First integrated assessment of potential cutaneous vitamin D production in Riyadh: A decade of ground based measurements and culturally tailored scenario modelling.

Despite abundant solar radiation year-round, vitamin D deficiency remains paradoxically widespread in Saudi Arabia - a persistent "sunshine paradox." This study presents the first integrated, decade-long assessment of potential cutaneous vitamin D production in Riyadh, combining high-resolution ground-based solar radiation measurements (2015-2025), rigorous clear-sky atmospheric correction, and culturally tailored scenario-based modelling that reflects real Saudi behavioural and physiological conditions. Vitamin D-effective irradiance (UVD) showed a strong seasonal cycle, with summer values ∼130% higher than winter. Cloud cover further modulated this baseline, reducing UVD by 34% under partly cloudy and 71% under overcast skies. Scenario modelling demonstrated that exposure duration and exposed body surface area (linked to clothing practices) are the most powerful modifiable determinants, producing up to seven- to eight-fold differences in daily IU production. Skin pigmentation and age imposed significant physiological constraints, while realistic population/lifestyle profiles revealed more than a ten-fold variation in vitamin D synthesis potential under identical seasonal UV availability. This work breaks new ground by moving beyond static prevalence studies to provide quantitative, context-specific evidence on how environmental UV availability interacts with modifiable human behaviours in a sun-rich environment. The findings offer a robust scientific foundation for developing season-specific, culturally appropriate sun-exposure guidelines and targeted supplementation strategies. Implementing these insights has substantial potential to reduce the public health burden of vitamin D deficiency across Saudi Arabia and similar MENA settings.