Hydroxy Acids Part I: What are Hydroxy Acids?

Alpha Hydroxy AcidsAbout 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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8 thoughts on “Hydroxy Acids Part I: What are Hydroxy Acids?

  1. Pingback: Hydroxy Acids Part I: What are Hydroxy Acids?-FutureDerm Post! v 0.03 « TheTripleHelixLiaison

  2. Pingback: Which Fruits and Vegetables are Best for Your Skin? + MORE Apr 28th

  3. BooBooNinja says:

    I like to think of myself as an informed consumer. I merely skimmed over this article when it was first posted. Nothing registered because I assumed I knew it all. :P I came back (after starting to read Part II) to read it more carefully and guess what? I’ve always considered SA a type of BHA … probably because good ol’ Paula Begoun equates the two (e.g.,when talking about the difference between AHAs and BHAs). Do BHA and SA have a similar function? I’m going to go read Part II now… perhaps you’ll address this.

  4. Hrm says:

    I thought that salicyclic acid was a BHA and not it’s own category? I also thought that citric was an AHA (well wikipedia calls it an AHA).

  5. John Su says:

    @Hrm

    A reader actually emailed me these very same questions, though she asked a few more follow up questions! Anyways, I will just copy and paste the relevant parts.

    “Technically speaking, citric acid can be considered as a BHA OR an AHA. If you look at the definitions of HAs above, you’ll see that:

    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.

    BHAs are: 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.

    Now, if you look at the chemical structure of citric acid (http://en.wikipedia.org/wiki/Citric_acid), you will see that the hydroxyl group “OH” can be either in the Alpha or Beta position of the “COOH” carboxyl group. How is that possible? As you can see from the photo, this compound has THREE carboxyl groups and ONE hydroxyl group. If you use either the left or right carboxyl groups, the hydroxyl group would be TWO carbons away, making it a BHA. If you use the carboxyl group on the top, then the hydroxyl group would be ONE carbon away, making it an AHA. So it’s all a matter or perception since this compound can be categorized as an AHA or a BHA depending on which carboxyl group you use to measure the distance of placement of the hydroxyl group.

    Perhaps citric acid is traditionally categorized as an AHA (since I believe AHAs were discovered before BHAs; but don’t quote me on that), but chemically and technically speaking, this compound can be either type of hydroxy acids.

    Does that make sense?

    And with SA not being a BHA, well I answer that in the second part of the series: http://www.futurederm.com/2012/05/10/hydroxy-acids-part-iii-common-misconceptions-of-hydroxy-acids/ As you will see in that article, SA being a BHA is a misnomer.

    I hope that helps!

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