| Summary: | Older adults are frequently cited as an at-risk population for vitamin D deficiency that may in part be due to decreased cutaneous synthesis, a potentially important source of cholecalciferol (vitamin D<sub>3</sub>). Previous studies found that cutaneous D<sub>3</sub> production declines with age; however, most studies have been conducted ex vivo or in the photobiology lab. The purpose of this study was to characterize the response of vitamin D metabolites following a 30-min bout of sun exposure (15-min each to the dorsal and ventral sides) at close to solar noon in younger and older adults. Methods: 30 healthy individuals with skin type II/III were recruited; a younger cohort, aged 20–37 (<i>n</i> = 18) and an older cohort (<i>n</i> = 12), age 51–69 years. Exposure was at outer limits of sensible sun exposure designed to enhance vitamin D synthesis without increasing risk of photo ageing and non-melanoma skin cancer. Serum D<sub>3</sub> concentration was measured at baseline, 24, 48 and 72 h post-exposure. Serum 25(OH)D was measured at baseline and 72 h post-exposure plus 168 h post-exposure in the older cohort. Results: D<sub>3</sub> increased in response to sun exposure (time effect; <i>p</i> = 0.002) with a trend for a difference in D<sub>3</sub> between cohorts (time*group; <i>p</i> = 0.09). By regression modeling of continuous data, age accounted for 20% of the variation in D<sub>3</sub> production. D<sub>3</sub> production decreased by 13% per decade. Despite changes in D<sub>3</sub>, however, serum 25(OH)D did not change from baseline to 72 or 168 h post exposure (<i>p</i> > 0.10). Conclusions: Serum D<sub>3</sub> concentration increased significantly in response to outdoor sun exposure in younger and older adults. While ageing may dampen cutaneous synthesis, sunlight exposure is still a significant source of vitamin D<sub>3</sub>.
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