Unsurprisingly, given the steady stream of information constantly pummelling the human brain, the tidbits that actually stick in your memory long-term aren't always what they should be. It's never the formula that calculates the area of a triangle; it's the absurd answer to an equally arcane question from a drunken game of Trivial Pursuit you played seven years ago. No, of course you can't remember the name of the first capital of Egypt despite once writing it on a blank map — but what self-respecting proponent of self-care could possibly forget that Cleopatra supposedly bathed in donkey milk on the regular to maintain her smooth, clear complexion?
The practice of bathing in milk, equine or otherwise, is something we — the non-Mariah Careys of the world — have largely done away with in present day. But, of all the useless knowledge there is to be had out there, Cleopatra's rumoured milk baths are far from the most useless. Dairy, particularly that which has gone sour, produces lactic acid, an alpha-hydroxy acid. And you know what AHAs do: They hold your skin's hand throughout the exfoliating process so that dead, used-up cells don't stick around as long, yielding a smoother, clearer, more resilient complexion.
What differentiates lactic acid from similar ingredients (like glycolic, malic, citric, and tartaric acids), however, is how gentle it is; some might even call it "weak." Dermatologist Ted Lain, MD, explains that the molecules that make up lactic acid are larger than the other AHAs, so it doesn't penetrate the skin as effectively to trigger that peeling action. If you have dry or sensitive skin, this is a good thing. "At low concentrations, lactic acid acts as a humectant, drawing moisture into the skin," says Dr. Lain. "It helps to both mildly exfoliate while also hydrating, which is the perfect combination."
That said, dramatic overnight results are hard to come by from pure lactic acid alone. Dr. Lain says that you can still expect tone and texture to improve at higher concentrations, and oily skin types can still bet on it to reduce pore-clogging debris and minimise the formation of blackheads and breakouts (it's the secret ingredient in Renée Rouleau's legendary Anti Bump Treatment) — but lactic acid works best in conjunction with friends.
Glycolic acid, retinol, and antioxidants make for particularly good pairings, but lactic acid's versatility is another one of its charms. In Sunday Riley's beloved Good Genes treatment, it's combined with squalane, aloe, and a handful of botanical extracts to smooth, retexturise, and hydrate; in the new Honeymoon Glow AHA Resurfacing Night Serum from Farmacy, it's supercharged with glycolic, citric, salicylic, and hyaluronic acids in a 14% AHA/BHA blend that's a little more potent than something Cleopatra may have submerged herself in.
And so the theory holds up: Milk, or at least one of its derivatives, can do really good things for your skin. We can't exactly credit Cleopatra with the discovery — accounts of her bathing habits have never been confirmed, and it wasn't until 1895 that German pharmacy Boehringer Ingelheim started producing lactic acid commercially. That's a lot less sexy of a story than one that involves an Egyptian lady pharaoh... even if she did marry at least two of her own brothers.