We were recently given a couple of samples of Mad Hippie’s Exfoliating Serum ($34.99) and my co-tester recently emailed me to ask when and how to use it. Could it be layered with a serum such as her AQ Serum, ditto for the other skin brighteners in her arsenal. This raised some interesting questions about exfoliating and brightening serums, particularly ones based on AHAs. So how should you use them and how do they work?

Exfoliating serums, masks, gels etc. are often based on alpha hydroxyl acids (AHAs). They are a group of weak acids typically derived from natural sources such as sugar cane, sour milk, apples and citrus that improve the skin’s appearance (evening out tone) and texture (smoothing). Glycolic acid is a typical one (from sugar cane) and Mad Hippie’s Exfoliating Serum has glycolic and lactic acids.

At the simplest level, they work by exfoliating the skin and helping make way for new, healthy skin cells. However, no one really knew how this actually worked. Until just every recently, that is. In a study published in the Journal of Biological Chemistry, a team of scientists from California and China used glycolic acid, the smallest and most biologically available AHA, to conduct some experiments and enter into keratinocytes, creating acidic conditions within the cell. The low pH encourages calcium ions to flow into the cell. The resulting calcium ion overload in the cell leads to its death and skin exfoliation.

Even if your eyes glazed over that nerdy science part, do take note of the key take away: the result is cell death and skin exfoliation. Now, this is important because it is rather different from what we’ve been told in the past about how glycolics and AHAs work – essentially that they loosen the glue-like substances that hold skin cells together, thus shedding the top layer of dull, damaged skin. If AHAs kill off the cell itself, then that gives us a big clue about how we should use them.

Sparingly, on their own and at night. Well that’s just my opinion, but while exfoliation is a good thing. Cell death probably shouldn’t be overdone. There is, after all, something called the Hayflick Limit – the number of times (50-odd) that a cell renews itself before dying off completely. Speeding up that process too frequently might not be a good idea. So perhaps products with glycolic should only be used once or, at most, three times a week, depending on the concentration of AHAs.

I would also think that layering a serum such as AQ might not work. Serums that have growth factors or peptides that signal to cells to do something would be wasting their time (and your money) if, when they got there, the cell has already croaked. Hence, my thinking that glycolic products are best used on their own. And at night, since they will make the skin more sun sensitive.

So while I will continue to use AHAs and think there are some great products with them (see my Five Best of 2012 with hydroxy acids), my regimen will be: sparingly, alone and at night.