According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Randomized Controlled Trial (RCT) with washout and delayed outcome measurement
A double-blind, placebo-controlled RCT in healthy adult humans where participants receive oral hyaluronan for 4 weeks, then stop, and skin hydration is measured daily for 14 days post-intervention using corneometry; placebo group receives inert capsule. Prospective longitudinal cohort with biomarker tracking
A prospective cohort study tracking skin hydration and biomarkers in humans taking oral hyaluronan over time, measuring hydration at baseline, end of intake, and weekly for 2 weeks after cessation, while simultaneously analyzing stratum corneum turnover (via tape-stripping and keratin markers) and fibroblast activity (via dermal biopsies or serum markers). Case-control study comparing prolonged hydration responders vs. non-responders
A case-control study comparing individuals who show sustained skin hydration (>14 days post-intake) after oral hyaluronan with those who do not, analyzing differences in stratum corneum turnover rates and fibroblast gene expression profiles. Human dermal fibroblast and stratum corneum model with hyaluronan exposure
An in vitro study exposing human dermal fibroblasts and reconstructed human epidermis to hyaluronan, then removing it, and measuring persistence of hydration-related gene expression and barrier function over 14 days. Systematic review and meta-analysis of existing RCTs on oral hyaluronan and skin hydration persistence
A meta-analysis pooling all available RCTs measuring skin hydration at multiple time points after cessation of oral hyaluronan, focusing on studies with at least 14-day post-intervention follow-up.