*3.3. Impact of Sun Exposure on Serum D3 and 25(OH)D*

In the 24 h following sunlight exposure, only one participant (younger cohort) had observable signs of erythema. Following the single bout of sun exposure, serum vitamin D3 concentration increased significantly in the 24 to 48 h post exposure (time effect; *p* < 0.002) with a trend for a difference in vitamin D3 response between older and younger individuals (time\*group; *p* = 0.09) and no group effect (*p* = 0.21) (Figure 1). The average increase was of 76.4 ± 83.7%, 88.1% ± 73.3 and 54.1 ± 52.5% at 24, 48 and 72 h, respectively, which were significantly greater than baseline at all points (*p* < 0.017). Peak concentration in serum D3 were generally observed between 24 and 48 h with a few participants, all in the younger cohort, experiencing slight upward trends in D3 concentration from 48 to 72 h. Overall there was a trend for older individuals to peak later (at 48 h) compared to younger individuals (at 24 h) (*p* = 0.0875), with one participant in the older cohort experiencing a peak at 24 h and nine at 48 h compared to seven participants in the younger cohort who peaked at 24 h and eight between 48 and 72 h. Two subjects in the older and three in the younger cohorts (16.7%) did not experience an increase in serum D3 at any time point. The average increase in D3 at peak was 9.8 ± 7.8 nmol/L (including the non-responders) and was not different between cohorts (7.7 ± 5.7 vs. 11.2 ± 8.8 nmol/L, older vs. younger, *p* = 0.25).

**Figure 1.** Vitamin D3 concentration following a single bout of sun exposure (time 0) in older (solid line) compared to younger (dashed lines) adults. Serum vitamin D3 concentration increased significantly post exposure (time effect; *p* < 0.002) with a trend for a difference in vitamin D3 response between older and younger individuals (time\*group; *p* = 0.09). Error bars represent ± SE.

Despite changes in serum vitamin D3 concentration, serum 25(OH)D concentration did not change following sunlight exposure and was not different between baseline and 72 h (*p* = 0.561) or baseline and 168 h post exposure (*p* = 0.237) (Figure 2).

**Figure 2.** 25(OH)D Concentration at baseline, 72 h and 168 h post exposure to natural sunlight for a total of 30 min in the Younger (Panel A) and Older (Panel B) Adult Cohorts. \* represents single data points that were greater than three times the interquartile range.
