Hydroxyacids are a class of organic acids widely used in cosmetic dermatology for their ability to exfoliate the stratum corneum, the outermost layer of the skin, and modulate the process of keratinisation, the formation of keratinocytes. These actions are primarily attributed to their low pH, which disrupts the ionic bonds between corneocytes (the dead skin cells in the stratum corneum), leading to a loosening and shedding of the superficial layers of the skin.
There are three main categories of hydroxyacids commonly used in skincare: alpha-hydroxy acids (AHAs), beta-hydroxy acids (BHAs), and polyhydroxy acids (PHAs).
Alpha-hydroxy acids (AHAs) are water-soluble acids derived from various natural sources, such as fruits and milk. Glycolic acid (from sugar cane) and lactic acid (from milk) are among the most commonly used AHAs in cosmetic formulations. Other AHAs include malic acid, citric acid, and tartaric acid. AHAs work primarily by reducing corneocyte cohesion in the lower layers of the stratum corneum, promoting a smoother, brighter complexion. They can also increase skin hydration and improve the appearance of fine lines and wrinkles over time by stimulating collagen production. The effectiveness of AHAs is generally concentration-dependent and pH-dependent, with lower pH values and higher concentrations typically yielding more pronounced exfoliation . However, higher concentrations can also increase the risk of irritation and sun sensitivity .
Beta-hydroxy acids (BHAs), with salicylic acid being the most prominent example, are oil-soluble acids. This oil solubility allows BHAs to penetrate into the pores of the skin, making them particularly effective in treating acne and blackheads by exfoliating within the hair follicle and helping to unclog pores. Salicylic acid also has anti-inflammatory properties, which can be beneficial for acne-prone and sensitive skin . While BHAs also exfoliate the surface of the skin, their primary advantage lies in their ability to address oil-related skin issues .
Polyhydroxy acids (PHAs) are structurally similar to AHAs but have larger molecular sizes and often multiple hydroxyl groups. Examples include gluconolactone and lactobionic acid . Due to their larger molecular size, PHAs generally do not penetrate the skin as deeply as AHAs, resulting in gentler exfoliation and reduced potential for irritation . PHAs also possess humectant (moisture-attracting) and antioxidant properties, making them suitable for sensitive skin types . Bionic acids, a type of PHA, such as lactobionic acid, have been shown to have antioxidant effects and may also inhibit glycation, a process that can contribute to skin ageing .
The modulation of keratinisation by hydroxyacids involves their influence on the turnover and differentiation of keratinocytes. By promoting the shedding of the outer layers of the stratum corneum, hydroxyacids encourage the production of new, healthier skin cells. This can lead to improvements in skin texture, tone, and overall radiance . In conditions like dry skin, hydroxyacids can help to normalise the desquamation process, preventing the build-up of dry, flaky skin cells .
Hydroxyacids also play a role in addressing other skin concerns beyond exfoliation and keratinisation. AHAs, for instance, have been shown to improve the appearance of photoaged skin by increasing epidermal thickness and stimulating the synthesis of collagen and glycosaminoglycans in the dermis. They can also help to disperse melanin, potentially reducing hyperpigmentation .
Formulating with hydroxyacids requires careful consideration of factors such as concentration, pH, and the overall product formulation to optimise efficacy while minimising the risk of irritation. The pH of the formulation is crucial, as lower pH values generally enhance the activity of hydroxyacids. However, very low pH can also lead to increased irritation. Buffering systems are often used in formulations to maintain an effective pH while improving tolerability .
Individuals using products containing hydroxyacids, particularly AHAs, should be advised to use sunscreen daily, as the removal of the outer layers of the skin can increase sun sensitivity . The level of exfoliation and potential for irritation can vary depending on the type and concentration of the hydroxyacid used, as well as individual skin sensitivity .
Hydroxyacids are also being explored for their potential antioxidant effects. Some, like lactobionic acid, have demonstrated the ability to scavenge free radicals . Furthermore, their ability to improve skin barrier function over time may indirectly contribute to skin protection against environmental damage . In conclusion, hydroxyacids are versatile ingredients in cosmetic dermatology that exert their effects primarily through exfoliation of the stratum corneum and modulation of keratinisation, largely due to their low pH. The three main types – AHAs, BHAs, and PHAs – offer different benefits and are suited for various skin types and concerns. Careful formulation and appropriate usage are essential to maximise the benefits of hydroxyacids while minimising potential side effects. Their role extends beyond simple exfoliation, encompassing improvements in skin texture, hydration, tone, and potentially offering antioxidant benefits.