The relation between sunscreen layer thickness and vitamin D production after ultraviolet B exposure: a randomized clinical trial.
Level 2 - randomized trial
Individual randomized controlled trial.
PubMed 22512875 · doi:10.1111/j.1365-2133.2012.11004.x
What was done
Thirty-seven healthy volunteers with fair skin types were randomized to five groups receiving varying thicknesses of an inorganic SPF 8 sunscreen: 0, 0.5, 1.0, 1.5, or 2.0 mg/cm² applied to the upper body (~25% body surface area). Twenty minutes post-application, participants received a fixed UVB dose of 3 standard erythema doses. This was repeated four times at 2- to 3-day intervals. Serum 25-hydroxyvitamin D3 [25(OH)D] was measured before the first exposure and 3 days after the final exposure.
What was found
Serum 25(OH)D increased in an exponential manner as sunscreen layer thickness decreased. The increase in 25(OH)D after irradiation was statistically significant for all thicknesses under 2 mg/cm² (P < 0.05). In the group receiving the standard recommended thickness of 2 mg/cm², the increase in 25(OH)D was not statistically significant (P = 0.16). Absolute baseline and post-treatment serum vitamin D concentrations were not reported in the abstract.
Why it matters
This trial shows that applying sunscreen at the standard recommended thickness (2 mg/cm²) substantially impairs cutaneous vitamin D synthesis, while the thinner layers typically applied by consumers in real-world settings still permit vitamin D production.
Limits
The sample size was small (37 participants distributed across five arms, averaging ~7-8 per group). The study was restricted to fair-skinned individuals, tested only one low-SPF (SPF 8) inorganic sunscreen formulation, used artificial UVB irradiation rather than natural sunlight, and did not report exact numerical serum vitamin D levels in the abstract.