Daily Photoprotection Mitigates Seasonal Skin Changes and Supports Skin Longevity: A 1-Year in Vivo 3D Multiphoton Microscopy Study.

INTRODUCTION:While the detrimental effects of intense sun exposure are well-recognized, the cumulative impact of daily moderate UV exposure remains less explored. This year-long study investigated the effects of seasonal variations on facial skin, aiming at highlighting subclinical skin changes using non-invasive in vivo multiphoton microscopy. We also explored the impact of applying daily a well-balanced photoprotection formula with enhanced UVA filtration. METHODS:Fifty-nine European descent women (aged 55-65, Fitzpatrick phototypes II or III), participated in this year long, real-life exposure study in Paris, France. Participants were randomized into two double-blind groups: one applying a daily broad-spectrum sun protection factor (SPF) /persistent pigment darkening (PPD) photoprotection product, the other applying the vehicle. In vivo assessments were conducted at four seasonal time points to measure the skin color and multiphoton parameters: melanin density and z-epidermal distribution, epidermal morphology, elastin, and fibrillar collagen densities in the papillary dermis (first 20 µm-thick dermal sublayer below the dermal-epidermal junction) and their combined 3D SAAID aging index, a marker of photoaging. RESULTS:The vehicle group exhibited a significant increase in skin color, melanin density, elastin density and a decrease in the 3D-SAAID aging index between winter and summer, reflecting seasonal UV impact. These changes were not observed in the photoprotected group. CONCLUSION:This study provides in vivo evidence that real-life sun exposure, even under moderate conditions, can induce subclinical skin changes. Daily use of sunscreen with enhanced UVA protection mitigated these effects, highlighting the importance of consistent photoprotection in preventing long-term UV-induced clinical aging signs and supporting skin longevity.