Green Tea

Green tea extract derived from the leaves of Camellia sinensis both protects the skin from UV-induced damage and improves photoaged skin. It is also the first botanical treatment for genital warts.

Effects


Grade Level of Evidence
A Multiple double-blind, controlled clinical trials.
B 1 double-blind, controlled clinical trial.
C At least 1 controlled or comparative clinical trial.
D Uncontrolled, observational, animal or in-vitro studies only.
Grade Effect Size of Effect Comments

A

Warts treatment

Strong

Clears genital warts in 50-78% of patients after 12-16 weeks of treatment, with a low rate of recurrence.

B

Rosacea treatment

Mild

Reduces facial redness in a topical formulation with resveratrol and caffeine, and may also prevent telangiectasias.

C

Smoother skin

Strong

Improved skin smoothness by 28% alone, and 50% in combination with lotus extract, after 60 days of treatment.

C

Photoprotection

Strong

Pretreatment of human skin with green tea polyphenols before UV exposure reduces DNA damage, oxidative stress and protects against immunosuppression and photoaging.

C

Less oiliness

Strong

Controls facial sebum levels by inhibiting 5α-reductase, an enzyme implicated in sebum production.

C

Wrinkle treatment

Moderate

Decreases wrinkling, especially for tannase-converted green tea extract. Also seems to accelerate rejuvenation with phototherapy.

C

Increased skin hydration

Moderate

Improved skin moisture content and reduced transepidermal water loss.

D

Antioxidant

Strong

Inhibits a multitude of UV-induced effects, including DNA damage, Langerhans cell damage and depletion, reactive oxygen species generation and lipid peroxidation.

D

Acne treatment

Moderate

Reduces inflammation, inhibits sebum production and decreases the viability of P. acnes.

D

Increased skin elasticity

Mild

Had definite effects on the viscoelastic properties of the skin, and increased elastic tissue content.

D

Enhanced barrier function

Mild

Reduced transepidermal water loss after twice daily application of an emulgel containing 20% green tea extract over 5 days.

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Scientific Research


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Table of contents:

1. Sources

Green tea extract is made by lightly steaming and drying the leaves of the plant Camellia sinensis.[1]

The main active ingredients in green tea are the polyphenols, which comprise 30-35% of the dry weight of green tea leaves.[2] These polyphenols are also known as catechins, and the 4 most important are epicatechin (EC), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epigallocatechin-3-gallate (EGCG).[3][4] Other green tea phenolic compounds include gallic acid (GA), gallocatechin (GC), catechin (C), p-coumaroylquinic acid (CA), and gallocatechin-3-gallate (GCG). Green tea also contains condensed and hydrolyzable tannins.[5]

While the ratio of each catechin is influenced by many intrinsic and extrinsic factors and even the technological processes used during production and extraction,[6][7][8][9] it is generally acknowledged that EGCG is the most abundant and most potent green tea catechin.[10][11]

2. Bioavailability

2.1 Stability

Green tea extract is among the more difficult botanicals to formulate as active ingredients in topical products.[12] Green tea catechins, like most antioxidants, are highly unstable and easily oxidized in an ambient environment.[13]

The stability of EGCG, the most plentiful and most powerful antioxidant extracted from green tea, is dependent on the pH, temperature, ionic strength and the solvent used. Degradation increases rapidly with increases in temperature, pH and ionic strength, while glycerin-based vehicles prolong the stability of EGCG in solution.[14]

EGCG is also highly photolabile -- model creams containing 1% EGCG decomposed by 69% after only 1 hour of exposure to solar-simulated sunlight.[15] The reduction in the antioxidant power is much lower than the extent of degradation however, suggesting the formation of products with antioxidant properties.[16]

The addition of some co-antioxidants such as α-lipoic acid and vitamin C exerts a stabilizing effect, decreasing the light-induced decomposition of EGCG, but other antioxidants such as butylated hydroxytoluene and vitamin E have no effect or even enhance photolysis.[16] The inclusion of UVB filters, particularly benzophenone-4 and to a lesser extent ethylhexyl methoxycinnamate, has also been shown to improve the light stability of EGCG in topical formulations.[15]

2.2 Permeation

Green tea catechins are by nature hydrophilic, limiting their passive diffusion through the hydrophobic stratum corneum.[17] This is especially true of EGCG and ECG, which have larger molecular sizes and pass through the skin less easily than EGC and EC, the smaller catechins.[18][19][20]

The structure and cutaneous metabolism of the various catechins also affects their permeation profiles.[19] EGC and EC are distributed in all layers of the skin, while EGCG and ECG, the galloyl catechins are usually localized to the skin surface and stratum corneum.[17] This is because the gallate group, despite being lipophilic, retards the permeation of EGCG and ECG through steric hindrance.[17][21] In addition, EGCG and ECG are also hydrolysed by esterases in the stratum corneum to EGC and EC.[18][19]

The extent of penetration of green tea catechins also relies on the vehicle used for topical delivery. A hydrophilic ointment containing 10% EGCG resulted in substantial intradermal uptake of up to 20% of the applied dose.[22] Terpenes, especially α-terpineol, have also been successfully used to enhance the skin uptake of green tea catechins by perturbing the stratum corneum.[23] Drug-in-adhesive patches also appear to improve the skin permeation of the catechins.[18] More recently, chitosan microparticles loaded with green tea extract have been shown to be promising carriers for aiding permeation, as they protected the catechins from enzymatic degradation.[19][24] Oil/water emulsions of catechin derivatives are also useful formulations.[25][20]

Gelling agents do not seem to significantly affect the permeability of EC however.[26] Likewise, EGCG encapsulated in liposomes did not seem to reach the deeper skin layers. It has been suggested that the galloyl catechins, particularly EGCG, have high affinity for and hence bind to the lipid bilayer of the liposomes, hindering their release from the carrier.[27]

Although green tea catechins are capable of penetrating through mouse and pig skin,[18][22][28] this does not happen with human skin,[22][26][29] which is ideal as it means that the catechins are retained at their sites of action within the skin.

2.3 Dosage

There is little standardization regarding the minimal concentration of green teas in cosmeceuticals.[13] Clinical trials have used concentrations ranging from 2-20%, with anti-aging formulations typically having lower concentrations, and preparations used in the treatment of warts having higher concentrations.

It has been suggested that 5% green tea extract is an effective concentration,[12] but this does not take into consideration the varying amount of green tea catechins in the extract.

Moreover, 10% green tea extract was also shown to provide almost complete UV protection in one study,[30] but another study noted that 10% green tea cream may cause too much irritation and sun sensitivity to be used commercially.[31]

3. Effects on the skin

Studies evaluating the efficacy of topical green tea extract in rejuvenating the skin have had mixed results.

One of the earliest trials, a 2-year, double-blind, placebo-controlled study of 56 women, sought to establish the anti-aging effects, if any, of oral supplementation with green tea capsules containing 175 mg of catechins. Clinical, quantitative and histologic analysis showed that such long-term supplementation with green tea polyphenols did not lead to superior improvements in photoaging parameters, compared to supplementation with placebo.[32]

Similarly, a combination regimen of 10% green tea extract cream and 300 mg twice-daily oral supplementation did not lead to significant changes in the clinical grading of photoaged skin compared to the placebo regimen after 8 weeks, although skin biopsies revealed histologic improvement in the elastic tissue content. This suggests that longer periods of treatment are required to achieve visible improvements to the skin.[31]

A cream containing 3% green tea extract by healthy male volunteers also did not produce significant improvements in skin elasticity compared to the base cream after 8 weeks of daily application to the cheek.[33]

However, a more recent vehicle-controlled study found that cosmetic formulations containing 6% green tea extract significantly increased skin moisture and significantly improved skin microrelief, due to a reduction in skin roughness.[34] An emulgel containing 20% green tea extract and 5% rose oil also significantly increased the water content of the skin and decreased transepidermal water loss, suggestive of an improved barrier function.[35]

5% green tea extract incorporated in a cosmetic emulsion also reduced skin roughness by 23%, scaliness by 14%, wrinkling by 12% and increased skin smoothness by 28% after 60 days of treatment. These improvements were significant when compared to baseline and are numerically higher than the effects of the placebo, but unfortunately no data were presented on a comparison between groups. It was also interesting that a combined green tea and lotus extract brought about even greater improvements over baseline in skin roughness (26% reduction), scaliness (21% reduction), wrinkling (23% decrease) and smoothness (50% increase), indicating a synergistic anti-aging effect.[36]

Curiously, tannase-converted green tea extract exhibited significantly higher free radical scavenging abilities compared to normal green tea extract. This has been advanced as an explanation for the greater anti-wrinkle effects of tannase-converted green tea extract, as 64% of subjects who applied tannase-converted green tea extract to their crow's feet reported marked or moderate improvement in wrinkles, while only 36% of subjects who applied normal green tea extract did so, in a comparative study.[37]

Also of note is a case study in which topical application of green tea was introduced into a facial rejuvenation program using red light. The motivation was that phototherapy generates reactive oxygen species,[38] control over the levels of which is crucial to prevent oxidative damage to cell components. Apart from the antioxidant effects of green tea constituents,[39] EGCG can also increase the cell survival rate of keratinocytes,[40] a surprising parallel to the effects of red light. This may explain why topical green tea appears to accelerate the rejuvenative effects of red light, realizing in 1 month the rejuvenated skin, reduced wrinkle levels and juvenile complexion that previously took 10 months of light treatment alone to achieve.[41] Larger, double-blind and controlled clinical trials are required to confirm these results, however.

3.2 Photoprotection

Topical green tea polyphenols has been shown to inhibit both UVB-induced pyrimidine dimer formation and erythema response in human skin. The protection provided was dose-dependent, and prevented damage even to the DNA of the deeper dermal cells.[42] This was not due to the green tea polyphenols acting as a sunscreen, as they did not absorb wavelengths within the UVB range in a spectrophotometric analysis.[30][43]

3% green tea extracts also provided human skin with partial protection against UV radiation-induced depletion of Langerhans cells, which is known to contribute to the pathogenesis of photocarcinogenesis.[44] A histologic examination supported this finding, disclosing that pretreatment with green tea polyphenols prior to UV exposure not only protected epidermal Langerhans cells from UV damage but also helped reconstitute the epidermal population of Langerhans cells.[30]

The application of EGCG before UV exposure also had multiple beneficial antioxidant effects, markedly decreasing the production of hydrogen peroxide and nitric oxide in the epidermis and dermis, inhibiting the infiltration of inflammatory leukocytes (the major producers of reactive oxygen species) into the skin, inhibiting epidermal lipid peroxidation and upregulating catalase activity.[45]

3.3 Acne treatment

Topical 2% green tea lotion has been shown to be effective in treating mild to moderate acne vulgaris, reducing lesion counts by 58% and decreasing acne severity by 39% after 6 weeks.[46]

Many of the constituents in green tea have anti-acne effects. ECG and EGCG, for instance, are selective inhibitors of 5α-reductase,[47] an enzyme implicated in sebum production.[48][49] True enough, 3% green tea extract reduced sebum production by 60% after 8 weeks.[50] Green tea extract also synergizes with lotus extract in controlling facial sebum levels.[51]

Furthermore, polyphenon-60, a green tea catechin compound, suppresses inflammation in acne by inhibiting TLR-2 expression and IL-8 secretion.[52] EGCG likewise has anti-inflammatory effects, decreasing the expression of pro-inflammatory cytokines from sebocytes in addition to inducing cytotoxicity in sebocytes via apoptosis.[53][54]

It is also important to note that EGCG directly decreases the viability of P. acnes, the bacterium that plays a major role in the induction and maintenance of the inflammatory phase of acne.[55] This may explain at least in part the findings of another study, which discovered that a herbal anti-acne moisturizer containing many plant extracts, including green tea, possessed the potential for inhibiting acne.[56]

Lipids present in green tea, such as linoleic acid and α-linoleic acid,[57] are also known to be effective in reducing the size of microcomedones in microcomedonal acne.[58][59]

3.4 Rosacea treatment

The results of 3 studies suggest that topical green tea extract may find utility in improving rosacea, which often presents with chronic facial erythema and telangiectasias.[60]

In the first study, a small, randomized, double-blind, controlled, split-face trial, a cream containing 2.5% EGCG inhibited two angiogenic growth and transcription factors, indicating that it may serve as a potential agent in preventing telangiectasias.[60] The second study found that a topical formulation with resveratrol, green tea polyphenols and caffeine successfully reduced facial redness after 6 weeks of twice-daily treatment.[61]

A double-blind, placebo-controlled trial on 60 women also demonstrated that 2% green tea extract in a hydrophilic cream was effective in treating papulopustular rosacea, as evidenced by a significantly higher reduction in mean inflammatory lesion count than the corresponding placebo cream, as well as the achievement of therapeutic success in 70% of patients treated with the green tea cream.[62]

3.5 Warts treatment

Numerous large, double-blind and placebo-controlled studies have verified the effectiveness of topical green tea extract in treating warts.

Sinecatechins is a defined green tea extract containing 55% EGCG that is also known by its trade names Veregen or Polyphenon E. It has been shown to successfully clear genital and anogenital warts in both men and women when incorporated in 10% or 15% ointment.[63][64][65][66] 10% ointment is also effective in clearing warts in immunocompetent patients.[67]

The clearance rate of wart lesions after treatment with sinecatechins is similar to that of other topically applied drugs like imiquimod and podophyllotoxin, but recurrences are seen less frequently after treatment with sinecatechins.[68] The treatment course requires dosing thrice daily for 12-16 weeks, and is not considered more effective than other existing treatments.[69]

The molecular mode of action underlying the clinical efficacy of sinecatechins has yet to be determined, but it has been postulated that the green tea extract may activate cellular immune reactions, induce cell cycle arrest and apoptosis, and inhibit HPV gene expression, resulting in the regression of the warts.[68]

4. Side Effects

4.1 Negligible toxicity

Safety studies on EGCG pareparations have established that oral administration of up to 2000 mg/kg to mice did not result in genotoxicity.[70] 500 mg/kg of EGCG was also not toxic to rats,[71] but led to morbidity in fasted dogs when administered as a single bolus dose.[72] However, this was not considered a realistic comparison to the human condition.[71] Teratogenicity and reproductive toxicity studies in rats also found no evidence of direct embryo-fetal toxicity, nor any adverse effects on reproduction and fertility.[73]

Furthermore, the fact that green tea catechins permeate into but not through human skin, supports its safety as a topical agent.[22][26][29]

4.2 Local skin reactions

Topical EGCG preparations cause minor dermal irritation in rats and guinea pigs, but not rabbits. Topical EGCG was also found to be a moderate dermal sensitizing agent in guinea pigs.[71]

The human studies where topical green tea extract was used for cosmetic purposes have reported that it is well-tolerated, with few adverse events.[31][34][36]

Ointments containing 15% sinocatechins, the green tea extract used in ointments for treating genital warts, however, causes moderate or severe reactions in most subjects with genital and/or perianal warts. These included phimosis, inguinal lymphadenitis, urethral meatal stenosis, dysuria, genital herpes, vulvitis, skin ulceration, erosions in the urethral meatus, and superinfection of warts and ulcers.[69][74]

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