- Ca2+ chelator leading to desquamation
- It can help with matrix remodelling depending on the concentration
- Its penetration is pH and concentration dependent
- Use it once or twice a week and start with a low concentration solution
- Always wear sunscreen as AHA can make your skin more sensitive to sunlight
Glycolic acid is one of the tried and tested ingredients that can help with skin roughness and epidermal spots. It is one of the well studied ingredient that shows great promise for anti-aging as it can stimulate matrix remodelling. However, because of its size and how it works, it is very easy to abuse it which may lead to irritation and barrier damage. On the other hand, if used properly, it can become an essential part of your weekly routine.
What is Glycolic Acid?
Glycolic acid (GA) is a alpha-hydroxy acid (AHA) which is a group of organic acids naturally found in fruits, milk, and other natural sources. AHAs have been a very popular ingredient in skincare and dermatological treatments for several years, if not decades. For this reason, GA and other AHAs are well-established in terms of research. The most common source of GA is sugar cane, and its structure is conducive to skin penetration since it has the smallest molecular structure compared to other AHAs (Sharad, 2013). Readily available skincare products will contain low concentrations of GA which is effective for desquamation of the top layer of the skin. Higher concentrations of GA ranging from 20-70% are used as peels typically conducted by a professional in a clinical setting.
How does Glycolic acid work?
In low concentrations of about 2-5%, GA weakens the cohesion of cells in the stratum corneum and is gentle enough to keep trans-epidermal water loss unaltered (a measure of barrier function) compared to normal skin. Interestingly, using GA at 4% twice a day for 3 weeks did not cause thinning of the stratum corneum (Fartasch et al., 1997). This may be attributed to GA’s ability to induce keratinocyte proliferation while promoting desquamation in a simultaneous manner (Denda et al., 2010).
Precautions: AHAs increase photosensitivity compared to baseline, and predispose the skin to damage caused by UVA and UVB rays (Tsai et al., 2000). It is recommended to use a broad-spectrum sunscreen when using AHAs.
Some of the main skin concerns that have been shown to improve with consistent GA use include
(Bernstein et al., 2001)
15 female participants ages 51-68 applied 20% GA to the posterior forearm twice a day for 3 months.
Hyaluronic acid levels and collagen gene expression was increased in the epidermal and dermal layers of skin treated with GA compared to vehicle-control.
30 participants aged 20-40 who had acne scars and Fitzpatrick skin types III-V. 15 of the participants had microneedling performed every 6 weeks for a total of 5 sessions. In the other 15 participants, 5 treatments of microneedling and 5 treatments of 35% GA were administered.
The main improvement of microneedling alone was 31.33%, while microneedling with GA treatment had a 62% improvement.
Glycolic acid and pH
Its penetration and activity is pH-dependent. It has a pKa of 3.8 so at this pH 50% will be charged and 50% will be neutral. When molecules are charged, they would have a harder time passing through the skin layers as a lot of proteins in the skin are also charged. What this means is that the lower the pH of your Glycolic acid, the more active it will be. If you have a solution of 4% Glycolic acid at 2.5 pH and another with 8% Glycolic acid at 4.5 pH, even though the 4% has less Glycolic acid, it would have more active (neutral) form which will penetrate deeper.
Does that mean I should throw away my Glycolic acid with higher pH?
No. It depends on your ultimate goal. If you are looking for light exfoliation or maintainance purposes, we would argue that the 4.5 pH would be okay as it can still work on the Stratum Disjunctum (uppermost layer of the Stratum Corneum).
Glycolic acid that didn’t penetrate would get a second or even a third chance at becoming neutral. Once the neutral-form Glycolic acid penetrate, the balance of charged vs uncharged species would shift, forcing the charged ones to become neutral. Likewise, in the skin, there are so many other charged species that can "ion pair" with the charged Glycolic acid causing them to penetrate together. When they reach the more neutral layer, the positive charge usually loses it's charge and releases Glycolic acid. So think of it as a timed-release Glycolic acid solution. Of course this is an oversimplification of skin penetration as there are more factors that would affect it but you get the general idea.
Why does it sting and burn?
One downside to fully active Glycolic acid at low pH is that it chelates Calcium ions and degrades desmosomes and tight junctions in the Stratum Granulosum layer if it reaches those deeper layers. Unfortunately those Tight Junctions are essential for a healthy barrier. They serve as a gate to the lower layers of the Epidermis and the Dermis. Due to its small size it is easy for Glycolic acid to breach and penetrate these layers. Subsequently, it is then very easy to stimulate pain receptors and initiate inflammation where it penetrates.
As it goes deeper into the dermis, it triggers more damage to the surrounding cells. However, the inflammatory signalling and pain stimulation serve as triggers for matrix remodelling (Collagen degradation, restructuring and production as well as HA upregulation). To add to that, most of the Stratum Disjuctum is already peeled of and most of the epidermal pigmentation and roughness would be gone, leaving you with smooth, baby skin.
So it really depends on what your goal is in using Glycolic acid. If your goal is barrier repair, obviously you should put high concentration, low-pH glycolic acid aside for now. Although we've mentioned that those irritation and inflammation can trigger matrix remodelling, they are still inflammation and you know by know that inflammation can lead to pigmentation. This is why there is always a risk of rebound pigmentation after peels or laser treatment.
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. https://doi.org/10.1046/j.1524-4725.2001.00234.x
Denda, S., Denda, M., Inoue, K., & Hibino, T. (2010). Glycolic acid induces keratinocyte proliferation in a skin equivalent model via TRPV1 activation. Journal of Dermatological Science, 57(2), 108–113. https://doi.org/10.1016/j.jdermsci.2009.11.007
Fartasch, M., Teal, J., & Menon, G. K. (1997). Mode of action of glycolic acid on human stratum corneum: Ultrastructural and functional evaluation of the epidermal barrier. Archives of Dermatological Research, 289(7), 404–409. https://doi.org/10.1007/s004030050212
Sharad, J. (2011). Combination of microneedling and glycolic acid peels for the treatment of acne scars in dark skin. Journal of Cosmetic Dermatology, 10(4), 317–323. https://doi.org/10.1111/j.1473-2165.2011.00583.x
Sharad, J. (2013). Glycolic acid peel therapy – a current review. Clinical, Cosmetic and Investigational Dermatology, 6, 281–288. https://doi.org/10.2147/CCID.S34029
Tsai, T.-F., Bowman, P. H., Jee, S.-H., & Maibach, H. I. (2000). Effects of glycolic acid on light-induced skin pigmentation in Asian and Caucasian subjects. Journal of the American Academy of Dermatology, 43(2, Part 1), 238–243. https://doi.org/10.1067/mjd.2000.104894