Article

Phytotherapy in Diabetes Mellitus: Mechanistic Insights and Perspectives

Introduction

Diabetes mellitus (DM) is a common metabolic disorder characterized by disturbances in carbohydrate, fat, and protein metabolism, leading to persistent fasting and postprandial hyperglycemia. It results from either absolute deficiency of insulin secretion or impaired insulin action, or both. Accordingly, DM has traditionally been classified into type I (insulin-dependent diabetes mellitus, IDDM) and type II (non-insulin-dependent diabetes mellitus, NIDDM), based on the degree of pancreatic dysfunction. This classification has historical recognition dating back to Ibn Sina, who described the condition in The Canon of Medicine.

DM is not limited to dysregulation of blood glucose; rather, it progressively involves multiple organ systems. Epidemiological and clinical evidence has established a strong association between chronic hyperglycemia and long-term complications affecting both macrovascular and microvascular systems, including coronary artery disease, cerebrovascular disease, renal failure, retinopathy leading to blindness, peripheral neuropathy, limb amputation, and increased risk of premature mortality.¹

Therapeutic management of DM includes lifestyle modification such as dietary restriction and physical activity, along with pharmacological interventions including oral hypoglycemic agents and insulin therapy. Given the multifactorial nature of DM, targeting multiple pathogenic pathways is considered more effective than single-target approaches, with improved therapeutic outcomes and reduced adverse effects.² Consequently, current management strategies for type II DM often involve combination therapy using insulin secretagogues and insulin sensitizers to achieve better glycemic control.³

Despite the availability of these pharmacological options, long-term management of type II DM remains challenging. Many patients gradually exhibit reduced responsiveness to conventional antidiabetic drugs, resulting in inadequate glycemic control. Furthermore, these agents are associated with several adverse effects, including severe hypoglycemia, lactic acidosis, idiosyncratic hepatic injury, neurological complications, gastrointestinal disturbances, headache, dizziness, and in rare cases, mortality.

Insulin therapy, essential for type I DM and advanced type II DM, is also associated with limitations. Prolonged exogenous insulin administration may lead to downregulation of insulin receptors due to receptor internalization and degradation following sustained hormone exposure, thereby contributing to reduced therapeutic efficacy over time.

Inhibition of glucose absorption

Postprandial hyperglycemia is a key contributor to type II diabetes mellitus (DM) pathogenesis, as sustained glucose elevation promotes non-enzymatic glycation of proteins, leading to chronic diabetic complications. Therefore, controlling postprandial glucose levels through inhibition of carbohydrate-digesting enzymes such as α-glucosidase is a well-established therapeutic strategy. α-Glucosidase, located in the intestinal brush border, catalyzes the final step in converting oligosaccharides and disaccharides into absorbable monosaccharides.⁴ In parallel, α-amylase, present in salivary and pancreatic secretions, initiates starch breakdown into maltose, which is subsequently processed by α-glucosidase. Hence, inhibition of these enzymes, particularly via natural compounds, is a major approach for managing postprandial hyperglycemia.

Several medicinal plants exhibit significant α-glucosidase and/or α-amylase inhibitory activity. For example, methanolic extract of Adhatoda vasica showed strong sucrase inhibition, attributed to vasicine and vasicinol.⁵ Similarly, Terminalia chebula and Tussilago farfara contain maltase-inhibiting phytoconstituents. Andrographis paniculata and its active compound andrographolide delay carbohydrate digestion and absorption by inhibiting α-glucosidase activity, while essential oils of Cedrus libani selectively inhibit α-amylase.

Nigella sativa demonstrates multi-target antidiabetic effects, including restoration of glucose homeostasis and improvement in glucose tolerance comparable to metformin.⁶ Beyond insulinotropic actions, its aqueous extract inhibits intestinal glucose absorption by downregulating SGLT1, the primary sodium-dependent glucose transporter, which also functions in glucose sensing regulation[26].
Tournefortia hartwegiana exerts antihyperglycemic effects via inhibition of intestinal enzymes such as α-glucosidase, maltase, and sucrase, thereby reducing carbohydrate absorption. Additional mechanisms include enhanced insulin secretion and modulation of pancreatic and extrapancreatic pathways, suggesting synergistic activity of multiple phytochemicals. Likewise, alkaloids from Scilla peruviana inhibit β-glucosidase and β-galactosidase, while Rheum emodi rhizome constituents such as chrysophanol derivatives exhibit strong α-glucosidase inhibition.⁷
Extracts of several Lebanese medicinal plants, particularly Marrubium radiatum and Salvia acetabulosa, demonstrate potent inhibition of both α-amylase and α-glucosidase. Clinically, Marrubium vulgare and various Salvia species have shown antidiabetic effects through enhanced insulin secretion, increased glucose uptake, and inhibition of intestinal glucose absorption.
Plantago ovata husk reduces intestinal glucose absorption without affecting insulin secretion, indicating a localized intestinal mechanism. Similarly, Salacia species exhibit multi-target effects including inhibition of α-glucosidase, aldose reductase, and pancreatic lipase, along with modulation of lipid metabolism via PPAR-α activation. Their bioactive compounds such as mangiferin, salacinol, and kotalanol significantly reduce postprandial glucose levels, in some cases surpassing acarbose efficacy.
Mangifera indica exerts hypoglycemic effects partly through mangiferin-mediated α-glucosidase inhibition. Likewise, triterpenoids from Lagerstroemia speciosa show moderate inhibition of α-glucosidase, with corosolic acid being the most active compound. Pentacyclic triterpenoids from Phyllanthus amarus and other plants also contribute to α-amylase inhibition.
Clinically validated extracts such as Pycnogenol® (pine bark extract) reduce HbA1c and improve endothelial function, primarily through α-glucosidase suppression rather than insulin stimulation. Fenugreek (Trigonella foenum-graecum) exerts antidiabetic effects via intestinal glycosidase inhibition and modulation of glucose, lipid, and energy metabolic pathways.
Additional plant sources including Cornus officinalis, Alismatis rhizoma, Carthamus tinctorius, Ficus deltoidea, and Swertia corymbosa demonstrate α-glucosidase inhibitory activity in vitro and in vivo. Grape seed extract delays carbohydrate absorption by inhibiting intestinal α-glucosidase and α-amylase, reducing postprandial glucose levels in clinical settings.
Coffee, particularly via chlorogenic acid, influences glucose metabolism by inhibiting intestinal glucose transport, stimulating GLP-1 secretion, and modulating glucose absorption pathways. Chlorogenic acid also inhibits glucose-6-phosphate translocase, further contributing to glycemic control.
Finally, several Mexican medicinal plants exhibit acarbose-like activity through α-glucosidase inhibition, with Cecropia obtusifolia showing strong glucose-lowering effects, likely due to synergistic actions of chlorogenic acid and other bioactive constituents affecting multiple metabolic targets.

Activation of the nuclear receptor PPAR-γ

The PPAR family comprises nuclear hormone receptors regulating lipid, carbohydrate, and glucose metabolism. It includes three isoforms (PPAR-α, PPAR-β/δ, and PPAR-γ with γ1 and γ2 subtypes) with tissue-specific distribution. PPARs are ligand-activated receptors; thiazolidinediones (TZDs) act as potent PPAR-γ agonists. Upon activation, PPAR-γ heterodimerizes with RXR and binds PPRE to regulate gene transcription.
PPAR-γ agonists improve glucose homeostasis by increasing GLUT-4 expression and translocation in adipocytes and reducing hepatic glucose output. They also redistribute adipose tissue from visceral to subcutaneous depots (“fatty acid steal” hypothesis), improving insulin sensitivity, and reduce lipotoxicity via enhanced fatty acid uptake.
Natural PPAR-γ agonists are being explored as nutraceutical alternatives. Cinnamon supplementation reduces HbA1c and improves insulin sensitivity in type II diabetes. Herbal combinations such as mulberry, ginseng, and banaba increase PPAR-α and PPAR-γ expression and restore metabolic balance in diabetic models. Rosemary and sage activate PPAR-γ via carnosol and carnosic acid.
Screening studies showed high PPAR activation potential among medicinal plants, with nearly half of tested extracts activating PPAR-γ and some showing pan-PPAR activity. North American ginseng modulates PPAR-related genes and improves lipid metabolism,⁸ while green tea catechins enhance PPAR-α/γ expression and insulin sensitivity[108]. Clematis species and honokiol from Magnolia also activate PPAR-γ and improve glucose uptake without adipogenesis.

Increasing adiponectin release

PPAR-γ also regulates adipocytokines, which are key mediators of insulin sensitivity. Reduced adiponectin levels are associated with diabetes and cardiovascular disease, often linked to genetic variations. Adiponectin exerts antidiabetic effects by enhancing skeletal muscle glucose uptake, activating IRS-1/PI3K signaling, stimulating AMPK-mediated fatty acid oxidation, and suppressing hepatic gluconeogenesis.
PPAR-γ activation increases adiponectin gene transcription, improving insulin sensitivity. Clinical studies confirm this effect: Ipomoea batatas increases adiponectin and insulin sensitivity, Agaricus blazei elevates adiponectin in diabetic patients, and Aronia melanocarpa improves adiponectin and cardiovascular outcomes.
Momordica charantia increases adiponectin-mediated glucose uptake. Plum extract enhances adiponectin and insulin sensitivity via PPAR-γ activation, while Salacia reticulata also increases adiponectin, contributing to metabolic benefits.

Glycogen metabolism

Hepatic glucose output is regulated by glycogenesis and lipogenesis, controlled by insulin signaling. Insulin stimulates glycogen synthase and inhibits glycogen phosphorylase, promoting glycogen storage. Insulin deficiency leads to glycogen depletion and weight loss due to increased proteolysis.
Caralluma sinaica restores glycogen levels and reverses diabetic weight loss. Panax ginseng improves glucose transport and glycogen metabolism. Momordica charantia enhances β-cell regeneration and glycogen storage. Tamarindus indica restores glycogen levels and improves glucose utilization.

Insulinomimetic and insulinotropic effect

Fenugreek increases insulin secretion and improves metabolic parameters similar to glibenclamide. Garlic enhances insulin secretion via β-cell stimulation and thiol-mediated protection. Ginger and garlic show insulinotropic rather than hypoglycemic effects, while Capsicum frutescens also enhances insulin secretion.
Asparagus racemosus and Ocimum sanctum stimulate insulin release and enhance β-cell function. Asparagus adscendens improves insulin secretion and glucose uptake. Stevia extracts exhibit antihyperglycemic and insulinotropic effects via stevioside, while Viscum album increases insulin secretion and lowers glucose.
Zizyphus spina-christi improves insulin release and glucose tolerance via saponins. Biophytum sensitivum acts via insulinotropic pathways. Pterocarpus marsupium and epicatechin enhance insulin secretion and glycogen synthesis.
A large number of medicinal plants exhibit insulin-mimetic activity, including Gymnema sylvestre, Trigonella foenum-graecum, and Momordica charantia. Fenugreek enhances glucose-stimulated insulin release via 4-hydroxyisoleucine, while Gymnema sylvestre promotes β-cell regeneration and insulin secretion.

Elevation of d-chiro-inositol

D-chiro-inositol (D-CI) acts as an insulin second messenger and improves glucose utilization. Reduced levels are associated with insulin resistance. Supplementation improves glycemic control in diabetic models.
Cucurbita ficifolia contains high D-CI levels and improves glucose tolerance and glycogen storage. Fagopyrum tataricum also enhances insulin sensitivity and glucose metabolism via D-CI.
Incretin mimetics and incretin enhancers
GLP-1 regulates glucose homeostasis by enhancing insulin secretion, suppressing glucagon, delaying gastric emptying, and supporting β-cell function. It is rapidly degraded by DPP-4. Therapeutic strategies include DPP-4 inhibition or GLP-1 analogues such as exenatide and liraglutide.
Fructans from chicory and agave increase GLP-1 production and improve metabolic parameters. Artemisia dracunculus improves incretin signaling and reduces gluconeogenesis. GLP-1 signaling via IRS-2 and PDX-1 enhances β-cell proliferation and insulin secretion.
Several medicinal herbs activate incretin pathways and improve insulin secretion, including Panax ginseng and Scutellaria baicalensis, through cAMP/PKA-dependent mechanisms.

Roles of endogenous opioids on glucose homeostasis

β-endorphin modulates glucose metabolism via opioid receptors in pancreatic β-cells. It enhances GLUT-4 expression and suppresses hepatic gluconeogenesis via MOR activation. Adrenal α1-adrenoceptor stimulation increases β-endorphin release.
In diabetic models, β-endorphin levels are elevated as a compensatory mechanism. Caffeic acid increases β-endorphin release and glucose uptake via α1A-adrenoceptors. Andrographolide enhances glucose uptake via similar pathways and GLUT-4 upregulation. Myricetin, syringin, and isoferulic acid also act via β-endorphin–opioid signaling to improve glucose metabolism.

Antioxidants

Oxidative stress contributes significantly to diabetic complications via ROS-mediated damage, protein glycation, and mitochondrial dysfunction. It exacerbates insulin resistance and β-cell dysfunction.
Antioxidant therapies reduce diabetic complications. Plants such as Ficus carica, Allium sativum, Azadirachta indica, and Momordica charantia improve antioxidant defenses and glycemic control Momordica grosvenori upregulates HO-1 and reduces oxidative stress.
Scutellaria baicalensis enhances antioxidant effects and improves insulin sensitivity. Aralia taibaiensis and Acanthopanax senticosus show antioxidant and antiglycation effects correlated with saponins. Albizzia lebbeck, Lycium barbarum, Strobilanthes crispus, and Silybum marianum demonstrate strong antioxidant and antidiabetic effects.
Several compounds, including puerarin and plantago extracts, protect β-cells from ROS damage. Morinda officinalis and Amaranthus esculentus also exhibit dual antioxidant and antidiabetic properties.
Isoorientin from Gentiana olivieri protects β-cells from oxidative damage. Ginseng and garlic exert antioxidant-mediated antidiabetic effects. Phyllanthus amarus, Camellia sinensis, and Punica granatum also reduce oxidative stress.
Aldose reductase inhibitors such as flavonoids and Salacia compounds reduce oxidative stress and diabetic complications.

Conclusion

Medicinal plants and their bioactive constituents regulate glucose metabolism through multiple mechanisms including enzyme inhibition, insulin secretion, PPAR activation, incretin modulation, antioxidant effects, and enhanced glucose utilization. Their multitarget actions, safety profile, and cost-effectiveness support their potential as adjunct or alternative therapies for diabetes management.

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