GlycerinvsUrea

Glycerin (glycerol) is a small three-carbon polyol (92 Da) that penetrates the stratum corneum via intercellular lipid and transcellular routes. It acts as a humectant through osmotic mechanisms—creating a concentration gradient that draws water from the atmosphere (when humidity >65%) and from deeper epidermal layers. Glycerin upregulates aquaporin-3 (AQP3), a glycerol-permeable water channel in keratinocyte membranes that facilitates water and glycerol transport. It supports the natural moisturizing factor (NMF)—amino acids, lactate, and urea that maintain stratum corneum water-holding capacity. Glycerin modulates keratinocyte differentiation and may enhance lipid lamellae organization. Mild antimicrobial activity occurs through osmotic stress. Clinical studies demonstrate hydration efficacy comparable to or exceeding hyaluronic acid.

At low concentrations (<10%), urea acts as a humectant — small molecule (60 Da) absorbing into stratum corneum, drawing water via hydrogen bonding to carbonyl and amine groups. Part of endogenous NMF (filaggrin degradation products with amino acids, lactate), highly biocompatible. Integrates into corneocyte envelope, supports aquaporin-3 water transport. At higher concentrations (>10%), denatures keratin (K1, K10) by disrupting hydrogen bonds in alpha-helical structure and disulfide bridges in cornified envelope, causing corneodesmosome degradation and desquamation. Keratolytic via direct protein denaturation, not enzymatic. Dual mechanism — humectant at low dose, keratolytic at high — versatile for hydration and hyperkeratotic conditions (psoriasis, keratosis pilaris).