Hydroxy Acids-What are they?
Hydroxy Acids Part I: What are Hydroxy Acids?
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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.