Repeated Moisturizer Use Narrows the Surface-to-Deep Hydration Gap over Time: The Hydration Gap Index (HGI)

Skin hydration is not uniform: it is low at the surface and rises with depth, and the standard capacitance probe (Corneometer) samples only the outermost layer. How the surface–deep relationship evolves under repeated moisturizer use is not captured by a single metric. Using the Hydration Gap Index (HGI = |surface − deep (0.5 mm)|), this relationship was tracked in 22 healthy adults, measuring surface hydration (Corneometer) and depth-resolved deep hydration (Moisturemeter D; 0.5–5.0 mm) at baseline, after one application, and after two weeks. Surface hydration rose sharply after one application (30.9 → 64.0 A.U.), then partially declined by two weeks (43.5 A.U.; all p < 0.001), whereas deeper readings were essentially unchanged (≤5.1%), consistent with a stable deep compartment. The per-subject surface–deep gap first widened (13.1 → 18.0) then narrowed to ≈5.1 A.U. (19/22 subjects; group-mean 2.0). Surface and deep compartments varied independently (baseline r = −0.14; change r = +0.05), and the surface-dominant response was reproduced with a different surface instrument in the same cohort (surface +102%, deep +4%). The apparent depth gradient steepened only through this shallow-layer gain (−3.66 → −3.93 A.U./mm, p < 0.0001). Thus, repeated use reflected equilibration of the superficial layer toward the 0.5 mm compartment, not a uniform rise; the HGI provides a compact index of this surface-to-deep equilibration.