Polyhydroxy acids are ingredients such as gluconolactone and lactobionic acid. They’re supposed to be as effective as AHAs, but less sensitizing.
Gluconolactone and lactobionic acid are chemically and functionally similar to AHAs (such as glycolic acid). The significant difference between them and AHAs is that gluconolactone and lactobionic acid have larger molecular structures, which limits their ability to penetrate the skin, resulting in less potential for sensitizing side-effects. Supposedly, this reduced absorption into the skin does not hamper their effectiveness.
Does that mean gluconolactone and lactobionic acid are better for your skin than AHAs in the form of glycolic acid or lactic acid? According to an Internet-published class lecture by Dr. Mark G. Rubin, a board-certified dermatologist and assistant clinical professor of dermatology at the University of California, San Diego, research on gluconolactone demonstrated only a “6% decrease in dermal penetration” in comparison to glycolic acid, which “isn’t a dramatic improvement.”
Gluconolactone may be slightly less sensitizing for some skin types, but this isn’t the magic bullet for exfoliation some cosmetics companies have been extolling.
Polyhydroxy acids can also function as antioxidants and may promote some amount of improvement in skin’s surface strength.
References for this information:
Clinics in Dermatology, September-October 2009, pages 495-501
Cutis, February 2004, Supplement, pages 3-13