Lactic AcidvsPetrolatum (Vaseline)

Lactic acid (90 Da, larger than glycolic acid) exfoliates via the standard AHA mechanism: chelating calcium at corneodesmosomes and promoting desquamation through protease activation. Unlike glycolic acid, lactic acid is a natural component of the skin's natural moisturizing factor (NMF) and functions as a humectant, drawing water into the stratum corneum through hygroscopic binding. It inhibits tyrosinase enzyme activity in melanocytes, providing mild brightening. At higher concentrations (10%+), lactic acid upregulates serine palmitoyltransferase and glucosylceramide synthase in keratinocytes, stimulating ceramide synthesis and improving barrier lipid composition. It also enhances filaggrin proteolysis to NMF components. This dual action—exfoliation plus barrier support—makes it the most moisturizing AHA and clinically useful for dry, sensitive, or barrier-compromised skin.

Petrolatum is a mixture of long-chain saturated hydrocarbons (C16-C32) that forms a semi-occlusive film—reducing transepidermal water loss (TEWL) by up to 99%. It penetrates intercellular spaces of the stratum corneum, filling gaps in the lipid lamellae and creating a hydrophobic barrier. This occlusive environment raises stratum corneum water content, activating endogenous repair: upregulation of antimicrobial peptides (defensins, cathelicidin LL-37), lamellar body secretion, and barrier lipid synthesis. The stratum corneum obtains oxygen from dermal circulation, not the atmosphere. Studies demonstrate accelerated wound healing and barrier recovery, contradicting infection myths.