Glycolic, lactic, mandelic — the alpha-hydroxy acids. A six-month study found they do more than slough the surface.
Alpha-hydroxy acids (AHAs) — glycolic, lactic, mandelic, citric — are the "acid exfoliants." They loosen the glue between dead surface cells so they shed more evenly, which is why skin looks smoother and brighter after consistent use.
But a 1996 study suggests they do more than surface work. People applied a 25% AHA lotion to one forearm and a placebo to the other for about six months. The AHA side showed a roughly 25% increase in skin thickness, along with improved elastic fibers and increased collagen density on biopsy — changes in the deeper dermis, not just the surface. [1]
Glycolic vs. lactic vs. mandelic — molecule size matters. Glycolic acid is the smallest molecule, so it penetrates fastest and hits hardest (most effective, most irritating). Lactic acid is larger and gentler, and is also a humectant, so it hydrates as it exfoliates — a good starting point. Mandelic acid is larger still and the gentlest, which is why it's popular for sensitive skin and deeper skin tones where aggressive exfoliation risks post-inflammatory marks.
What this means in practice. AHAs make skin more sun-sensitive — the FDA specifically advises sunscreen with AHA products — so they're best at night with diligent SPF by day. Start with a lower percentage a few times a week. And resist stacking every active at once: AHAs plus a strong retinoid plus vitamin C on the same night is a fast track to a compromised barrier.
The 25% concentration in that study is higher than most leave-on consumer products, so don't expect a gentle 5% toner to do exactly the same thing — but the direction of effect is real and well replicated.