Skin concentration of 7-dehydrocholesterol is not a limiting factor for vitamin D3 synthesis in older versus younger adults, and a similar response occurs to UVR in these age groups

Skin exposure to ultraviolet radiation (UVR) triggers conversion of precursor 7-dehydrocholesterol (7DHC) to vitamin D3(cholecalciferol), which then enters the bloodstream. The aim of this study is to assess if skin 7DHC concentration differs between younger and older adults, and to explore the impact of solar simulated UVR (SSR) on 7DHC (in skin) and vitamin D3 in serum) in these age groups.
Younger (n=10, 18-40 years) and older (n=11, 65-89 years) adults of phototype I-III were exposed to a sub-erythemal dose of SSR (95% UVA, 5% UVB, 1.3 standard erythemal dose) over 35% body surface area, in the UK winter. Six 5mm buttock skin punch biopsies were taken: two from unexposed skin, two immediately post-UVR, and two at 24h post-UVR. Blood was taken at baseline, 24h and 7d post-UVR. Skin and serum samples were assayed using LC-MS/MS. Baseline skin 7DHC concentrations were (mean ± SD) 0.22 ± 0.07 and 0.25 ± 0.08 µg/mg in younger and older adults, respectively. Immediately post-UVR, 7DHC concentrations were 0.27 ± 0.10 and 0.22 ± 0.08 µg/mg in younger and older adults respectively, and 24h post-UVR were 0.27 ± 0.08 and 0.28 ± 0.13 µg/mg, respectively. Baseline serum vitamin D3 concentrations in younger adults were 1.5 ± 1.5 versus 1.5 ± 1.7 nmol/L in older adults. 24h post-UVR they were 3.1 ± 2.0 nmol/L in younger versus 2.5 ± 2.0 nmol/L older adults, and 7 days post-UVR were 2.0 ± 2.1 versus 1.7 ± 1.2 nmol/L, respectively. No significant difference was observed in any parameter between age groups.
Our study did not observe changes in skin 7DHC concentration after UVR and did not affect serum vitamin D3 concentrationin healthy older adults relative to younger adults. The early vitamin D3 biosynthetic pathway does not appear to differ between these age groups.