α-hydroxy acids, or
alpha hydroxy acids (
AHAs), are a class of chemical compounds that consist of a
carboxylic acidCarboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -COH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called...
substituted with a
hydroxy groupIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
on the adjacent carbon. They may be either naturally occurring or synthetic. AHAs are well-known for their use in the cosmetics industry. 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.
α-hydroxy acids, or
alpha hydroxy acids (
AHAs), are a class of chemical compounds that consist of a
carboxylic acidCarboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -COH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called...
substituted with a
hydroxy groupIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
on the adjacent carbon. They may be either naturally occurring or synthetic. AHAs are well-known for their use in the cosmetics industry. 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. They are also used as
chemical peelA chemical peel is a body treatment technique used to improve and smooth the texture of the facial skin using a chemical solution that causes the dead skin to slough off and eventually peel off. The regenerated skin is usually smoother and less wrinkled than the old skin. Thus the term chemical...
s available in a dermatologist's office, beauty and health spas and home kits, which usually contain a lower concentration. Although their effectiveness is documented numerous cosmetic products have appeared on the market with unfounded claims of performance.
Many well-known α-hydroxy acids are useful building blocks in organic synthesis: the most common and simple are
glycolic acidGlycolic acid is the smallest α-hydroxy acid . This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used in various skin-care products. Glycolic acid is found in some sugar-crops....
,
lactic acidLactic acid , also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It was first isolated in 1780 by a Swedish chemist, Carl Wilhelm Scheele, and is a carboxylic acid with a chemical formula of C
3H
6O
3...
,
citric acidCitric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things...
,
mandelic acidMandelic acid is an aromatic alpha hydroxy acid with the molecular formula C
8H
8O
3. It is a white crystalline solid that is soluble in water and most common organic solvents.-Discovery and name:...
.
Cosmetic Applications
Understanding skin structure and cutaneous aging is helpful to a discussion of the topical action of AHAs. Human
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
is comprised of two principal components, the avascular
epidermisEpidermis may refer to:* Epidermis , in plants, the outermost layer of cells covering the leaves and young parts of a plant* Epidermis , in vertebrates, the outermost layer of the skin...
and the underlying
vascularIn zoology, "vascular" means "related to blood vessels", which are part of the Circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
dermisThe dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis. Structural components of the dermis are collagen, elastic fibers, and extrafibrillar matrix ....
. Cutaneous aging, while having epidermal concomitants, seems to involve primarily the dermis and is caused by
intrinsic and extrinsic agingIntrinsic aging and Extrinsic aging are terms used to describe cutaneous aging of the skin and other parts of the integumentary system, which while having epidermal concomitants, seems to primarily involve the dermis. Intrinsic aging is influenced by internal physiological factors alone, and...
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 acidGlycolic acid is the smallest α-hydroxy acid . This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used in various skin-care products. Glycolic acid is found in some sugar-crops....
(from sugar cane),
lactic acidLactic acid , also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It was first isolated in 1780 by a Swedish chemist, Carl Wilhelm Scheele, and is a carboxylic acid with a chemical formula of C
3H
6O
3...
(from sour milk),
malic acidMalic acid is an organic compound with the formula HO
2CCH
2CHOHCO
2H. This dicarboxylic acid is the active ingredient in many sour or tart foods. Malic acid is found mostly in unripe fruits....
(from apples),
citric acidCitric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things...
(from citrus fruits) and
tartaric acidTartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste, and is used as an antioxidant. Salts of tartaric acid are known...
(from grape wine). For any topical compound, including AHA, it must penetrate into the skin where it can act on living cells.
BioavailabilityIn pharmacology, bioavailability is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered intravenously, its bioavailability is 100%...
(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 acidGlycolic acid is the smallest α-hydroxy acid . This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used in various skin-care products. Glycolic acid is found in some sugar-crops....
having the smallest molecular size is the AHA with greatest
bioavailabilityIn pharmacology, bioavailability is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered intravenously, its bioavailability is 100%...
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 corneumThe stratum corneum is the outermost layer of the epidermis, composed of large, flat, polyhedral, plate-like envelopes filled with keratin, which is made up of dead cells that have migrated up from the stratum granulosum. From the Latin for horned layer, this skin layer is composed mainly of dead...
. 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
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. Care needs to be taken to avoid irritation as this may result in worsening of
melasmaMelasma is a tan or dark skin discoloration. Although it can affect anyone, melasma is particularly common in women, especially pregnant women and those who are taking oral or patch contraceptives or hormone replacement therapy medications...
or other pigmentary problems. Newer formulations combine glycolic acid with an amino acid such as
arginineArginine is an α-amino acid. The
L-form is one of the 20 most common natural amino acids. Its codons are CGU, CGC, CGA, CGG, AGA, and AGG. In mammals, arginine is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health...
and form a time-release system that reduces the risk of irritation without affecting glycolic acid efficacy. The use of an anti-irritant like
allantoinAllantoin is a chemical compound with formula C
4H
6N
4O
3. It is also called 5-ureidohydantoin or glyoxyldiureide. It is a diureide of glyoxylic acid...
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.
In higher concentrations, between 10 and 50%, its benefits are more pronounced but are limited to temporary skin smoothing without much long lasting results. 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 acidTrichloroacetic acid is an analogue of acetic acid in which the three hydrogen atoms of the methyl group have all been replaced by chlorine atoms....
).
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. 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
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
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
photosensitivityPhotosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.-Interpretation in medicine:Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...
to the sun.