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Title: The risks and benefits of cutaneous sunlight exposure
Author: Felton, Sarah Jane
ISNI:       0000 0004 5994 0363
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2016
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Background: Recommendations to restrict summer sunlight exposure to prevent skin cancer apparently conflict with requirements to protect bone health through adequate 25-hydroxyvitamin D (25[OH]D) levels, as provided by cutaneous ultraviolet (UV)B exposure. Furthermore, sunlight exposure promotes a feeling of euphoria that is felt to drive further sun-seeking behaviour. Objectives: My principal objective was to examine health risk (DNA damage) and health benefits (25[OH]D gain, and potential cutaneous photoprotection) following low-level summer sunlight exposures in people of light (phototype II) and darker (phototype V) skin. A further objective was to evaluate serum endocannabinoid levels, potential drivers of mood elevation, following these exposures, and to assess for evidence of tanning addiction in a cross-section of psoriasis patients who had received similar low dose UV exposures, as medical phototherapy. Methods: During wintertime, 10 white Caucasians and 6 South Asians aged 18 to 60 years, from Greater Manchester, UK, received a simulated summer's sunlight exposures, specifically 1.3 standard erythemal dose, thrice weekly for 6 weeks, whilst casually dressed. Serum and urine samples and skin colour measurements were taken at baseline, Monday, Wednesday and Friday of week 1 and then weekly, and buttock skin that had received differential UVR exposures was biopsied for immunohistochemical analysis. Phototype II individuals, who are at higher risk of sunburn, were subsequently challenged with 2X minimal erythema dose (MED) UVB on small areas of simulated summer-exposed and photoprotected skin. Separately, a link to an online tanning questionnaire survey was sent to all members of the National Psoriasis Foundation (USA) during my USA field trip. Results: The simulated summer resulted in 50% gain in 25(OH)D for both phototype groups, but significantly more cutaneous DNA damage (cyclobutane pyrimidine dimers, CPD) in phototype II than V (p<0.0001). There was no accumulation of cutaneous CPD after 6 weeks compared with a single UVR exposure in either group, while phototype V individuals had repaired a greater proportion of their CPD 24 hours after final UVR exposure (p<0.0001). Urinary oxidative DNA damage was higher in phototype II throughout the simulated summer (p=0.002) and unaffected by UVR. All individuals had significant skin darkening, and in phototype II, stratum corneum thickness increased significantly (p<0.05). This tanning response provided significant photoprotection against a pro-inflammatory UVB (2X MED) challenge, as shown by reduced erythema and neutrophil influx in skin exposed to the simulated summer than in photoprotected skin (p<0.05 for both). Serum endocannabinoid (2-arachidonoyl glycerol) levels increased significantly during the simulated summer in both phototype groups (p<0.01), peaking at week 2-3. The cross-sectional study of 1,832 psoriasis patients revealed 34% had used sunbeds; 11% of current users fulfilled diagnostic criteria for addictive-like tanning behaviour: female sex, younger age, younger age at psoriasis diagnosis, severe disease and prior phototherapy were significant risk factors for addiction. Discussion: These findings should assist public health guidance on safe sunlight exposure and highlight the need for distinct guidance targeted to different phototype groups. Furthermore, individuals with psoriasis, in particular those who previously received regular UVR exposure, are at high risk of tanning addiction that may be driven by the endocannabinoid system.
Supervisor: Berry, Jacqueline ; Rhodes, Lesley Sponsor: Not available
Qualification Name: Thesis (M.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: sunlight ; skin ; ultraviolet radiation ; photoprotection ; endocannabinoid ; DNA damage ; tanning addiction