHOP, C/EBP homologous protein; COL1, collagen type I; ERK, extracellular signal-regulated kinase; Fas, Fas cell surface death receptor; GRP78, glucose-regulated protein 78; HO-1, heme oxygenase-1; IL-1β, interleukin-1 beta; IL-6, interleukin-6; IRE1, inositol-requiring enzyme 1; JNK, c-Jun N-terminal kinase; M-CSF, macrophage colony-stimulating factor; MDA, malondialdehyde; MSCs, mesenchymal stem cells; NF-κB, nuclear factor kappa B; Nrf2, nuclear factor erythroid 2–related factor 2; PARP, poly(ADP-ribose) polymerase; PBMCs, peripheral blood mononuclear cells; PERK, protein kinase RNA-like endoplasmic reticulum kinase; PTH, parathyroid hormone; QE, quercetin; RANK, receptor activator of nuclear factor κB; RANKL, receptor activator of nuclear factor κB ligand; ROS, reactive oxygen species; Runx-2, runt-related transcription factor 2; SOD, superoxide dismutase; TGF-β1, transforming growth factor beta 1; TNF-α, tumor necrosis factor alpha; TRAP, tartrate-resistant acid phosphatase; VEGF, vascular endothelial growth factor; bFGF, basic fibroblast growth factor.

. Overall, most published studies indicate that quercetin promotes osteoblast differentiation, although a small number of reports show contrasting results. Such inconsistencies may stem from differences in the cell models employed and the concentrations of quercetin applied. Additionally, existing evidence indicates that quercetin consistently suppresses osteoclast differentiation, growth and maturation. Future investigation into the molecular mechanisms responsible for quercetin’s osteogenic effects is warranted.

5. In Vivo Evidence on the Impact of Quercetin on Bones

The potential of QE in promoting bone health has been the subject of extensive research, with numerous in vivo studies consistently demonstrating its protective properties in various models of osteoporosis and osteopenia. These experiments have examined the ability of quercetin to counteract reductions in bone mass in animals experiencing deficiencies in sex hormones, diabetes induced by streptozotocin, or glucocorticoid-induced decreases in bone density [4]. Ovariectomised (OVX) female rats have been the most commonly used model of postmenopausal osteoporosis [5]. As demonstrated by Yuan et al. [81], oral administration of QE at a dose of 50 mg/kg/day for eight consecutive weeks in OVX rats produced a notable improvement in bone quality parameters. Specifically, the supplementation of quercetin resulted in a significant enhancement of femoral bone mineral density, trabecular number (Tb.N), and trabecular thickness (Tb.Th), indicating a marked attenuation in estrogen deficiency-induced bone loss. Furthermore, biomechanical analysis of the femur demonstrated enhancements in both bone elasticity and maximal loading capacity, indicative of enhanced structural integrity and resistance to fracture. A subsequent study reported that the administration of continuous QE at an equivalent daily dose over a 30-day period resulted in a decline in the enzymatic activities of acid phosphatase and alkaline phosphatase, which are markers associated with bone resorption and formation, respectively. Concurrent with these biochemical changes, serum calcium and phosphorus concentrations increased significantly, indicating a positive modulation of mineral metabolism and an overall shift towards bone preservation [93]. In another experiment, the osteoprotective potential of quercetin was subjected to further evaluation in comparison with alendronate, the standard pharmacological therapy for postmenopausal osteoporosis [94]. This study employed a rat model of mandibular osteoporosis induced by ovariectomy to assess the relative efficacy of the two treatments in mitigating bone degeneration under estrogen-deficient conditions. The animals received either QE (50 mg/kg/d) or alendronate (6.25 mg/kg/w) by oral gavage for a period of eight weeks. The administration of the aforementioned substances to control groups occurred via intraperitoneal injection following bilateral ovariectomy, and the injection consisted of chloroquine (10 mg/kg, twice weekly) or the cytokine release inhibitor MCC950 (10 mg/kg, thrice weekly). It has been demonstrated that the ingestion of QE has a number of significant effects on the body. These include limiting body weight gain, improving bone microarchitecture, increasing bone mass and bone mineral density, reducing trabecular spacing and decreasing osteoclast numbers. The ability of QE to inhibit interleukins 1β and 18 was found to result from its capacity to downregulate the NLRP3 inflammasome pathway, thereby preventing osteoclast differentiation. Furthermore, QE was found to suppress autophagy and reduce the number of tartrate-resistant acid phosphatase-positive osteoclasts in OVX rats. These findings suggest that QE can preserve skeletal structure and mitigate bone loss in rats with osteoporosis. In a seminal study, Feng et al. [95] elucidated the molecular mechanisms underlying QE’s effects on the activation of the Wnt/β-catenin pathway and the suppression of NF-κB signaling. These observations counteract the inhibitory effect of tumor necrosis factor alpha (TNFα) on BMSC osteogenesis. In OVX-induced osteoporotic mice, QE significantly mitigated bone loss and structural damage and restored serum Metastasis Associated Lung Adenocarcinoma Transcript 1 (Malat1) levels. These findings indicate that QE has the capacity to reverse TNFα-induced impairment of BMSC differentiation and osteoporosis-related bone deterioration, a process that is dependent on Malat1. This suggests that QE has the potential to be a valuable therapeutic agent for the treatment of osteoporosis.

QE has also been reported to possess bone-preserving properties when administered via nanoparticles or hydroxyapatite bioceramic microspheres in models of osteoporosis. The utilization of delivery systems has been shown to enhance the bioavailability and bioactivity of therapeutic agents through several mechanisms, including the prolongation of half-life, the optimization of particle size, and the facilitation of controlled release [96]. In female albino rats subjected to bilateral osteoarthritis, the oral administration of 10 or 50 milligrams of QE-loaded phytosome nanoparticles per kilogram of body weight for 30 days had a marked effect on bone metabolism, as indicated by reduced bone turnover markers and increased serum mineral content [93]. In a separate investigation, QE was incorporated into hydroxyapatite bioceramic microspheres to create a composite delivery system designed for localized and sustained release within bone tissue. These QE-loaded hydroxyapatite microspheres were implanted into surgically induced bone defects located in the distal femoral diaphysis of OVX rats, a well-established model for postmenopausal osteoporosis. After an eight-week implantation period, microstructural analysis revealed a pronounced improvement in bone mineral density and trabecular thickness (Tb.Th) at the defect site, indicating enhanced new bone formation and mineralization. Moreover, the study demonstrated that QE exerted a dual biological effect by promoting the proliferation of BMSCs and stimulating their osteogenic differentiation. In parallel, QE upregulated the expression of angiogenic factors, thereby facilitating the coupling of osteogenesis and angiogenesis in the repaired tissue. Importantly, treatment with QE restored the receptor activator of nuclear factor kappa B ligand (RANKL)/osteoprotegerin (OPG) balance in a dose-dependent manner, suggesting a regulatory effect on bone remodeling. Mechanistically, several signaling cascades, including the extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and protein kinase B, were implicated in mediating these effects. Available evidence also points to potential crosstalk among these pathways, contributing to the integrated regulation of osteoblastic activity and vascularization. Furthermore, the hydroxyapatite microspheres featuring a micro-nano hybrid surface provided a controlled and sustained release of QE, which amplified its osteoinductive capacity and angiogenic potential in vivo. It can be concluded that hydroxyapatite-based microspheres have the potential to function as effective carriers for the delivery of QE. Additionally, they can be considered as prospective bone graft substitutes for the purpose of promoting osteoporotic bone regeneration [48]. In contrast, the dietary supplementation of phytochemical blends in ovariectomised aged Fischer 344 rats failed to show any anti-osteoporotic effects [5]. In one study, the experimental design consisted of two distinct treatment groups, each receiving a diet supplemented with a specific combination of bioactive compounds. The first group was fed a high-dose formulation containing 1000 mg/kg of QE, 2400 IU/kg of vitamin D₃, 500 mg/kg of genistein, and 200 mg/kg of resveratrol. The second group received a low-dose diet, which included 2000 mg/kg of QE, 2400 IU/kg of vitamin D₃, 1000 mg/kg of genistein, and 400 mg/kg of resveratrol. Despite four weeks of dietary intervention, neither formulation produced a statistically significant change in femoral bone mineral density or bone microarchitectural parameters. The lack of noticeable bone-protective effects, in contrast to findings from other studies, may be attributed to potential antagonistic or competitive interactions among the constituent polyphenols and micronutrients in the mixture, which could have diminished QE’s osteoprotective efficacy [97]. Further insight into the structure–activity relationship of QE derivatives was provided by research on quercitrin, a naturally occurring QE glycoside formed through conjugation with the deoxy sugar rhamnose. In an experiment conducted by Xing et al. [98], ovariectomized female Sprague-Dawley rats were orally administered quercitrin at doses of 50, 100, or 200 mg/kg/day for a period of 60 days. The treatment elicited marked improvements in bone mechanical and biochemical parameters: quercitrin significantly elevated bone mineral density at the distal femur, increased maximum energy absorption, and enhanced fracture load and stiffness at the femoral neck.

Fayed et al. [99] provided further valuable insights into the anti-osteoporotic properties of isoquercitrin by investigating its effects in OVX rat model. In their study, isoquercitrin was administered orally at varying doses, with the highest dose (60 mg/kg/day) producing the most pronounced improvements in bone strength. Specifically, this treatment significantly enhanced lumbar vertebral compression strength, suggesting that isoquercitrin contributes to increased bone density and improved biomechanical resilience of osteoporotic bone tissue. Moreover, biochemical analyses revealed that treatment with isoquercitrin led to a reduction in bone turnover markers, indicating a beneficial modulation of bone remodeling dynamics. This implies that isoquercitrin can suppress excessive bone resorption while promoting bone formation, thereby restoring the balance between these two crucial processes in osteoporotic conditions. The choice of isoquercitrin in this study was supported by earlier findings highlighting its superior pharmacokinetic and pharmacodynamic characteristics compared to its parent flavonol, quercetin, and its glycoside analog, quercitrin. Previous research has shown that isoquercitrin possesses greater bioavailability, allowing for more efficient absorption and systemic distribution, as well as higher antioxidative capacity, which enhances its ability to counteract oxidative stress—a critical factor contributing to osteoblast dysfunction and bone loss. These properties make isoquercitrin a particularly promising flavonoid derivative for the prevention and management of osteoporosis [100].

There is a robust body of evidence indicating a strong correlation between diabetes mellitus and osteopenia, a heightened propensity for bone fractures, and a protracted healing process following bone fractures. The potential of QE to mitigate diabetes-associated bone loss has been investigated using streptozotocin (STZ)-induced diabetic rat models in two independent studies. In the first investigation, experimental diabetes was induced via a single intraperitoneal (i.p.) injection of STZ at a dose of 50 mg/kg. Following eight weeks of hyperglycemia, the diabetic rats received QR treatment (15 mg/kg/day, i.p.) for a subsequent period of four weeks. Evaluation of bone structural and mechanical parameters revealed that QE administration significantly enhanced bone volume fraction, trabecular number, and trabecular thickness. Moreover, biomechanical testing indicated improved maximum load-bearing capacity at both the femoral mid-diaphysis and femoral neck. Concurrently, treated animals exhibited elevated calcium and magnesium concentrations in bone tissue compared with untreated diabetic controls. In addition to these skeletal effects, QE effectively lowered blood glucose levels while restoring plasma insulin content, underscoring its dual role in glycemic regulation and bone protection under diabetic conditions [101]. In a subsequent study, a modified diabetic model was established in male Sprague-Dawley rats through intraperitoneal administration of STZ at 100 mg/kg on two consecutive days. After four weeks of diabetes development, the animals received QE at doses of either 30 mg/kg or 50 mg/kg daily for an eight-week treatment period. Untreated diabetic rats demonstrated markedly elevated serum glucose levels, disrupted bone remodeling, and deterioration of bone microarchitecture. Histomorphometric analyses revealed thinning and disorganization of trabecular bone, accompanied by reductions in mechanical strength and alterations in serum bone turnover markers. Remarkably, QR supplementation reversed many of these degenerative changes, improving structural integrity and restoring bone formation–resorption balance toward values observed in healthy controls [102].

Elevated production of ROS, in conjunction with disrupted glucose metabolism, severely impairs the bioenergetic efficiency and adenosine triphosphate (ATP) synthesis capacity of MSCs derived from osteoporotic bone. These alterations compromise osteogenic potential and cellular regeneration within the osteoporotic microenvironment. To address these metabolic and oxidative challenges, Chen et al. [103] engineered a multifunctional implant system by fabricating calcium carbonate–quercetin–chromium (CaCO₃-QR-Cr) nanoparticles through ion coordination chemistry and embedding them into a ROS-responsive gelatin/chitosan coating applied to a titanium substrate (designated Ti/Gel/CaCO₃-QR-Cr). The conceptual design aimed to synergistically enhance ATP production and mineralization in osteoporotic MSCs by integrating the antioxidative capacity of QE with the glucose metabolism-modulating properties of the QR-Cr complex. The Ti/Gel/CaCO₃-QR-Cr interface was engineered to undergo controlled degradation in response to elevated ROS levels characteristic of osteoporotic bone tissue. This degradation behavior facilitated a sustained, localized release of CaCO₃-QE-Cr nanoparticles into the surrounding microenvironment for over 28 days. Functionally, this controlled release system effectively scavenged excess intracellular and extracellular ROS, thereby alleviating oxidative stress, while concurrently enhancing glucose uptake and metabolic flux in osteoporotic MSCs. As a result, cellular bioenergetics were restored, and osteogenic differentiation was markedly improved, leading to enhanced formation of mineralized extracellular matrix. In in vivo assays, titanium implants modified with the Ti/Gel/CaCO₃-QE-Cr composite coating significantly accelerated new bone formation and improved bone-implant contact area compared with unmodified controls. These findings demonstrate not only increased osteogenic activity around the implant site but also superior mechanical stability and integration between the titanium surface and host bone under osteoporotic conditions. This study introduces an innovative biomaterial platform that couples antioxidative and metabolic regulation for the targeted restoration of osteoporotic bone function. It also establishes a promising foundation for the rational design of next-generation titanium-based implants capable of actively promoting osseointegration in compromised metabolic bone environments.

Glucocorticoid therapy is among the most frequent causes of secondary osteoporosis [104]. The experimental studies were conducted in which glucocorticoids were administered subcutaneously (s.c.) on repeated occasions to induce an osteoporotic phenotype in rats. In the first test, female Sprague-Dawley rats were given a drug that made them lose bone. Then they were given a different drug called QE. QE treatment had a positive effect on a number of bone-related factors, including increasing Tb, Th, the number of osteoblasts and the moment of inertia. It also strengthened the bone and increased osteocalcin levels, as this study pointed out [105]. A promising strategy for treating osteoporosis is represented by topical delivery of quercetin via deformable transfersomes [106]. These transfersomes exhibit favorable physicochemical properties, including high entrapment efficiency, small particle size, an appropriate zeta potential and a low polydispersity index. The demonstration of excellent skin permeation in rat models was made by the transfersomes when incorporated into chitosan films. In a glucocorticoid-induced osteoporosis rat model, bone loss was evident by day 30. Compared to the positive control group, substantial improvements were seen in the treatment initiated on day 45, which involved the use of chitosan films loaded with quercetin transfersomes. It is interesting to note that the thickness, length, and density of the thigh bone (the femur) increased a lot. This increase was also accompanied by higher levels of calcium, phosphorus, an enzyme called alkaline phosphatase, and another enzyme called tartrate-resistant acid phosphatase in the blood. The tensile strength of osteoporotic femurs was restored to levels comparable with those of healthy controls, and histomicrographic analysis revealed reduced bone disruption and lytic lesions. These outcomes reflect quercetin’s ability to suppress osteoclastogenesis and osteoblast apoptosis, thereby increasing osteoblast numbers and enhancing bone mineralization. Overall, QE-loaded transfersome–chitosan films represent a superior alternative to oral QE administration, offering convenient ‘anytime, anywhere’ application—such as via a wristband—even during work, thereby improving patient adherence. Subsequent research employed network pharmacology analyses alongside in vivo models to elucidate the mechanistic basis of QE anti-osteoporotic effects [107]. This approach identified 55 putative molecular targets linked to QE’s therapeutic action, with the TNF signaling pathway emerging as a central mediator of its pharmacological benefits. Molecular docking simulations confirmed particularly robust binding affinities for IL1 β, RelA, and NFKB1 (ranking as the top three), providing a molecular rationale for QR’s capacity to modulate inflammatory and bone remodeling cascades in osteoporosis. Complementary validation through zebrafish models and quantitative PCR assays yielded results consistent with these predictions, reinforcing the pathway’s involvement. Collectively, these findings offer a comprehensive framework for QE’s anti-osteoporotic mechanism, laying a robust foundation for targeted mechanistic investigations and potential clinical translation. Beyond the common forms of osteoporosis associated with estrogen deficiency, diabetes, or aging, quercetin has also demonstrated therapeutic potential in bone loss induced by other pathological conditions. Osteoporosis and an increased susceptibility to fractures frequently occur as secondary complications in patients suffering from chronic liver diseases such as primary biliary cholangitis and hepatic cirrhosis. These conditions disrupt mineral metabolism and bone remodeling due to systemic inflammation, oxidative stress, and altered bile acid homeostasis. To explore quercetin’s protective efficacy under such hepatic stress, a rat model of cirrhosis-induced osteoporosis was established through complete bile duct ligation, which reproducibly induces cholestasis and subsequent bone deterioration [108]. In this model, administration of QE at a dose of 150 µmol/kg/day for four weeks markedly mitigated bone loss associated with cirrhosis. Morphological and biomechanical assessments revealed that quercetin treatment significantly improved both trabecular and cortical bone parameters, including increased bone volume, enhanced trabecular connectivity, and restoration of cortical thickness. Furthermore, mechanical testing indicated improved bone strength and resistance to fracture in quercetin-treated rats relative to untreated bile duct ligation controls. These findings highlight quercetin’s capacity to counteract cirrhosis-related bone deterioration, likely through its antioxidative, anti-inflammatory, and metabolic regulatory effects. They underscore the broader therapeutic potential of quercetin in managing secondary osteoporosis arising from systemic pathological stress beyond metabolic or endocrine origins [108].

Bone defects can be defined as fractures or malformations of the bone. The etiology of these conditions may be attributed to factors such as injury, tumors or infections. Recent studies have demonstrated the efficacy of QE in promoting bone healing in rats and rabbits. Song and his team [109] made a hole in the heads of female rats that was 4 mm wide. They put a special material called a scaffold into the hole. This special material was made from QE, silk, and hydroxyapatite. They also put BMSCs into the scaffold. A special type of X-ray called a ‘microcomputed tomography scan’ was used to analyze the bone. This scan was done six weeks after the implant. The scan showed that the bone was much stronger and thicker than normal. Histological assessments further confirmed that the defect sites treated with MSC-seeded QE/silk fibroin/hydroxyapatite scaffolds exhibited increased bone matrix thickness, boosted collagenous tissue formation, and enhanced tissue ingrowth. Another study looked at what happens when a special sponge made from duck’s foot collagen and hydroxyapatite is used to treat problems in the head and neck area. This sponge had a really good effect on the problems it was used to treat. Eight weeks after surgery, the treated defects showed increased bone mineral density, bone volume and new bone formation [110]. Córdoba et al. [111] conducted a study investigating the osteoregulatory effects of QE-functionalized implants using a rabbit tibial defect model. In this experiment, bilateral cylindrical drill-hole defects were created in the proximal tibiae of rabbits, and four titanium implants coated with a nanostructured layer of QE were inserted into each tibia. The quercetin nanocoating was designed to provide a localized and sustained release of the flavonoid at the bone–implant interface, thereby modulating local bone remodeling processes. Molecular analyses revealed that the expression of key osteoclast-related genes—including cathepsin K, vacuolar-type H⁺-ATPase, and matrix metalloproteinase-9—was markedly downregulated in bone tissue surrounding the QE-coated implants compared with uncoated controls. In parallel, serum analysis demonstrated a significant reduction in RANKL concentrations, indicating suppression of osteoclastogenesis and bone resorption activity. Complementary findings were reported by Babosová et al. [112], who evaluated the osteogenic influence of systemically administered QE in healthy, five-month-old female rabbits. QE was delivered intramuscularly at doses of 10 µg/kg or 100 µg/kg, three times per week over a defined treatment period. Quantitative histomorphometric analyses revealed a dose-dependent increase in cortical bone thickness in treated animals relative to controls, indicating that quercetin enhances cortical bone apposition even under physiological (non-pathological) conditions. These observations further support QE’s dual modulatory action—suppressing bone resorption while promoting bone formation—across both systemic and localized modes of administration.

The majority of studies conducted on this subject have demonstrated that the application of QE technology results in enhanced bone strength and health in individuals with low bone density, holes in the bone, and broken bones by increasing bone thickness and strength. The delivery of QE is typically accomplished through either oral administration, parenteral injection, or direct muscular injection. However, two studies reported unexpected negative or null results, which may be attributable to interference from co-administered phytochemicals attenuating quercetin’s bone-protective effects (this requires further confirmation) or the limited impact of QE on bone parameters in healthy, non-osteoporotic animals. The outcomes of the present study are extremely encouraging, and thus, researchers are currently conducting further investigations into the mechanisms by which QE operates directly on bone cells and the processes that regulate bone metabolism [43]. Other cases of quercetin’s effects on bone health, not described in the text, are included in

Table 2. The effect of quercetin on bone conditions.

Type of Animal	Intervention	Findings	Ref.
OVX rats; 8-week-old female Sprague-Dawley	15 mg/kg/day	↑ serum calcium, bone weight, bone volume, trabeculae volume, the total number of osteocytes and osteoblasts, LC3, beclin1, caspase 3 ↓ total number of osteoclasts, serum osteocalcin, Bcl-2	[113]
OVX rats; 10-week-old female Wistar	QE 50 mg/kg/day + Dasatinib mg/kg/day	↑ femur trabecular bone microarchitecture, BV/ TV, trabecular number ↓ Tb.Sp, SMI, SnC, p16	[114]
OVX rats; adult 3-month-old female Sprague-Dawley rats	QE 15 mg/kg/day  +  Alendronate 5 µg/kg/day	↑ BMD, BV/TV, Tb.N, osteocyte, osteoblast ↓ Tb.Sp, Beclin-1, Caspase-3	[115]
Iron overload-induced osteoporosis mouse model; 6–8-week-old female C57BL/6	QE 50 or 100 mg/kg/day	↑ BV/TV, Tb.Th, Tb.N; ↓ SMI	[59]
STZ-NA-induced diabetic rats; 200–250 g male Wistar	QE 50 mg/kg/day	↑ BMD, Tb.Bv/Tb.Tv, Tb.N, Tb.Th, Ct.Th, Tb.Sp; ↓ SMI	[116]
Osteoporosis in orchiectomy mice; 8-week-old male C57BL/6	QE 75, 150 mg/kg/day	↑ bone mass, bone strength, bone microstructure, stride length and frequency, insulin-like growth factor-1, high-density lipoprotein, GPRC6A, phospho-AMPK/AMPK; ↓ phospho-mTOR/mTOR	[117]
Ti particle-induced osteolysis in female C57BL/6 adult mice	QE 2 or 5 mg/kg/day	↑ BV/TV ↓ total porosity, erosion area, osteoclast number	[118]
Ti particle-induced osteolysis in 6-week-old male BALB/C mice (weighing 18 ± 5 g)	QE 50 or 100 mg/kg/day	↑ bone area ↓osteolysis, osteoclast number, PERK, IRE1, GRP78, CHOP, cleaved caspase-12, cleaved caspase-3, Bcl-2	[78]
Zinc oxide nanoparticles (600 mg/kg/day, 5 days); male Wistar albino rats weighing 170–200 g	QE 200 mg/kg/day	↑ Bone ALP, TNF-α ↓ CTX, NO *; DNA damage, IL-6, CRP	[77]

↑ increase/upregulation, ↓ decrease/downregulation. Abbreviations: AMPK, AMP-activated protein kinase; mTOR: mammalian target of rapamycin; ALP, alkaline phosphatase; BMD, bone mineral density; BV/TV, bone volume/total volume; Bcl-2, B-cell lymphoma 2; CHOP, CCAAT/enhancer-binding protein homologous protein; CRP, C-reactive protein; Ct.Th, cortical bone thickness; CTX, C-terminal telopeptide of type 1 collagen; DNA, deoxyribonucleic acid; GPRC6A, G protein-coupled receptor family C, Group 6, Subtype A; GRP78, glucose-regulated protein; IL-6, interleukin-6; IRE1, inositol-requiring enzyme 1; NO *, nitric oxide radical; OVX, ovariectomized; p16, multiple tumor suppressor 1; PERK, protein kinase RNA-like endoplasmic reticulum kinase; SMI, skeletal muscle mass index; SnC, senescent cell; STZ-NA, streptozotocin nicotinamide; Tb.Bv/Tb.Tv, trabecular volume fraction; Tb.N, trabecular number; Tb.Sp, trabecular separation/spacing; Tb.Th, trabecular thickness; TNF-α, tumor necrosis factor-α.

.

In general, QE exhibits pronounced bone-protective effects in estrogen-deficient states, particularly in ovariectomized rodent models, where it improves BMD and trabecular microarchitecture, and reduces osteoclast number and bone loss markers [94,119]. In one recent OVX study, these effects were linked to modulation of the intestinal flora–SCFA–inflammatory axis [119]. These actions may be partly related to ER-dependent osteogenic signaling and inhibition of RANKL-driven osteoclastogenesis [42,61]. Although most studies have been conducted in female hyperestrogenic models, limited data from male diabetic and orchiectomized animals also suggest beneficial skeletal effects [101,102,116,117]. In diabetic osteoporosis models, quercetin improves bone remodeling, microarchitecture, and selected metabolic parameters related to glucose homeostasis [101,102,117]. Protective effects have also been reported in glucocorticoid-induced osteoporosis [105] and in retinoic acid-induced bone loss models [120]. However, the available evidence remains predominantly preclinical, and human data are still lacking [5].

6. Conclusions

QE and its glycosides consistently enhance bone quantity and quality across diverse osteopenic models, including estrogen deficiency, diabetes, and glucocorticoid excess. In ovariectomized rodents, QE increases bone mineral density, trabecular number/thickness, cortical thickness, and mechanical strength of long bones/vertebrae, while normalizing histomorphometric indices. Analogous benefits occur in diabetic/glucocorticoid-induced osteoporosis, restoring microarchitecture, load-bearing capacity, and turnover markers like ALP/acid phosphatase. In vitro, QE promotes osteoblastogenesis from bone-marrow/adipose MSCs, boosting proliferation, ALP activity, mineralization, and markers (Runx2, osterix, osteocalcin). It exerts anti-resorptive effects by elevating OPG, suppressing RANKL, and downregulating osteoclast genes (NFATc1, TRAP, cathepsin K). Local QE delivery via scaffolds, microspheres, or nanocoatings accelerates critical-size defect repair and osseointegration by increasing bone volume/implant contact and curbing peri-implant osteoclasts. Preclinical safety data reveal a wide therapeutic window, with glycosides (quercitrin, isoquercitrin) often outperforming aglycone due to better bioavailability. QE acts pleiotropically on osteoblasts/osteoclasts, redox (Nrf2), inflammation (NF-κB), and angiogenesis, positioning it as an adjunct for osteoporosis prevention/treatment.

7. Future Perspectives

QE’s low solubility, rapid metabolism, and ~2–17% oral bioavailability necessitate advanced formulations like nanoparticles, transfersomes, hydroxyapatite composites, and graphene systems for sustained bone-targeted release. QE–vitamin D interactions warrant exploration for synergistic osteoblast/osteoclast regulation, given the limited current evidence [97,121,122]. Critical needs include human RCTs assessing optimal dosing/regimens, long-term safety, and head-to-head comparisons with bisphosphonates/denosumab on BMD, microarchitecture, fractures, and patient outcomes. Nutraceutical applications in at-risk groups (postmenopausal and diabetic) and bioactive implants for osteoporotic defects represent near-term priorities. Mechanistic studies should clarify metabolite contributions, age/sex/pathology modulators, and interactions with standard therapies.