alpha-hydroxy acid

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Glycolic acid, an alpha hydroxy acid

α-hydroxy acids, or alpha hydroxy acids (AHAs), are a class of chemical compounds that consist of a carboxylic acid substituted with a hydroxy group on the adjacent carbon. They may be either naturally occurring or synthetic. AHAs are well-known for their use in the cosmetics industry^[1]. They are often found in products claiming to reduce wrinkles or the signs of aging, and improve the overall look and feel of the skin^[1]. They are also used as chemical peels available in a dermatologist's office, beauty and health spas and home kits, which usually contain a lower concentration^[specify]. Although their effectiveness is documented^[2] numerous cosmetic products have appeared on the market with unfounded claims of performance^[3]. Many well-known α-hydroxy acids are useful building blocks in organic synthesis: the most common and simple are glycolic acid, lactic acid, citric acid, mandelic acid.

Cosmetic Applications

Understanding skin structure and cutaneous aging is helpful to a discussion of the topical action of AHAs. Human skin has two principal components, the avascular epidermis and the underlying vascular dermis. Cutaneous aging, while having epidermal concomitants, seems to involve primarily the dermis and is caused by intrinsic and extrinsic aging factors.

Alpha-hydroxy acids are a group of organic carboxylic compounds. AHAs most commonly used in cosmetic applications are typically derived from food products including glycolic acid (from sugar cane), lactic acid (from sour milk), malic acid (from apples), citric acid (from citrus fruits) and tartaric acid (from grape wine). For any topical compound, including AHA, it must penetrate into the skin where it can act on living cells. Bioavailability (influenced primarily by small molecular size) is one characteristic that is important in determining compound's ability to penetrate the top layer of the skin. Glycolic acid having the smallest molecular size is the AHA with greatest bioavailability and penetrates the skin most easily; this largely accounts for the popularity of this product in cosmetic applications.

Epidermal effect: AHA's have a profound effect on keratinization; which is clinically detectable by the formation of a new stratum corneum. It appears that AHAs modulate this formation through diminished cellular cohesion between corneocytes at the lowest levels of the stratum corneum.

Dermal effects: AHAs with greater bioavailability appear to have deeper dermal effects. Glycolic acid, lactic acid and citric acid, on topical application to photodamaged skin, have been shown to produce increased amounts of mucopolysaccharides and collagen and increased skin thickness without detectable inflammation, as monitored by skin biopsies^[4]

Alpha Hydroxy Acids at different concentrations

In low concentrations, 5 - 10% as is found in many over the counter products, glycolic acid reduces cell adhesion in the top layer of the skin. This action promotes exfoliation of the outermost layer of the skin accounting for smoother texture following regular use of topical GA. This relatively low concentration of glycolic acid lends itself to daily use as a monotherapy or a part of a broader skin care management for such conditions as acne, photo-damage, wrinkling as well as melasma.^[5][6] Care needs to be taken to avoid irritation as this may result in worsening of melasma or other pigmentary problems. Newer formulations combine glycolic acid with an amino acid such as arginine and form a time-release system that reduces the risk of irritation without affecting glycolic acid efficacy^{[citation needed]}. The use of an anti-irritant like allantoin is also helpful. Because of its safety, glycolic acid at the concentrations below 10% can be used daily by most people except those with very sensitive skin^{[citation needed]}.

In higher concentrations, between 10 and 50%, its benefits are more pronounced but are limited to temporary skin smoothing without much long lasting results^{[citation needed]}. This is still a useful concentration to use as it can prepare the skin for more efficacious glycolic acid concentrations (50 - 70%) as well as prime the skin for deeper chemical peels such as TCA peel (trichloroacetic acid).

At higher concentrations, 50 - 70% applied for 3 to 8 minutes under the supervision of a physician, glycolic acid promotes slitting between the cells and can be used to treat acne or photo-damage (such as mottled dyspigmentation, melasma or fine wrinkles). The benefits from such short contact application (chemical peels) depend on the pH of the solution (the more acidic the product, or lower pH, the more pronounced the results), the concentration of GA (higher concentrations produce more vigorous response), the length of application and prior skin conditioning such as prior use of topical vitamin A products. Although single application of 50 - 70% GA will produce beneficial results, multiple treatments every 2 to 4 weeks are required for optimal results^{[citation needed]}. It is important to understand that glycolic acid peels are chemical peels with similar risks and side effects as other peels. Some of the side effects of AHAs chemical peeling can include hyper-pigmentation, persistent redness, scarring as well as flare up of facial herpes infections ("cold sores").

Safety

AHAs are generally safe when used on the skin as a cosmetic agent using the recommended dosage. The most common side-effects are mild skin irritations, redness and flaking. The severity usually depends on the pH and the concentration of the acid used. Chemical peels tend to have more severe side-effects including blistering, burning and skin discoloration, although they are usually mild and go away a day or two after treatment.

The FDA has also warned consumers that care should be taken when using AHAs after an industry-sponsored study found that they can increase photosensitivity to the sun.^[7]

See also

• Beta hydroxy acid
• Omega hydroxy acid
• Salicylic acid, a β-hydroxy acid

References

External links

• http://www.foodsafety.gov/~dms/cos-aha.html