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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Lactic acid shares glycolic acid’s exfoliating abilities and also affects the lipid structures of the skin, offering additional moisturizing benefits. “Lactic acid penetrates the skin and changes the structure of the lipids in the stratum corneum,” says Manzo. “It does that by aligning them differently, which traps moisture in. So lactic acid is a very effective tool for skin hydration, whereas glycolic is a more effective treatment active for lightening and desquamation.”
While AHAs are among the most proven actives in skin care, it is important to remember that they will not get rid of deep wrinkles. “They are good for superficial wrinkles where you’re removing a few layers of skin, because this reveals the younger skin underneath and addresses tone and texture,” says Manzo. “From that perspective AHAs, specifically glycolic acid, work very well. But they will not get rid of deep wrinkles.”
For this reason, it’s necessary to manage patients’ expectations about what a topical can do for them, says Laurel Morton, MD, of SkinCare Physicians in Chestnut Hill, Massachusetts. “They provide significant results, but not dramatic results,” she says.
Dr. Morton also points out that although AHAs are tolerable across all skin types, there are still some considerations for patients on both ends of the spectrum. “People with darker skin types are going to be more prone to discoloration from any inflammation, so proceed cautiously,” she says. “Patients who have fair skin may not have long-term issues, but they must be very careful about sun protection; they’ll be a little more prone to irritation when using acids in the presence of sunlight.”
At NeoStrata, formulators often choose to work with glycolic acid for antiaging (with citric acid added for antioxidant benefits). For oily and acneic skin they use mandelic acid, “which is a more oil-soluble form of glycolic acid, e.g., phenyl glycolic acid,” says Green. “All AHAs have proven to be tolerable across the full Fitzpatrick skin type range, but for sensitive skin types and postprocedure care, we use Polyhydroxy Acids (PHAs), which are still in the AHA family but have more water-attracting hydroxyl groups on the molecule; they’re inherently more moisturizing and conditioning, and less stinging than the traditional AHAs.”
One of the most widely used PHAs in skin care is gluconolactone. In “Applications of hydroxy acids: classification, mechanisms and photactivity” (Clinics of Cosmetic Investigative Dermatology, November 2010), Andrija Kornhauser, et al, noted that PHAs offer antiaging and skin-smoothing benefits comparable to AHAs, as well as some additional advantages: They function as humectants, moisturizers and antioxidants.
Because PHAs do not cause the irritation seen with glycolic acid, they have become popular actives in postprocedure skincare formulations. “PHAs have been found to be compatible with clinically sensitive skin, including rosacea and atopic dermatitis, and can be used after cosmetic procedures,” wrote Pearl E. Grimes, MD, et al, in “The Use of Polyhydroxy acids in photoaged skin” (Cutis, February 2004), adding that “PHA-containing products were shown to be compatible with African American, Caucasian and Hispanic/Asian skin and provided significant improvements in photoaging in these populations.”
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Beta Hydroxy Acids
Like AHAs, BHAs are carboxylic acids with one hydroxyl group, but the hydroxyl group is attached to the ß-position of the carboxyl group. True BHAs, such as benzoic and butyric acids, are not commonly used in skin care. “Benzoic acid is an active in some prescriptions, like Bensal HP and Whitfields Ointment. It works with salicylic acid to deliver desquamation in addition to controlling infection,” says Manzo. “We do some formulations for plastic surgeons and dermatologists if they want that specific blend, but it’s pretty rare.”
Some acids, including malic acid and citric acid, are considered BHAs because they have one carboxyl group in the a-postion and one in the ß-position. Similarly, salicylic acid is not a true BHA—its hydroxyl and carboxyl groups are directly attached to a benzene ring—but it is often included in this group.
Salicylic acid is indicated primarily for acne because it is a keratolytic (peeling) agent, but it has other benefits as well. “It has some antiaging properties and decreases pigmentation too,” says Dr. Halem. “Especially in patients who have acne and a lot of the acne stains, it helps treat both the acne and the resulting hyperpigmentation.”
“There are two goals of salicylic acid on skin: down regulate oil through sebaceous interference, and desquamation,” adds Manzo. “And at 1% you get a nice anti-inflammatory property out of it.”
The secret to formulating an effective hydroxy acid topical lies in buffering, pKA and pH levels. “In order for AHAs to absorb, they need to be in their acid form—not their neutralized form. The chemist must formulate around the acid’s pKa, which is the relative strength of that specific acid,” says Green. “For example, glycolic acid has a pKa of 3.8, so if a product is formulated at a pH of 3.8, approximately half of the glycolic acid would be available as free acid and half would be present as neutralized salt, which would not penetrate as readily. You maximize effectiveness by balancing the pH of the formulation, bearing in mind the pKa of the acid. Recognizing that pH is a logarithmic scale, as one increases pH there is a fast decline in the amount of the free acid form; there is a sensitive formulating balance required to achieve effectiveness and tolerability.”
In addition, the acid can lose potency from the time it leaves the bottle to when it’s applied to the skin. Adding water to a glycolic cleanser to form a lather, for example, will dilute the acid and decrease its effectiveness. “A big problem with cleansers that claim to have glycolic acid is that they self-buffer before getting to the face and therefore don’t work as well,” says Manzo. “So look for buffered glycolic acids in cleanser form that have a pH between 3 and 3.5; that will work much better than a cleanser that starts at 4 and is then diluted for a resulting pH of 6.”
He notes that AHAs do work nicely in lotion and especially serum forms. “AHA is a very robust chemical. The ability to combine lactic and glycolic acids, buffer it well and formulate them to get both hydration and cellular removal properties is tricky, but we can do it,” says Manzo. “At Skinprint, we’re adding time-released retinoids to AHA formulations as well, so the retinoid is released over a six to eight hour period plus you get all the AHA properties. These combinations are very potent.”
Salicylic acid differs from AHAs in that it is oil-soluble. “If you have an oil-soluble compound and you’re putting it into an aqueous base, you have to formulate it such that it precipitates onto the skin in the closest way it can so it gets into the sebaceous glands,” says Manzo.
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Over-the-counter salicylic acid-based products typically contain 1% to 2% of the acid. “The 2% is generally for people with acne and it suppresses the sebaceous glands,” says Manzo. “The 1% allows the skin to turn over and regulate, and acts as an anti-inflammatory. Anything over 2% is prescription. We make up to 15% for sebaceous hyperplasia and issues where patients have rhinophyma on the nose—at high levels it works extremely well to suppress those concerns, particularly in the nose area.”
AHAs can be easily incorporated into homecare routines, as they are available in various formulations. “I recommend AHAs in the form of cleansers, serums or moisturizers—it depends on the patient’s preference, so I offer all of them,” says Dr. Halem. “A cleanser is still effective even though it is washed off, but I like to add a serum that’s going to stay on all day. As long as you’re incorporating it somehow, that’s where benefits come in.”
Dr. Morton suggests combining AHAs with other actives for optimal results. Patients can use AHAs with a retinol for antiaging, or with bleaching agents for pigmentation concerns, for example. “For antiaging, I would recommend an antioxidant, like vitamin C or resveratrol, and a glycolic acid either in lotion form or serum form, and then I think retinols are important too,” says Dr. Morton. “Patients can alternate the glycolic and the retinol every other night so as not to cause too much irritation. By combining things, you get an overall better result.”
Dr. Halem notes that patients, especially those with sensitive skin, should be cautious when starting an AHA product. “Any time patients start glycolic acid, they should only use it a couple times a week and increase as they go to make sure they don’t get too dry or irritated. Someone who is super sensitive might want to use it less frequently,” she says. “And I personally don’t recommend it for the undereye area; the skin is a little too delicate for that.”
Patients using glycolic acid-based topicals must be diligent about using sun protection—especially if they are also applying a retinol—because it can make skin more sensitive to sun exposure.
Manzo reminds practitioners that they must be aware of factors affecting the bioavailability of active AHAs and BHAs, particularly when it comes to cream cleansers. “Creamy cleansers deliberately provide an occlusion by precipitating waxes on the skin to prevent the barrier function from releasing water. If the next step in the regimen is to apply an active, the waxy substance isn’t going to let it penetrate,” he says.
Laura Beliz is the associate editor of MedEsthetics.
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