Hydroxy Acids are ubiquitous in aesthetic medicine. Alpha- and beta-hydroxy acids (AHAs/BHAs) are the most widely used ingredients for chemical peels; they have also become popular topical agents for antiaging, hyperpigmentation and acne. In recent years, they have been joined by polyhydroxy acids (PHAs) as a less irritating alternative for patients with sensitive skin. “I recommend AHAs for everybody—antiaging, acne, rosacea patients. It’s really one of the best things you can do for the skin,” says Monica L. Halem, MD, FAAD, a dermatologist in private practice in New York City.
Here, we look at the science behind hydroxy acids in topical skin care.
AHAs and the Skin
The main purpose of AHAs is desquamation, or the removal of the outer layer of the skin. “There are five primary AHAs: glycolic, lactic, malic, maleic and mandelic,” says Robert Manzo, chemist and founder of Skinprint skin care. “Anything that is not glycolic is really window dressing. The other ones are nice, but they’re all variants of glycolic acid, and from a pure chemistry and pure functional standpoint, glycolic acid provides the most effective desquamation.”
AHAs are carboxylic acids with one hydroxyl group attached to the a-position of the carboxyl group. “The corneocytes on the surface of the stratum corneum are attached to the skin and each other by desmosomes. Glycolic acid specifically targets the chemistry of a desmosome bond to release those surface cells,” says Manzo. “Of the AHAs, it is the smallest molecule and offers the best removal.”
In addition to its exfoliating properties, glycolic acid has been shown to accelerate collagen synthesis. While the exact mechanism of action is unknown, a February 2003 study in Experimental Dermatology (supplement) outlined the effects of the acid on in vitro and ex vivo human skin biopsies. Yuri Okano, et al, found that glycolic acid directly accelerates fibroblast collagen synthesis and regulates extracellular matrix degradation, both of which contribute to the recovery of photodamaged skin.
A study on the use of glycolic acid for acne scarring (International Journal of Dermatology, October 2000), highlighted its ability to lighten areas of hyperpigmentation. The authors posited that the acid both disperses pigment in lesions as a result of desquamation and reduces new melanin formation in the melanocytes by inhibiting tyrosinase. For the study, Zulal Erbagci, et al, compared the outcomes of two groups of patients with atrophic acne scars—one group was treated with bi-weekly serial glycolic acid peels and the other with long-term daily use of topical low-strength glycolic acid. They concluded that, “Long-term daily use of glycolic acid is effective on scars and may be recommended for patients who cannot tolerate the peeling process.”
“I think glycolic acid should be a staple in skin care,” says Dr. Halem. “It gently removes dead skin cells revealing fresh skin, decreases pore size, hydrates, stimulates collagen and elastin, eliminates fine lines and wrinkles, and helps treat acne.”
“AHAs have renewing effects across multiple layers of skin,” says Barbara Green, head of R&D and clinical research at NeoStrata. “There’s a loosening of the bonds between surface cells, encouraging exfoliation at the very top layers. In the epidermis there is enhanced cellular turnover and a more even distribution of pigmentation for improved skin tone and clarity. Studies have shown that AHAs can also increase collagen and hyaluronic acid.”
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