The Influence of Solar-Simulated UV Radiation on Circulating 25(OH)D3, 24,25(OH)2D3 and Their Ratio in Younger and Older Adults

<b>Background:</b> In addition to the well-known vitamin D metabolites 25(OH)D and 1,25(OH)₂D, the catabolite 24,25(OH)₂D may also reflect vitamin D status and influence biological and skeletal processes. However, the effects of UVR-induced synthesis on 24,25(OH)₂D levels and the 25-VMR (24,25(OH)₂D3:25(OH)D3 ratio) remain unclear. <b>Objectives:</b> We aimed to assess how a single standardised UVR dose influences the production of 25(OH)D3, 24,25(OH)₂D3, 1,25(OH)₂D3 and 25-VMR, with a comparison between younger and older adults being conducted to explore potential age-related differences in vitamin D metabolism. <b>Methods</b>: A total of 11 young (18-40 years; 7M, 4F) and 10 older (65-89 years; 6M, 4F) skin type I-III volunteers received a single sub-erythemal dose of solar simulated UVR (SSR) (95% UVA: 320-400 nm, 5% UVB: 290-320 nm, 1.3 standard erythemal dose) during winter time in the UK (vitamin D trough season), exposing approximately 35% of the body surface area. The Blood was assayed for 25(OH)D3, 24,25(OH)₂D3 and 1,25(OH)₂D3 using LC-MS/MS at baseline, 24 h and 7 days following UVR exposure. <b>Results:</b> There was a significant increase in 25(OH)D3 from baseline (44 ± 22 nmoL/L) to 24 h post-UVR (48 ± 22 nmoL/L) in the combined age group (<i>p</i> = 0.044), but no significant differences were found in 24,25(OH)₂D3 in the combined group, or between young and older volunteers for both metabolites. 1,25(OH)₂D3 concentrations were higher in young groups (163 ± 60 pmoL/L) than in older (105 ± 38 pmoL/L) groups at 7 days post-UVR (<i>p</i> = 0.044). The 25-VMR decreased from baseline (9 ± 3) to 24 h post-UVR (7.5 ± 2.1) in the combined group (<i>p</i> = 0.003). <b>Conclusions:</b> Our data suggest that a single sub-erythemal UVR challenge does not influence 24,25(OH)₂D3 concentration in younger and older adults at 24 h and 7 days post-UVR and that the significant difference seen in the 25-VMR between baseline and 24 h post-UVR is due to the increase in 25(OH)D3 concentration post-UVR. This is in line with vitamin D oral supplementation studies, and indicates that low doses of UVR trigger the metabolic pathway, without affecting the catabolic pathway.