About the author: FutureDerm.com is proud to introduce John Su on our staff as a Contributing Writer. John is an established skin care expert and aspiring dermatologist. He also runs a blog, The Triple Helix Liaison, dedicated to providing unbiased, meaningful, and insightful information about skin care. For his full bio, please visit our About page.
Along with retinoids and vitamin C, hydroxy acids (HA) are among some of the most well-documented topical compounds in the field of dermatology. While glycolic and salicylic acid remain the most prevalent and ubiquitous (1), several new additions have been making waves in the cosmetic and medical industries such as lactobionic acid (2). But before we discuss specific hydroxy acids, we need to know what they are.
What defines a hydroxy acid?
The most basic definition of an HA is a carboxylic acid, which is an organic acid that has at least one carboxyl (carbon double-bonded to oxygen) group. However, that general definition includes unrelated compounds like retinoic acid, L-ascorbic acid, and azelaic acid (3). Therefore, further qualifications need to be identified.
How do they differ from each other?
There are four types of HAs: Alpha HA (AHA), Beta HA (BHA), Salicylic Acid (SA), and Poly-HA (PHA).
- AHAs are carboxylic acids with one hydroxyl group attached at the “Alpha” position of the carboxyl group, meaning that the two functional groups are separated by ONE carbon atom. AHAs include the glycolic, lactic, and phytic acids.
- BHAs are carboxylic acids with one hydroxyl group attached at the “Beta” position of the carboxyl group, meaning that the two functional groups are separated by TWO carbon atoms. BHAs include the citric, malic, and tropic acids.
- SA is a carboxylic acid with both the hydroxyl and carboxyl groups attached directly to an aromatic benzene ring, rather than along a linear carbon chain.
- PHAs are carboxylic acids with multiple hydroxyl groups, with at least one attached to the “Alpha” position of the carboxyl group. PHAs include gluconolactone and lactobionic acid.
And that’s a wrap! I hope this clarified the structural differences between the various HAs that are encountered in cosmetic formulations. I know this post is short compared to my usual novels, but I’ve decided to break down this topic discussion into FOUR parts. Next week, we’ll be talking about HOW the various HAs are used, and WHAT their mechanisms of action are. Part III will be about the common misconceptions of HAs, and Part IV will be product reviews and recommendations! So stay tuned! Don’t forget to enter my brush giveaway!
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