Poplar

Poplar (Populus spp.) as a Dietary Supplement and Medicinal Natural Ingredient

1. Identity: Botanical Names, Natural Sources, and Common Forms

1.1 Taxonomy and Botanical Identity

The genus Populus belongs to the Salicaceae family and consists of dioecious, deciduous, and commercially important forest tree species widely spread over the Northern Hemisphere. Some particularities that characterize the Populus genus are that it includes more than 40 species and is widespread especially in Europe and Asia. Poplars (Populus spp., Salicaceae) are widely distributed throughout the world, and the genus includes about 100 species and many hybrids.

The most medicinally significant species include:

• Populus nigra L. — Black poplar. Found from northern Europe to central Asia and the coast of North Africa.
• Populus balsamifera L. — Balsam poplar, also called Balm of Gilead. Balsam poplar and black poplar (Populus balsamifera L. and Populus nigra L.) buds are the primary source of propolis used by bees.
• Populus tremuloides Michx. — Quaking aspen. It stretches from Newfoundland and Labrador to British Columbia and Yukon.
• Populus tremula L. — European aspen, distributed across Europe and Asia.
• Populus alba L. — White poplar.

1.2 Plant Parts Used

Many parts of the poplar tree, such as the buds, leaves, and bark, can be used as active ingredients for the pharmaceutical industry. A total of one hundred and fifty-nine constituents have been isolated from different plant parts like leaves, bark, buds, and roots of Populus species. The leaf-buds (Populi gemmae) are among the most pharmacologically studied materials.

1.3 Pharmacopeial Recognition

Unlike willows, poplars are not widely included in pharmacopeias. Leaves of poplars (Populi folium) are included in the national part of the Polish Pharmacopeia. In the case of buds, some species such as P. nigra L., P. balsamifera L., P. canadensis Marsh., P. laurifolia Ledeb., and P. suaveolens Fisch. are plant sources of Populi gemmae in the Russian Pharmacopeia.

1.4 Common Preparations and Dosage Forms

Poplar is prepared and sold in a variety of forms for medicinal and supplemental use:

• Tinctures and fluid extracts: Alcoholic or hydroalcoholic extracts of buds or bark, used internally or topically.
• Decoctions and teas: Hot-water preparations of bark, leaves, or buds.
• Ointments and salves (Unguentum Populeon): Poplar ointment (Unguentum Populeon) was used in folk medicine to relieve skin inflammation, burns, and wounds, as well as muscle and rheumatic pain. One of the old formulas using this plant in the past was the Populeon ointment, specially indicated in hemorrhoids.
• Essential oils and resinous extracts: Obtained from bud exudates, particularly from P. nigra and P. balsamifera.
• Standardized dry extracts: Used in modern cosmetic and pharmaceutical research.

Poplar is approved by the German Commission E for the topical treatment of minor cuts and abrasions, hemorrhoids, sunburns, frostbite, and other skin care needs.

2. Traditional and Historical Use

2.1 European Traditions

In European traditional medicine, black poplar buds were generally recommended as a remedy in respiratory infections (cough, laryngitis, bronchitis, sore throat) and also in the treatment of dermatological problems (psoriasis), rheumatic inflammations, and hemorrhoids. Traditionally, P. nigra buds were used to prepare ointments for the treatment of inflammations and hemorrhoids (Gerarde, 1597).

The buds were used traditionally as a tincture against asthma and other respiratory ailments, such as bronchitis, cough, trachea, laryngitis, sore throat, and influenza. In addition, the buds were used to treat gout, pulmonary hemorrhage, and as an ointment in the treatment of dermatological disorders such as ulcerations, hemorrhoids, anal pain, fissures, and rheumatic inflammation.

Leaf buds were used as expectorant and in the treatment of dermatitis. In addition, the leaves and bark of Populus species are known as antirheumatic herbal medicines.

The poplar buds were traditionally used in the eastern regions of Poland for treatment of cold. Poplar buds have been used for the preparation of the Populeon ointment, and they have been recommended in infusion for internal use for their sudorific and diuretic properties, and the tincture in some disorders of the urinary system.

2.2 North American Indigenous and Settler Traditions

Quaking aspen (Populus tremuloides) was formerly used in traditional medicine by Native American nations of Québec for its various therapeutic functions, either as a vermifuge, to treat venereal disease, as an anti-arthritis agent, or as a cure for colds.

Nineteenth-century doctors focused on Quaking Aspen as an excellent remedy for intermittent fever, which most likely refers to malaria, characterized by fevers that come and go with periodicity.

Poplar bark was described as tonic and febrifuge and was used in intermittent fever with advantage in eclectic medical practice.

2.3 Traditional Uses Across Cultures

Traditionally, Populus species are used in the treatment of rheumatism, arthritis, lower back pains, urinary complaints, digestive and liver disorders, debility, anorexia, fevers, and also to relieve the pain of menstrual cramps.

Previous reviews discussed the use of poplar-type propolis in folk medicine for the treatment of many diseases including skin regeneration, wound healing, local anesthesia, treatment of colds, flu, infections, and cardiovascular diseases.

3. Key Constituents and Active Compounds

3.1 Phenolic Glycosides (Salicylates)

The glucosides populin and salicin are constituents common to the barks of nearly all species of Populus, including P. tremuloides, P. tremula, and P. alba.

The genus Populus, including Populus deltoides, is characterized by the presence of salicin, salicortin, pyrocatechol, 1-p-coumaroyl-β-d-glucoside, and tremulacin. Salicin is a commonly found phenolic glycoside in the bark, leaves, and buds of more than 100 Populus species, with anti-inflammatory, antiangiogenic, and antitumor activities.

In black poplar bark extracts, salicylic acid glycosides, salicortin, salireposide, benzoic acid derivatives, populin (salicin-6-benzoate), and tremulacin (salicin-2-benzoate) have also been identified. Additionally, phytochemical investigations have revealed the presence of tannins, triterpene derivatives, wax, α- and β-amirenol, glucose, and fructose.

3.2 Flavonoids and Flavanones

It has been suggested that the presence of a large number of flavonoids and phenolic compounds are responsible for most of the biological and pharmacological activities of Populus nigra L. buds. Literature reports that black poplar buds contain flavones (e.g., apigenol and chrysin) and flavanones (e.g., pinocembrin and pinostrombin). Beside this class of phytochemicals, other important representatives are phenolic compounds, such as caffeic and ferulic acids and their derivatives.

The main flavonoids in analyzed extracts were galangin and pinocembrin. The highest content of galangin was found in the extract of balsam poplar buds, which accounted for about 23.5% ± 1.12% of the total amount of active compounds.

3.3 Phenolic Acids and Phenylpropanoids

Propolis and balsamic poplar bud extract are dominated by p-coumaric acid, while black poplar is dominated by caffeic acid.

Populus nigra L. is rich in a wide variety of phytochemicals found in its aerial parts, including chrysin, α-curcumene/ar-curcumene, caffeic acid phenethyl ester, salicin, caffeic acid, chlorogenic acid, p-coumaric acid, ferulic acid, isoquercetin, quercetin, luteolin, kaempferol, apigenin, cinnamic acid, rosmarinic acid, ellagic acid, hemiterpenes, monoterpenes, sesquiterpene hydrocarbons, 1,8-cineole, eugenol, pinostrobin, pinocembrin chalcone, and pinobanksin-3-acetate.

3.4 Terpenoids and Essential Oils

More than 48 phytocompounds have been described in the essential oils obtained from Populus nigra L. buds. Cadinen, cineol, curcumene, bisabolene, farnesol, humulene, and acetophenone were detected in significant concentration.

Essential oils extracted from P. balsamifera buds contained mono- and sesquiterpenoids, including alpha-bisabolol (a major cytotoxic agent). Other terpenes such as cadinenes, cineol, curcumene, bisabolene, and humulene were also detected.

3.5 Tannins and Other Constituents

High value-added products can be obtained from poplar bark owing to its richness in phenolic compounds. Three main types of molecules can be obtained from barks: tannins, lignin, and cellulose, though bark generally presents a lower cellulose content in comparison to log wood.

Quaking aspen trees are rich in polyphenols, but also in sterols, steryls, triterpenes, and alkaloids.

4. Mechanisms of Action

4.1 Salicylate-Mediated Anti-inflammatory and Analgesic Mechanisms

Salicin itself does not possess anti-inflammatory properties until it is metabolized into salicylic acid in the blood and gastrointestinal tract. Interestingly, some derivatives of salicylic acids showed differential inhibitory effects on COX-2 in murine macrophages.

Willow bark as well as poplar leaves are a rare source of particularly valuable glucosides and glucoside esters of salicylic alcohol (saligenin). The product of the metabolism of this class of natural salicylates in the human body is salicylic acid, which is formed only in the blood by the oxidation of released, free salicylic alcohol.

4.2 Antioxidant Mechanisms

Antioxidant activity has been evaluated by DPPH, ABTS, FRAP, and CUPRAC methods and all extracts showed antioxidant activity, with results correlating with the amounts of phenolic compounds and flavonoids in the extracts.

In addition to salicylic derivatives, poplar leaves contain flavonoids and phenolic acids, which have anti-inflammatory effects based on different mechanisms of action. The presence of the above-mentioned classes of active compounds may result in synergistic action and cause a stronger therapeutic effect. The synergism of salicylic and flavonoid compounds was noted in clinical studies evaluating the anti-inflammatory effect of extracts from willow bark from the Salicaceae family.

4.3 NF-κB and Related Signaling Pathways

Anti-inflammatory activity of P. deltoides leaf extract has been investigated, including the mechanism underlying its anti-inflammatory activity in LPS-induced RAW 264.7 macrophages, specifically via modulating NF-κB and p38/JNK pathways.

Feeding with poplar bud 50% fraction could reduce the levels of interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), monocyte chemotactic protein-1 (MCP-1), and cyclooxygenase-2 (COX-2) in liver homogenate.

4.4 Antimicrobial Mechanisms

Studies of antimicrobial activity have shown that poplar extracts have a growth inhibitory effect against Staphylococcus aureus and Candida albicans, with balsam poplar bud extract showing the most significant effect.

Three pure metabolites were isolated from P. nigra ethyl acetate extract and identified. Among them, p-hydroxy benzoic acid showed antibacterial activity while methyl p-coumarate showed antifungal activity against Malassezia furfur.

5. Scientific Evidence by Area of Use

5.1 Anti-inflammatory Activity

Evidence level: Preclinical (in vitro and animal models); no robust human clinical trials identified for poplar specifically.

Results from a study on Populus nigra flower buds ethanolic extract showed a moderate antioxidant activity (40%), but potent anti-inflammatory activity (49.9%) on carrageenan-induced mice paw edema, and also, as revealed by histopathologic examination, complete protection against AlCl₃-induced hepatic toxicity. This was an animal model study.

The potential protective role of pinocembrin and pinostrobin and extracts from buds of P. nigra and P. × berolinensis against AgNPs-induced inflammation and cytotoxicity in HGF-1 cells was demonstrated. The antioxidant properties of poplar bud extracts were demonstrated, with P. × berolinensis buds showing the highest activity in both the in vitro model and in the bioautographic tests. This was a cell-culture (in vitro) study.

A wound healing study found that treatment with Populus nigra ointment (10 and 20%) promoted wound contraction of 97.37 ± 1.19 and 97.28 ± 0.91%, respectively. Anti-inflammatory potential was illustrated by lipoxygenase and cyclooxygenase inhibition (52.80 ± 0.2 and 53.88 ± 2.55%, respectively). This was an animal (rat excision) model.

5.2 Antimicrobial and Antifungal Activity

Evidence level: In vitro only; no human clinical trials identified.

Minimal inhibition concentration (MIC) and minimal bactericidal/fungicidal concentration (MBC/MFC) testing demonstrated that water extracts of P. tremuloides had the best antimicrobial activity by a weak to moderate inhibition of growth of all eight tested microorganisms in addition to having a bactericidal effect on three of them. The quaking aspen methanol extract also displayed antimicrobial activity but to a lower level than the water extract.

A study found that Populus nigra L. bud extract presents important antioxidant activity due to its rich phytochemical composition. Regarding biological activity, results showed that poplar bud extract presents a significant inhibitory activity against Gram-positive bacteria and a dose-dependent decrease of MCF-7 tumor cell viability with an IC₅₀ of 66.26 μg/mL, while not affecting healthy cells.

Ethyl acetate extract of P. nigra showed antibacterial effects against S. aureus, S. epidermidis, and C. acnes at concentrations of 0.025%, 0.025%, and 0.1%, respectively.

5.3 Antioxidant Activity and Skin Aging

Evidence level: In vitro; some cosmetic application research, no robust human clinical trials identified specifically for poplar.

Black poplar extracts exhibit biological activities sought after by pharmaceutical and cosmetic industries. Among these activities, in vitro antioxidant properties of bud extracts have been reported, as well as anti-inflammatory and antimicrobial properties.

The buds of P. nigra constitute a renewable natural residue and a valuable source of bioactive compounds responsible for antioxidant, anti-inflammatory, antifungal, hepatoprotective, vasorelaxant, antimicrobial, anti-hyperuricemic, neuroprotective, antidiabetic, and antitumor properties. Several findings have elucidated that bud extract has beneficial effects against skin infections and anti-aging potential.

5.4 Antidiabetic and Metabolic Effects

Evidence level: Animal (rodent) model studies only; no human clinical trials identified.

In order to evaluate the effect of poplar buds on type-2 diabetes, crude extract and 50% fraction of poplar buds were used to feed streptozotocin-induced type-2 diabetic mice. The results showed that the 50% fraction could increase insulin sensitivity and reduce insulin resistance, as well as decrease the levels of fasting blood glucose, glycated hemoglobin, and glycosylated serum proteins in diabetic mice. Compared with the model control group, the 50% fraction-treated group showed significant decreases of malondialdehyde (MDA) and increases of superoxide dismutase (SOD) in serum and liver homogenate. Moreover, the 50% fraction could significantly decrease total cholesterol (TC), alleviate abnormal lipid metabolism, and enhance antioxidant capacity in the serum.

This study demonstrated that poplar buds are effective in ameliorating the abnormalities in glycometabolism, dyslipidemia, and inflammation caused by type-2 diabetes mellitus. However, it remains unclear what specific active ingredients in the 50% fraction are responsible for the antidiabetic activity. Besides, more evidence is required to explain why a low dosage of 50% fraction was better than a high dosage in regulating insulin and glycated hemoglobin. Future research was indicated to focus on the specific active ingredients and appropriate dose of 50% fraction for regulating glucose level in type-2 diabetes mellitus.

5.5 Hepatoprotective Activity

Evidence level: Animal model studies only.

To evaluate antioxidant, anti-inflammatory, hepatoprotective, and vasorelaxant activities of Populus nigra flower buds ethanolic extract, antioxidant and anti-inflammatory activities were assessed using the ABTS test and the animal model of carrageenan-induced paw edema. Protection from hepatic toxicity caused by aluminum was examined by histopathologic analysis of liver sections. The results demonstrated complete histopathological protection against aluminum chloride-induced hepatic toxicity in animal models.

5.6 Anticancer and Antitumor Activity

Evidence level: In vitro cell-line studies only; no human clinical evidence.

Populus nigra extract was tested in vitro on lung cancer A 549, NCI-H 292, and breast cancer MCF-7 cell lines, which indicated its anticancer activity. Other studies have highlighted antitumor effects of various Populus species.

Phenomena of early apoptotic events at the maximum concentration tested (150 μg/mL) were detected by Annexin V-PI double staining in MCF-7 cells. The extract induced G0/G1 phase cell cycle arrest. In addition, P. nigra extract showed antiangiogenic potential on the chorioallantoic membrane. These are preclinical, in vitro findings and do not constitute evidence of anticancer efficacy in humans.

5.7 Vasorelaxant and Cardiovascular Effects

Evidence level: In vitro vascular tissue preparations; no human clinical trials identified.

In Algeria, traditional uses of Populus nigra include the treatment of many conditions related to endothelial dysfunction such as inflammations, arthritis, bronchitis, and respiratory tract diseases. Laboratory studies found vasorelaxant effects of P. nigra ethanolic extract on porcine coronary artery preparations, though these findings remain confined to ex vivo tissue models.

5.8 Wound Healing and Dermatological Use

Evidence level: Animal model studies; historical evidence supports topical use, which has received Commission E approval.

Treatment with Populus nigra ointment (10 and 20%) promoted wound contraction of 97.37 ± 1.19 and 97.28 ± 0.91%, respectively. The antioxidant marker enzymes, catalase and superoxide dismutase, were upgraded with respective treatments. Myeloperoxidase activity was repressed, indicating anti-inflammatory potential. Moreover, a significant increase in respective levels of hydroxyproline and hexosamine was triggered, reflecting high regeneration of collagen in the scarred tissue.

This was conducted using an excision wound model in rats. The German Commission E approval for topical wound and skin care applications is based on traditional use evidence combined with available pharmacological data rather than controlled human trials.

6. Body Systems and Health Areas Associated with Poplar

• Musculoskeletal system: Traditionally used in the treatment of rheumatism, arthritis, and lower back pains.
• Respiratory system: The buds were used traditionally as a tincture against asthma and other respiratory ailments, such as bronchitis, cough, trachea, laryngitis, sore throat, and influenza.
• Urinary tract: Traditionally used in urinary complaints. The tincture was used in some disorders of the urinary system.
• Skin and wound healing: Poplar leaf-buds (Populi gemmae) are used traditionally as anti-inflammatory agents for the treatment of skin injuries or cough.
• Digestive and hepatic systems: Traditionally used in digestive and liver disorders, debility, and anorexia.
• Cardiovascular and endothelial function: Preclinical research has investigated vasorelaxant properties.
• Metabolic regulation: Animal model data suggests effects on blood glucose and lipid metabolism, without human evidence.
• Analgesic and antipyretic applications: Black poplar has many traditional uses due to its analgesic–antipyretic, anti-inflammatory, diuretic, astringent, and cicatrizing action, and recent clinical studies have demonstrated new important pharmacological effects.

7. Dosage Forms and Reported Dosages

The appropriate dose of poplar depends on several factors such as the user's age, health, and several other conditions. At this time there is not enough scientific information to determine an appropriate range of doses for poplar.

The following dosages and concentrations appear in published experimental research:

• Topical ointments (wound healing studies): Wound healing potential was assessed for black poplar ointment containing 10 or 20% of Populus nigra ethanolic flower buds extract using the excision model in rats.
• Antidiabetic animal studies: Crude extract and a 50% fraction of poplar buds were prepared from 40 g of powdered buds via ultrasonic ethanol extraction and administered to streptozotocin-induced diabetic mice. The specific oral doses effective in those animal studies have not been translated to human equivalents in the literature reviewed.
• Antimicrobial studies (P. nigra roots): Ethyl acetate extract showed antibacterial effects against skin-associated bacteria at concentrations of 0.025% and 0.1%.
• Anticancer in vitro (P. nigra buds): Poplar bud extract showed a dose-dependent decrease of MCF-7 tumor cell viability with an IC₅₀ of 66.26 μg/mL, and early apoptotic events were detected at the maximum concentration tested (150 μg/mL).
• Alcoholic bud extracts (traditional formulations): Alcohol extracts of balm of gilead bud are marketed and sold by a number of nutritional manufacturers with instructions for internal use for up to 2 mL/day, with no known reported side effects.

No standardized human clinical dosage for poplar-specific preparations has been established in the peer-reviewed literature.

8. Safety Considerations and Drug Interactions

8.1 Salicylate Sensitivity and Aspirin Allergy

The German Commission E reports state its use has no known drug interactions and no restrictions applicable during pregnancy or lactation, with the precaution of use in individuals who are allergic to propolis or aspirin (salicylic acid).

Salicylates in these trees can cause allergic reactions in people who are sensitive to aspirin. This is an AHPA-BSH Class 1 herb indicating it is relatively safe; however, allergic reaction to aspirin occurs in approximately 1% of the population, with greater likelihood in individuals who suffer from other allergic reactions such as hives, asthma, and sinus infections. In these individuals, serious side effects can include Reye's syndrome, severe liver inflammation, brain damage, and death.

8.2 Gastrointestinal Considerations

Willow, aspen, and birch have not been demonstrated to cause peptic ulcer or gastrointestinal bleeding at usual therapeutic doses. However, they can cause nausea, which is usually easily remedied by taking them with food. These observations regarding the closely related Salicaceae family are relevant to poplar preparations given their shared salicylate chemistry.

8.3 Children and Reye's Syndrome

There is no information available about natural salicylates and Reye's syndrome, but for the sake of greatest safety, they should be avoided in children with influenza until more information is available.

8.4 Drug Interactions

The organs of poplars (bark, leaves, buds) contain anti-inflammatory salicylate-like phenolic glycosides, and for this reason, poplar's organs are also used in folk medicine to treat gout. Because of their salicylate content, poplar preparations share the theoretical interaction profile of other Salicaceae-family plants. Caution is warranted in individuals also taking anticoagulant or antiplatelet medications, though the degree of interaction risk may differ from that of pharmaceutical aspirin because salicin is metabolized differently.

Poplar should be avoided in individuals with asthma, salicylate sensitivity, or gastrointestinal ulcers.

8.5 Overall Evidence and Limitations

Several studies have depicted antioxidant, anti-inflammatory, antibacterial, antifungal, antidiabetic, antitumor, hepatoprotective, and hypouricemic properties, and effects on melanin production. All these lead to the conclusion that black poplar buds are a valuable and important source of bioactive compounds responsible for a wide range of therapeutic uses, being a promising candidate as a complementary and/or alternative source for a large number of health problems.

Despite this breadth of preclinical interest, the overwhelming majority of evidence for poplar's health effects derives from in vitro cell culture experiments and animal model studies. Rigorous, randomized, placebo-controlled human clinical trials examining the efficacy and safety of poplar-specific preparations are lacking in the peer-reviewed literature. The Commission E approval for topical use and the long history of traditional application in European and North American traditions remain the strongest practical basis for the herb's use at this time.

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