The effect of daily sunscreen application on vitamin D: findings from the open-label, randomised, controlled Sun-D Trial

Sunscreen reduces vitamin D production in experimental studies. It is uncertain whether this translates to 'real-world' settings. We aimed to dtermine if routinely applying high-SPF sunscreen for one year reduces serum 25-hydroxyvitamin D [25(OH)D] concentration. We conducted a population-based, open-label, randomised, controlled trial in Australians aged 18-70 years, who were not routinely using sunscreen/vitamin D. Participants were randomised (1:1) using stratified, computer-generated permuted block randomisation to routine application of SPF 50+ sunscreen on days the UV Index was forecast to reach ≥3 (intervention), or discretionary sunscreen use (control), for ∼1 year. We measured 25(OH)D concentration at baseline (winter/spring 2022), and at end of summer and winter 2023; we excluded from analyses participants with no post-baseline concentrations (n=11). The primary outcome was change in 25(OH)D concentration from baseline. Vitamin D deficiency (25(OH)D concentration <50 nmol/L) in the final sample was an exploratory outcome. We analysed data using mixed-effects models and Poisson regression. We stratified our analysis by baseline 25(OH)D concentration, residential UV radiation zone, skin exposure (derived from time outdoors and clothing use), and personal UV radiation exposure (derived by combining UV radiation zone and skin exposure). Sample and data analysis were performed blind to randomisation group. Trial registration: Australian New Zealand Clinical Trials Registry (ACTRN12621001752853). Between June 30, 2022, and November 29, 2022, we randomised 639 participants (intervention, n=319; control, n=320). 628 participants were analysed (intervention, n=312; control, n=316; median age, 52 years (IQR 40-64)). 415 (66%) identified as female, 210 (33%) as male, 3 (0.5%) used another term. Baseline 25(OH)D concentration was balanced (mean (SD): intervention, 63.5 (21.9) nmol/L; control, 62.1 (22.8) nmol/L). Adjusted mean differences from baseline were 1.6 nmol/L (intervention) and 6.8 nmol/L (control) (between-group treatment effect, -5.2 nmol/L; 95% CI, -7.2, -3.2). The treatment effects were consistent across almost all subgroups. Vitamin D deficiency (final sample) was higher in the intervention (n=139 [46%]) compared with the control group (n=115 [37%]) (prevalence ratio, 1.33, 95% CI, 1.14, 1.55). Routinely applying high-SPF sunscreen results in lower 25(OH)D concentrations than would be observed with discretionary sunscreen use. Regular sunscreen users may need supplementary vitamin D.