Hydroxy acids have been used in skin care for decades, both in clinical practice and in cosmetic skin care applications. Their continued use points to their effectiveness as skin care ingredients. The most common hydroxy acids are alpha hydroxy acids (AHAs) and beta hydroxy acids (BHAs); however, recent research has led to the discovery of a newer class of hydroxy acids called polyhydroxy acids (PHAs).
Alpha Hydroxy Acids (AHAs)
The most common AHAs are glycolic acid and lactic acid. Malic acid and citric acid are also often considered to be AHAs, though their chemical composition allows them to be both AHAs and BHAs. Alpha hydroxy acids are found naturally in many foods including sugar cane (glycolic acid), fermented dairy products (lactic acid), citrus fruits (citric acid) and apples (malic acid). Alpha hydroxy acids have multiple anti-aging effects on the skin; however, they are probably most recognized for their exfoliating properties.
Exfoliating the skin helps to reduce the appearance of fine lines and wrinkles and smoothes the skin, helping it to maintain a more youthful appearance. Alpha hydroxy acids are a chemical form of exfoliation. They appear to work by reducing cellular calcium  and disrupting the ability of cells to stick together, allowing them to peel off more easily. When used at low concentrations, AHAs are generally mild enough that they can promote exfoliation without disrupting the skin’s barrier function .
In addition to exfoliation, AHAs have several other effects that help fight the appearance of aging. For instance, it has been reported that glycolic acid increases skin cell growth , enhances collagen synthesis [4, 5], promotes hyaluronic acid production , and reduces signs of damage caused by overexposure to the sun . Similarly, lactic acid has been shown to promote collagen synthesis  and reduce the signs of photodamage  while citric acid has been shown to enhance skin thickness . Additionally, combinations of lactic acid and glycolic acid can help support the skin’s natural barrier function .
AHAs, either individually or in combinations have multiple skin benefits that can help maintain a more youthful appearance. However, at high concentrations, AHAs can also make the skin more sensitive to sunlight, making it more susceptible to damage from the sun [10, 11]. Because of this, the U.S. FDA has indicated that AHAs are safe in cosmetic products when used at levels of 10% or less and in products at a pH of 3.5 or higher; however, they also recommend the use of daily sun protection to further minimize the risk of sun sensitivity when using products containing AHAs.
Beta Hydroxy Acids (BHAs)
Salicylic acid is generally considered a BHA, though chemically it does not meet the definition of one. Salicylic acid is clearly the most common BHA used in cosmetic and skin care products. This includes products to reduce the visible signs of aging, for exfoliation, and to treat acne, calluses and other skin conditions.
Salicylic acid is particularly useful for the treatment of acne because of its ability to penetrate oils and its anti-bacterial properties. These properties allow salicylic acid to penetrate into the pores where it can help exfoliate and cleanse the pores as well as suppress bacterial growth. Because of these actions, salicylic acid is approved for use in over-the-counter acne treatments at doses up to 2%.
In addition to its benefits for acne sufferers, salicylic acid has multiple benefits for maintaining a younger-looking appearance. One recent study reported that sodium salicylate, a sodium salt of salicylic acid, enhanced synthesis of structural proteins in the skin, reduced wrinkle depth, and reduced skin roughness . Another derivative of salicylic acid, capryloyl salicylic acid, was shown to reduce the appearance of fine lines and wrinkles and reduce the appearance of hyperpigmentation at doses of 5 – 10% . Furthermore, a 2006 study reported that topically applied salicylic acid suppressed the formation of sunburn cells . It is clear from these studies, that salicylic acid has skin health benefits beyond its well known use for acne treatment.
Polyhydroxy Acids (PHAs)
Polyhydroxy acids are considered the new generation of AHAs. Polyhydroxy acids, like gluconolactone and lactobionic acid, have been shown to have activities similar to AHAs, but with less irritation to the skin. The PHA most commonly used in skin care products is gluconolactone. In addition to its similarities to AHAs, gluconolactone has been shown to have antioxidant and moisturization benefits for the skin . In one study, gluconolactone was shown to reduce the appearance of wrinkles in the crow’s-feet area of the eye and enhance skin elasticity . A separate study reported that gluconolactone protects against UV radiation and does not cause an increase in sunburn cells after exposure to UV radiation .
Overall, it is clear that hydroxy acids have earned their place in skin care. These ingredients have multiple benefits that can help alleviate certain conditions like acne as well as fight the visible signs of aging. Research continues to show the benefits of AHAs and BHAs for skin health, while also leading to the discovery of new hydroxy acids like the PHAs, which offer similar benefits with fewer risks of irritation.
Do you use hydroxy acids in your daily skin care regimen?