Concurrent beneficial (vitamin D production) and hazardous (cutaneous DNA damage) impact of repeated low-level summer sunlight exposures.

Background: The concurrent impact of repeated low-level summer sunlight exposures on vitamin D production and cutaneous DNA damage, potentially leading to mutagenesis and skin cancer, is unknown.

Objectives: This is an experimental study (i) to determine the dual impact of repeated low-level sunlight exposures on vitamin D status and DNA damage/repair (via both skin and urinary biomarkers) in light-skinned adults; and (ii) to compare outcomes following the same exposures in brown-skinned adults.

Methods: Ten white (phototype II) and six South Asian volunteers (phototype V), aged 23-59 years, received 6 weeks' simulated summer sunlight exposures (95% ultraviolet A/5% ultraviolet B, 1·3 standard erythemal doses three times weekly) wearing summer clothing exposing ~35% body surface area. Assessments made were circulating 25-hydroxyvitamin D [25(OH)D], immunohistochemistry for cyclobutane pyrimidine dimer (CPD)-positive nuclei and urinary biomarkers of direct and oxidative (8-oxo-deoxyguanosine) DNA damage.

Results: Serum 25(OH)D rose from mean 36·5 ± 13·0 to 54·3 ± 10·5 nmol L (14·6 ± 5·2 to 21·7 ± 4·2 ng mL ) in phototype II vs. 17·2 ± 6·3 to 25·5 ± 9·5 nmol L (6·9 ± 2·5 to 10·2 ± 3·8 ng mL ) in phototype V (P < 0·05). Phototype II skin showed CPD-positive nuclei immediately postcourse, mean 44% (range 27-84) cleared after 24 h, contrasting with minimal DNA damage and full clearance in phototype V (P < 0·001). The findings did not differ from those following single ultraviolet radiation (UVR) exposure. Urinary CPDs remained below the detection threshold in both groups; 8-oxo-deoxyguanosine was higher in phototype II than V (P = 0·002), but was unaffected by UVR.

Conclusions: Low-dose summer sunlight exposures confer vitamin D sufficiency in light-skinned people concurrently with low-level, nonaccumulating DNA damage. The same exposures produce minimal DNA damage but less vitamin D in brown-skinned people. This informs tailoring of sun-exposure policies.