What are hydroxy acids and why are they used in skincare?

By Anna Huang

The recent years have seen a surge in the popularity of skincare products, with consumers growing more interested in the type of ingredients used in skincare products and their effects. Fuelled by growing communities on social media, providing discussion platforms for skincare trends and routines, the formulations of skincare products have come under greater scrutiny. Amongst the most popular and widely used compounds in skincare formulations are hydroxy acids (HAs), which are famous for their exfoliating properties and have shown to be effective for improving wrinkles, skin elasticity and tone and for treating photoaging (Moghimipour, 2012).

Two main classes which constitute the family of HAs are α-hydroxy acids (AHAs) and β-hydroxy acids (BHAs). AHAs are hydrophilic weak carboxylic acids with one or more hydroxyl group attached to the α-carbon. They include naturally occurring acids such as glycolic acid and lactic acid, which are most commonly used even though citric acid and malic acid are also similar in functionality (Tran et al., 2015). AHAs are found in many foods and milk sugars, however, those used in cosmetic products are usually produced synthetically. Due to their exfoliating properties, AHAs are used for treating acne, scars, skin dryness, pigmentation and wrinkles.

The exact mechanisms are still unclear, but it is assumed that when applied topologically on to the skin, AHAs interfere with the ionic bonding between skin cells by reducing the concentration of calcium ions in the epidermis (Moghimipour, 2012). Because desmosomes (weakly adhesive) and adherens junctions are calcium dependent, the removal of calcium ions from cell adhesions through chelation results in a loss of calcium ions from the cadherins of these junctions (Kornhauser, 2010). Subsequently, cutaneous cellular adhesions are weakened causing the shedding of dead skin cells, resulting in an exfoliating effect (Kornhauser, 2010; Moghimipour, 2012; Tran et al., 2015). Furthermore, the reduced calcium ion concentration in the epidermis also tends to promote cell growth and slow cell differentiation, which can help improve the appearance of wrinkles and give the skin a more youthful appearance (Tran et al., 2015). Additionally, glycolic acid contributes to the antiaging properties of AHAs by increasing hyaluronic acid (which hydrates skin) and collagen gene expression in the epidermal and dermal skin layers (Bernstein et al., 2001).

BHAs work in a similar fashion to AHAs, but with a difference in solubility. BHAs are lipophilic in contrast to AHAs which are hydrophilic and require facilitated transport for uptake into cells. BHAs are thus able to penetrate the skin through sebaceous follicles (cutaneous glands which secrete sebum). This is especially useful for individuals with more oily skin and can also achieve a deeper level of exfoliation (Moghimipour, 2012).

Salicylic acid (SA) is another type of HA, which falls under neither the AHA nor BHA category (although it is often falsely described as a BHA) (Kornhauser, 2010). SA, like BHAs, is lipophilic, but contains an aromatic benzene ring to which both the hydroxyl and the carboxyl group are attached. It is also used as an exfoliating agent and as treatment for acne and photoaging (Kornhauser, 2010). SA has been found to be photoprotective when applied topologically and to be able to protect skin carcinogenesis caused by UV radiation, thus preventing photoaging (Bair et al., 2002). SA is also an effective treatment for acne due to its antibacterial and anti-inflammatory effect (Kornhauser, 2010).

HAs are rightfully widely hailed ingredients in skincare products. However, it must be said that the beneficial results can be expected only when used in moderation. When used at high concentrations or in excess, HAs (especially AHAs) can disrupt the cohesion of the cells in the outermost skin barrier (corneocytes), which protect the underlying cells from UV damage, and thus result in skin irritation and photosensitivity (Tang & Yang, 2018) .

AHAs and BHAs work best at concentrations ranging from 1% – 2% and SA is commonly formulated at concentrations ranging from 2% – 4% (Kornhauser, 2010; Tang & Yang, 2018).

References

Bair, W. B. 3., Hart, N., EInspahr, J., Liu, G., Dong, Z., Bowden, G. T. & Alberts, D. (2002) Inhibitory effects of sodium salicylate and acetylsalicylic acid on UVB-induced mouse skin carcinogenesis. Cancer Epidemiology. 11 (12), 1645–1652.

Bernstein, E. F., Lee, J., Brown, D. B., Yu, R. & Van Scott, E. (2001) Glycolic Acid Treatment Increases Type I Collagen mRNA and Hyaluronic Acid Content of Human Skin. Dermatologic Surgery. 27 (5), 429-433.

Kornhauser, A. (2010) Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clinical, Cosmetic and Investigational Dermatology. 3 (default), 135-142. Available from: https://search.datacite.org/works/10.2147/ccid.s9042. Available from: doi: 10.2147/ccid.s9042. 

Moghimipour, E. (2012) Hydroxy Acids, the Most Widely Used Anti-aging Agents. Jundishapur Journal of Natural Pharmaceutical Products. 7 (1), 9-10. Available from: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3941867&tool=pmcentrez&rendertype=abstract. Available from: doi: 10.17795/jjnpp-4181. 

Tang, S. & Yang, J. (2018) Dual Effects of Alpha-Hydroxy Acids on the Skin. Molecules (Basel, Switzerland). 23 (4), 863. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29642579. Available from: doi: 10.3390/molecules23040863. 

Tran, D., Townley, J. P., Barnes, T. M. & Greive, K. A. (2015) An antiaging skin care system containing alpha hydroxy acids and vitamins improves the biomechanical parameters of facial skin. Clinical, Cosmetic and Investigational Dermatology. 8 9-17. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25552908. Available from: doi: 10.2147/CCID.S75439.