GA, a phenolic compound, is found in various plants, including black tea, green tea, quince, pomegranate, oranges, grapes, and berries. Given the wide range of sources containing GA, various extraction techniques, including hot water extraction, supercritical CO2 extraction, solid-phase extraction, and ultrasonic-assisted extraction, have been developed (Du et al., 2009; Murga et al., 2000; Palma et al., 1999; Pawar and Surana, 2010).

Researchers have found that hot water extraction is the primary method for extracting antioxidants. Pawar et al. optimized the extraction parameters forextracting GA from Caesalpinia decapetala. The ideal conditions were an extraction temperature of 65–70°C, an extraction period of 48 h, and a solvent mixture of ethanol to water (70:30). Under these optimal conditions, the maximum yield of GA was 17.85% (Pawar and Surana, 2010). However, the extraction of GA from Emblica officinalis, by the preparation of molecularly imprinted microspheres and nanoparticles through precipitation polymerization and subsequent elution with hot water, significantly increased the extraction yield to 28% (Pardeshi et al., 2014). Additionally, GA was extracted from pomegranate rind using solid-phase extraction with surface-imprinted polymers on magnetic carbon nanotubes with a yield of 3.16 mg/g (Hao et al., 2015). The optimum extraction yield of GA (8.57 mg/g) was achieved by preparing hydrophilic molecularly imprinted chitosan and employing solid-phase microextraction methods, using the response surface methodology strategy (Li and Row, 2019). Khodaie et al. successfully extracted GA (50.54%) from the seeds of the sumac species Rhus coraria using supercritical CO₂ with ethanol as a co-solvent (Khodaie and Ghoreishi, 2021). In addition, when GA was extracted from the leaves of Ficus auriculata, alkaline water and ultrasonic-assisted extraction were used. This method was environmentally friendly and safe. The results showed that the extraction yield with weak alkaline water was 284.2 mg/L, second only to 50% methanol (Baite et al., 2021).

It have been found that GA was synthesized through the shikimic acid pathway (Fernandes and Salgado, 2016; Wu et al., 2022; Zhang et al., 2009). Researches showed that shikimate dehydrogenase (SDH) was an essential enzyme in the shikimate pathway for GA synthesis. The aroE mutant strain of Escherichia coli could produce GA through functional complementation with plant-derived SDH. In transgenic tobacco (Nicotiana tabacum) expressing SDH from walnut (Juglans regia), accumulation of GA was increased by 500% (Muir et al., 2011). Additionally, with the high-yield, high-titer synthesis of 3-dehydroshikimic acid from glucose using recombinant E. coli, GA was efficiently synthesized from 3-dehydroshikimic acid by choosing the appropriate solvent (such as acetic acid) and optimizing the catalytic system (such as using Cu (OAc)₂ and ZnO). And the yield of GA was significantly increased to 67% (Kambourakis and Frost, 2000).

Additionally, in nature or within plants, GA was synthesized through the degradation of tannic acid via tannase, a glycoprotein esterase. This enzyme was produced by fermentation with various microorganisms, particularly fungi from the Aspergillus and Penicillium genera (Aguilar-Zárate et al., 2015; Dhiman et al., 2018). For example, the novel Penicillium roqueforti strain could produce both tannase and GA simultaneously (Andrade et al., 2018). Moreover, the marine Aspergillus awamori strain BTMFW032, under deep fermentation conditions, was able to simultaneously produce GA and tannase, resulting in a 15-fold increase in the yields of both tannase and GA (Beena et al., 2011). Furthermore, other microorganisms such as Rhodotorula pilimanae A45.2 and Bacillus spheroides have also been noted for their abilities to co-produce GA and tannase, with the latter achieving high efficiency in converting tannic acid to GA (90.80% crystallization), making it the most potent bacterial tannase producer for GA synthesis (Kanpiengjai et al., 2020; Raghuwanshi et al., 2011). Recent research discovered that a high-activity PobA variant, Y385F/T294A-PobA, was developed from the hydroxyphenylacetic acid hydroxylase of Pseudomonas aeruginosa. This variant exhibited a molar conversion rate of up to 93%, providing a promising pathway for the biomanufacturing of GA and its derivatives (Chen et al., 2017). In summary, GA is primarily synthesized through the shikimic acid pathway, and in some cases, it is also produced as a by-product of tannic acid decomposition.

It has been established that GA can ameliorate lipid accumulation by inhibiting lipogenesis, a critical strategy for lowering lipid levels. GA reduced the activity of transcription factors involved in lipid synthesis. Particularly noteworthy is its targeting of the acetyl-CoA carboxylase (ACC) family, which acts as a rate-limiting enzyme in fatty acid metabolism, thereby reducing overall lipid synthesis (Chen et al., 2019; Yeudall et al., 2022). GA strongly activated the AMPK-ACC axis in a dose-dependent manner, thereby inhibiting lipid synthesis. In mice fed high-fat diet (HFD), GA reduced the mRNA expressions of the lipidogenesis marker ACC, FAS (Fatty Acid Synthase)/B-actin, ACC/B-actin and Acetyl-CoA Carboxylase Beta (Sousa et al., 2020). Futhermore, in HepG2 cells, GA treatment activated AMPK signaling and reduced mRNA expression of key transcription factors in fatty acid synthesis, including SREBP-1c, LXRα, and ACCα, as well as fatty acid transporter proteins such as CD36 and FATP2, which mitigated palmitic acid-induced lipid accumulation (Tanaka et al., 2020a). In summary, GA primarily inhibited lipogenesis and improved lipid profiles through the AMPK pathway.

Researches have demonstrated that GA mitigated lipid accumulation by enhancing lipid metabolism which involved s several key pathways.

On one hand, GA directly promoted lipid metabolism. Firstly, GA treatment promoted fatty acid metabolism. Specifically, GA increased beta-oxidation of fatty acids and elevated levels of ketone body metabolites which are produced during fatty acid catabolism. This process enhanced lipolysis, thereby reducing hepatic cholesterol accumulation. In mice with HFD-induced non-alcoholic fatty liver disease (NAFLD), GA treatment raised ketone body levels in both serum and urine, enhanced beta-oxidation in the liver, and reduced excessive fat accumulation in intracellular vacuoles. Additionally, GA treatment decreased liver triglyceride (TG), cholesterol, and fatty acid levels (Chao et al., 2014a). Peroxisome proliferator-activatedve receptor alpha (PPARα) regulates genes involved in lipid metabolism, influencing lipid homeostasis (Bougarne et al., 2018). In HFD mice, GA could also significantly reduce the expression of PPARα. It downregulated the expression of the lipogenic enzyme monounsaturated fatty acid synthase (SCD-1) and upregulated the expressions of key genes for triglyceride synthesis, such as sterol regulatory element-binding protein 2 (SREBP-2) and the cholesterol synthesis gene β-hydroxy-β-methylglutaryl-CoA synthase (HMGCS), which led to the inhibition of cholesterol synthesis and the reduction of lipid accumulation (Chao et al., 2020; Lee et al., 2021).

Secondly, GA treatment enhanced triglyceride metabolism. Pancreatic lipase was essential for breaking down triglycerides into glycerol and fatty acids, which prevented the absorption and digestion of triglycerides (Modanwal et al., 2024). In diet-induced obese mice, GA inhibited triglyceride uptake and digestion by decreasing pancreatic lipase activity (Oi et al., 2012). Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are key enzymes in lipolysis (Grabner et al., 2021). Studies have shown that GA increased ATGL expression in the perirenal adipose tissue of HFD rats, leading to decreased hypertriglyceridemia and fat accumulation (Huang et al., 2018). In bovine subcutaneous adipocytes, GA reduced TG levels b increasing the expression of lipolysis-related genes such as ATGL and HSL, and activating the metabolic regulator AMPKα (Jin et al., 2022). Moreover, GA promoted triglyceride metabolism by inducing autophagy. According to Singh et al., autophagy is a critical pathway for regulating cellular lipid levels by digesting lipid droplets in autophagic lysosomes, thus reducing TG storage (Grabner et al., 2021). In HepG2 cells, an increase in autophagosome formation, marked by the conversion of LC3-I to LC3-II, led to a reduction in oleic acid (OA)-induced TG accumulation (Doan et al., 2015).

On the other hand, GA treatment can also indirectly enhance lipid metabolism. Firstly, GA treatment can indirectly enhance lipid metabolism by inhibiting inflammation. Increasing evidences suggested that inflammation in adipose tissue triggered lipolysis, leading to the excessive release of free fatty acids and subsequent hepatic lipid accumulation (Badmus et al., 2022; van Dierendonck et al., 2022). By reducing inflammatory responses, GA potentially lowered the risk of abnormal lipid metabolism and related diseases. In dust-exposed HFD-induced rats, GA pretreatment notably reduced the expression of NF-κB, interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α), and the serum levels of triglycerides, cholesterol, and low-density lipoprotein (LDL) (Fanaei et al., 2021). In the white adipose tissue of diet-induced obese mice, GA diminished the expressions of Il6, Nos _2, Ptgs _2, Adgre ₁ and Srebf ₁ . Moreover, in a co-culture of 3T3-L1 adipocytes and RAW 264 macrophages, treatment with GA resulted in a significant reduction in the expression of TNF, CCl₂ and NOS₂. Consequently, GA may be mitigate adipose tissue inflammation by suppressing the expression of inflammatory mediators, either within the white adipose tissue or in macrophages (Tanaka et al., 2020b).

Secondly, GA treatment can enhance lipid metabolism by improving mitochondrial function. Mitochondria are crucial for cellular energy production and lipid metabolism (Pernas, 2024). Moreover, GA can promote lipid metabolism by improving mitochondrial function, a key factor in cellular energy production and lipid metabolism. Specifically, GA significantly increased the expression of PGC1α target genes such as mitochondrial transcription factor A, nuclear respiratory factor-1, and nuclear respiratory factor-2, thereby upregulating fatty acid β-oxidation enzymes like carnitine palmitoyltransferase-I, and promoting fatty acid oxidative metabolism in mitochondria (Doan et al., 2015). Furthermore, in rats induced with HFD, GA reduced body weight, adipose tissue weight (peritoneal and epididymal), triglyceride (TAG) level, low-density lipoprotein cholesterol level, phospholipids, and total cholesterol level by enhancing antioxidant activity (Hsu and Yen, 2007). For example, in db/db mice fed with a high-fiber diet, GA enhanced intracellular antioxidant defenses, inhibited reactive oxygen species (ROS) production, and boosted hepatic antioxidant enzymes (GSH, SOD, GST), thereby promoting lipid metabolism (Hsu and Yen, 2007; Lee et al., 2021; Punithavathi V. et al., 2011).

Thirdly, GA improved lipid metabolism by increasing thermogenesis, which was associated with the AMPK/Sirt1/PGC1α pathway. Brown adipose tissue (BAT) is integral to lipid metabolism as it increased heat production through enhanced energy metabolism (Jung et al., 2019). Bioinformatics analyses have identified SIRT1 as a significant player in thermogenesis. PGC-1α is known to regulate thermogenesis in BAT (Rao et al., 2014; Zhang J. et al., 2022). In the BAT of HFD-fed mice, treatment with GA elevated the expression of SIRT1 and PGC1-alpha mRNA, enhancing thermogenesis and improving overall body metabolism (Paraíso et al., 2019). Furthermore, GA regulated key thermogenic factors primarily by activating the AMPK pathway. GA treatment activated this pathway and significantly upregulated the expression of UCP1, a crucial regulator of heat production in BAT. It also increased the expression of energy expenditure-related genes such as UCP3, PGC1β, and β3-Adr, contributing to body weight reduction in HFD-induced obese mice (Doan et al., 2015).

In conclusion, GA significantly promoted lipid metabolism both directly, by enhancing fatty acid and triglyceride metabolism, and indirectly, through the inhibition of inflammation, improvement of mitochondrial function, and stimulation of thermogenesis.

GA has been demonstrated to reduce lipid accumulation by inducing adipocyte apoptosis, a process that entails the regulated, active death of adipocytes through apoptosis pathways. GA triggers adipocyte apoptosis via two primary pathways, outlined as follows:

GA can induce adipocyte apoptosis through the intrinsic (mitochondrial) pathway. In 3T3-L1 preadipocytes, GA increased the expression and release of mitochondrial cytochrome c into the cytoplasm, which significantly upregulated the expression of apoptosis-related enzymes, such as caspase-3 and caspase-9, thereby reducing cell viability (Hsu F.-L. et al., 2007). Additionally, in subcutaneous preadipocytes, GA altered the expression ratio of pro- and anti-apoptotic members of the Bcl-2 family, decreasing anti-apoptotic proteins like Bcl-2 and Bcl-XL, and markedly increasing pro-apoptotic proteins such as Bax, Bad, and Bak, which reduces the viability of these cells (Jin et al., 2022). Indeed, the extrinsic apoptosis pathway, activated by death receptors, is crucial for cellular apoptosis. This pathway is primarily mediated by fas cell surface death receptor/fas ligand (Fas/FasL) and significantly modulated by the p53 protein (Mustafa et al., 2021; R. Zhao et al., 2020). Similarly, GA modulated the Fas/FasL pathway, contributing to p53-mediated induction of apoptosis in adipocytes. In 3T3-L1 preadipocytes, GA elevated the expressions of Fas, FasL, and p53, diminished adipocyte viability, and decreased lipid accumulation (Hsu C.-L. et al., 2007).In summary, GA disrupted adipocyte viability and induces apoptosis via both the mitochondrial and death receptor pathways.

GA indirectly regulated lipid levels by promoting adipocyte differentiation. Torres et al. have found that this is a process that increases the production of adipokines (such as lipocalin) and enhances oxidase activity (Pérez-Torres et al., 2021). A key factor in this process is PPARγ, a ligand-activated transcription factor from the nuclear receptor family, pivotal in adipocyte differentiation (Hernandez-Quiles et al., 2021). In diet-induced obese mice, GA reduced serum triglyceride concentrations and significantly increased PPARγ protein level in white adipose tissue, thereby enhancing adipocyte differentiation (Bak et al., 2013). Additionally, in GA-treated mouse 3T3-L1 cells, expressions and levels of both adiponectin, a crucial protein hormone for this process—and fatty acid-binding protein-4 (Fabp4), a target of PPARγ and a marker of adipocyte differentiation were increased. Furthermore, GA treatment boosted the secretion and expression of lipocalin, reduced adipocyte viability, and ameliorated lipid accumulation (Makihara et al., 2016; Tanaka et al., 2020b).

In conclusion, it was evident that GA promoted adipocyte differentiation, leading to enhanced production of adipokine, which in turn indirectly influenced lipid levels.

Research showed that GA preserved vascular health by preventing endothelial cell apoptosis and dysfunction, making it a key strategy against AS. Initially, GA can directly reduce apoptosis. In various mouse models, DNA (Cytosine-5)-Methyltransferase 1 (DNMT1) was crucial for cell survival, and proteasome inhibitors can prevent endothelial cell apoptosis and reduce mortality, significantly aiding in endothelial protection (Wesley et al., 2024; Zhao et al., 2024). In EAhy926 and HBEC-5i cells exposed to a mixture of homocysteine, adenosine, and tumor necrosis factor (TNF) (DL Hcy Ado TNF), GA effectively restored DNMT1 expression, reduced the activity of the chymotrypsin-like proteasome, significantly inhibited caspase-3 expression, and alleviated apoptotic effects and granule formation (Kam et al., 2014).

Secondly, GA can damage endothelial cells by reducing oxidative stress. Endothelial dysfunction, a precursor in atherosclerosis pathogenesis, was primarily driven by oxidative stress (Panda et al., 2022). GA acted as an antioxidant to counteract oxidants generated in the peroxidase cycle. In human microvascular endothelial cells (HMEC-1) exposed to high concentrations of hydrogen peroxide, GA reduced oxidants produced in the peroxidase cycle, and significantly enhanced cell viability (Serrano et al., 2010). Furthermore, in a cultured human umbilical vein endothelial cell model of oxidative stress (o-toluene-3-phenol), GA at relatively low concentrations enhanced endothelial cell viability and significantly reduced o-toluene-3-phenol-induced cytotoxicity. This was primarily attributable to an elevation in total intracellular GSH levels in the presence of low concentrations of GA. However, no enhancement in SOD or catalase activity was observed (Goszcz et al., 2017). In addition, recent studies have shown that GA can also ameliorate endothelial cell injury by inhibiting endothelial cell mitochondrial dysfunction, mainly through the ERK/CypD/NOX 4/Poldip 2 pathway. In Ang II-induced HUVECs, GA inhibited Extracellular Signal-Regulated Kinase (ERK) phosphorylation, Cyclophilin D (CypD) expression, the interaction of NOX4 and Poldip 2, and lowered mitochondrial ROS levels, resulting in an increase in the mitochondrial membrane potential of the HUVECs cells, thereby alleviating endothelial mitochondrial dysfunction (Sun et al., 2023). Thus, GA may inhibit oxidative stress and ameliorate endothelial cell dysfunction by increasing total GSH levels and mitochondrial membrane potential.

Inhibition of platelet activation and aggregation constituted a key anti-atherosclerosis mechanism of GA in the treatment of cardiovascular diseases.This effect was primarily associated with platelet P-selectin. In both normolipidemic C57/Bl6 and aged atherosclerotic ApoE-deficient mice, GA improved vascular cell adhesion molecule function and inhibit platelet activation prior to platelet activation. Further research has found that GA inhibited the interaction between P-selectin and platelets in HL60 cells, thereby preventing platelet activation (Appeldoorn et al., 2005). Futhermore, in platelet-rich plasma stimulated by ADP or U46619 (a thromboxane A2 analog), GA reduced the adherence of platelets to white blood cells and inhibited the formation of platelet-leukocyte aggregates. It was shown that GA reduced intracellular Ca²⁺ level by inhibiting phosphorylation of Protein Kinase C alpha (PKCα) and p38 mitogen-activated protein kinase (p38 MAPK) in platelets, as well as phosphorylation of Akt and glycogen synthase kinase 3 beta (GSK3β), leading to a significant reduction in ADP- and U46619-induced platelet aggregation and platelet P-selectin expression in a concentration-dependent manner (Chang et al., 2012b). In conclusion, GA improves platelet aggregation by inhibiting the expression of P-selectin, which may be related to PKC α/p38 MAPK and Akt/GSK3β pathways.

Additionally, GA could reduce the initial activation of platelets by inhibiting thrombin activity. Negrier et al. identified thrombin as a key regulatory factor in platelet activation and aggregation, enhancing adhesion and aggregation between platelets by promoting fibrin formation (Negrier et al., 2019). In platelet aggregation assays involving thrombin-stimulated platelets, GA demonstrated a reduction in aggregation by approximately 35%. Subsequent studies confirmed that GA significantly inhibited thrombin by directly binding to the thrombin protein, thereby rapidly stabilizing its conformation and reducing platelet aggregation (Zhang et al., 2022b).

In summary, GA reduces platelet activation and aggregation by inhibiting P-selectin and thrombin activity.

Firstly, GA inhibited the proliferation of VSMCs by inducing their apoptosis. In cultured VSMCs from rat aortas, GA treatment inhibited oxidative stress caused by hydroxyl radicals and promoted apoptosis, characterized by cytoplasmic shrinkage, vesicle formation, and nuclear condensation, thereby inhibiting the proliferation of vascular smooth muscle cells (Qiu et al., 2000). Secondly, Clark et al. have demonstrated that GA decelerated cell cycle progression in VSMCs through reducing signaling pathways associated with growth, mobility, and senescence, thereby inhibiting the over-proliferation of VSMCs (Clark et al., 2022). In TNF-α-stimulated A7r5 rat aortic VSMC, GA inhibited the expression and phosphorylation of the cytoskeletal proteins ras homolog family member A (RhoA), Ras-related C3 botulinum toxin substrate 1 (Rac1), and cell division cycle 42 (CDC42). This resulted in a reduction in TNF-α-induced cell migration. Furthermore, GA inhibited the phosphorylation of PI3K, Akt and ERK, and regulated the expression of inflammatory proteins (reducing the activation of NF-κB and Ras, and increasing the expression of iNOS and Kinase Suppressor of Ras 2), and increased the expression of Phosphatase and Tensin Homolog deleted on Chromosome 10 (PTEN), thereby inhibiting TNF-α-induced cell proliferation (Chung et al., 2020). Furthermore, in OA-treated VSMC, GA treatment activated AMPK and eNOS, and inhibited fatty acid synthase (FAS), decreased the levels of cytokinin B1 and cytokinin-dependent kinase 1 (CDK1), and increased the levels of Kip1/p27 and Cip1/p21, which led to the accumulation of the G2/M phase of the cell cycle, thereby slowing down cell cycle progression and inhibiting VSMC proliferation. The specific inhibitor of AMPK, Compound C, was observed to reduce GA-induced eNOS activation and nitric oxide production, thereby underscoring the pivotal role of AMPK in this process (Ou et al., 2013). In summary, GA inhibited the RhoA/CDC42 and PI3K-Akt-ERK1/2 pathways and activated the AMPK-eNOS-FAS signaling pathway, thereby suppressing the migration and proliferation of VSMCs (Figure 3).

Gerst et al. identified that mitigating β cell damage and fostering β cell regeneration significantly boosts insulin secretion from these cells (Gerst et al., 2021; Rady et al., 2022).Thus, strategies to restore β cell functionality typically involved promoting cell proliferation and minimizing damage (Lewis and Wells, 2021; Lu et al., 2020; Tomita, 2017). Researches have demonstrated that GA primarily enhanced regeneration of β cells and reduced β cell damage, thereby augmenting insulin secretion. The detailed mechanisms are elaborated below.

Initially, GA promoted the regeneration of pancreatic β cells and enhance insulin secretion. In diabetes rats, GA treatment resulted in a dose-dependent decrease in blood glucose level, as evidenced by lowered AUC_glucose, insulin levels, and Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) indices (Abdel-Moneim et al., 2017; Variya et al., 2020). Additionally, in male Wistar rats with diabetes induced by streptozotocin (STZ), GA improved pathological alterations in pancreatic islet cells, promoted β-cell regeneration, and boosted insulin output. From a mechanistic standpoint, GA promoted pancreatic β-cell regeneration through its antioxidant properties (Latha and Daisy, 2011). In STZ-induced diabetic male albino rats, GA enhanced antioxidant capacity by altering biochemical conditions in pancreatic tissues. It decreased the activity of pancreatic peroxidases, purinergic enzymes, detoxifying enzymes (GST), heme biosynthesizing enzymes (δ-ALA-D), and glycolysis enzymes (LDH), which led to reduced level of blood glucose and decreased pancreatic weight (Kade et al., 2014). Furthermore, in a rat model of STZ-induced diabetes, GA significantly reduced thiobarbituric acid-reactive substances and lipid hydroperoxides, enhanced activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Histopathological analysis showed that GA restored pancreatic beta cells and increased insulin secretion (Punithavathi V. R. et al., 2011).

Secondly, GA enhanced insulin secretion by ameliorating damage of β-cells. Recent studies have demonstrated that secreted frizzled-related protein 4 (SFRP4) is a potential biomarker for β-cell dysfunction in type II diabetes, associated with reduced insulin secretion (Bukhari et al., 2019; Zhang et al., 2020). In diabetic mice induced by a high-calorie diet, oral administration of GA significantly lowered serum SFRP4 level, which led to reductions in body weight and blood glucose level (Bukhari et al., 2022). Furthermore, in RINm5F β cells exposed to high glucose, palmitate esters, or both, GA enhanced cellular survival pathways by upregulating Bcl-2 activity, downregulating caspase-3 activity, reducing DNA damage, weakening nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling, and increasing the expression of insulin mRNA, thereby promoting insulin secretion (Sameermahmood et al., 2010). Fibrin deposition (also known as islet amyloid polypeptide) has been identified as one of the factors leading to the death of islet β cells (Salazar Vazquez et al., 2020). Also, GA prevented the formation of pancreatic amyloid fibrils, thereby protecting pancreatic β cells and enhancing insulin secretion. In vitro, GA could interacted with natural insulin, inhibiting the key nucleation process essential for the growth of fibrils, thus hindering the formation of amyloid fibers and safeguarding the insulin structure and curtailing protofibril formation (Jayamani and Shanmugam, 2014).

Caturano et al. showed that addressing insulin resistance could significantly improve blood glucose control and was a feasible strategy for diabetes management (Caturano et al., 2024; Wang et al., 2019). In streptozotocin-induced experimental type 2 diabetic rats fed a high-fat diet, GA significantly reduced plasma insulin and HOMA-IR levels and normalised changes in insulin resistance levels, thereby reducing body weight and fasting blood glucose (Gandhi et al., 2014). Therefore, GA can lower blood glucose by alleviating insulin resistance GA attenuated insulin resistance primarily by activating the protein kinase B (Akt) signalling pathway. In fructose-induced diabetic rats, GA increased the expression of proteins related to hepatic insulin signalling, including insulin receptor (IR), insulin receptor substrate 1 (IRS-1), phosphoinositide 3-kinase (PI3K), Akt and glucose transporter 2 (GLUT2) (Huang et al., 2016). Furthermore, in experimental type 2 diabetic rats induced by a high-fat diet and streptozotocin, GA treatment lowered the levels of glucose-6-phosphatase and fructose-1,6-bisphosphatase, facilitating glucose absorption. This was associated with a significant increase in the expression of peroxisome proliferator-activated receptor gamma (PPARγ) mRNA. Molecular docking studies showed that PPARγ interacted well with GLUT4, GLUT1, PI3K and p-Akt, indicating that GA could attenuate insulin resistance by activating the PPARγ-PI3K/p-Akt -GLUT4 signalling pathway (Gandhi et al., 2014; Huang et al., 2018; Variya et al., 2020). It was noteworthy that in fructose-induced diabetic rat model, treatment with GA increased expressions of Glut4, PPAR-γ and pAkt proteins, but not phosphorylated AMP-activated protein kinase (pAMPK) in the epithelial white adipose tissue, suggesting that GA attenuated insulin resistance through the Akt signalling pathway (Variya et al., 2020). However, pAMPK was not restored, which further suggested that GA attenuated insulin resistance through the Akt signalling pathway.

Numerous studies have demonstrated that type 2 diabetes could be therapeutically managed by reducing intestinal glucose absorption, primarily through the inhibition of intestinal carbohydrate hydrolases such as α-amylase, and sodium-dependent glucose transporter protein 1 (SGLT1) (Cefalo et al., 2019; Gong et al., 2020). GA has been shown to be effective in inhibiting and delaying intestinal absorption of glucose. In diabetic albino rats, GA treatment significantly decreased α-amylase activity, thereby disrupting the hydrolytic absorption of starch-like compounds, which led to a marked reduction in situ intestinal glucose absorption (Abdel-Moneim et al., 2022). In addition, in a glucose transport assay using Caco-2 cell monolayers, GA demonstrated specific inhibitory effects on SGLT1, reducing intestinal glucose absorption primarily by suppressing the transport of low concentrations of glucose (5 mM). This effect was further validated in a 2DG transport assay (Wang et al., 2021). In conclusion, GA effectively reduced in situ intestinal glucose absorption, primarily by suppressing the activity of the intestinal carbohydrate hydrolase α-amylase and by downregulating the levels of glucose transporter proteins in the gastrointestinal tract.