UVB exposure

During wintertime in latitudes greater than ∼38°, the angle of the sun is too oblique for UVB rays to pass through ozone, so little or no vitamin D is dermally synthesised. This vitamin D winter increases with latitude; it lasts about 3 months in Athens, Greece (37°N), ∼6 months in Cork, Ireland or London, UK (both at 51°N) and 7–8 months in Helsinki, Finland (at 60°N) and Tromsø, Norway (at 70°N). Personal behaviour also limits UVB exposure, even in sunny countries. As excessive sun exposure is the principal risk factor for most skin cancers, public education campaigns recommend limiting exposure to sunlight.

Improved adherence to sun safety recommendations and awareness of the links between excessive sun exposure and skin cancer, as well as premature wrinkles, has led to the widespread use of sunscreen and inclusion of sun protection factor (SPF) ingredients in cosmetic products. Correct application of a product containing an SPF of 15 almost completely prevents cutaneous skin production of pre-vitamin D3 (Matsuoka et al. 1988). Dermal synthesis of vitamin D is less efficient in older than in younger adults (Holick 2008). Discreet clothing habits limit sun exposure particularly in veiled women and long working hours spent indoors mean that most adults rely on weekends and vacation to spend time outdoors during the day.

Furthermore, melanin in skin reduces the penetration of UVB and thus contributes to lower vitamin D status in dark-skinned individuals. These factors reemphasise the need for public health strategies to exploit food-based solutions for prevention of vitamin D deficiency. However, the question of whether a minimal risk approach to UVB exposure would enable vitamin D production without increasing the risk of skin cancer is outstanding. While such infor- mation would not offset the dietary requirement for deficiency prevention during winter, it could have a massive impact on the dietary requirements for vitamin D for health given the more powerful ability of skin synthesis to increase serum 25(OH)D concentrations relative to dietary intake.

ODIN will provide new data to test whether there is a minimal or threshold UVB exposure level that enables subcutaneous vitamin D synthesis and avoids risk of skin cancer. In particular, it will test the traditionally held notion that 6–8% and 15% of body surface exposure (i.e. face and hands, and face, hands and arms), to obtain one minimal erythemal dose (MED: the minimum amount of time to cause slight reddening of the skin without burning after which vitamin D production ceases; typically ∼15 minutes for White Europeans) of summer sunlight is enough to increase serum 25(OH)D above 30 nmol/l and importantly, whether such exposure induces significant DNA damage in the skin. These data will be of huge value in informing vitamin D public health policy in the future. In addition, meteorological data will be used in ODIN to develop models to predict serum 25(OH)D concentrations across populations in different geo- graphical locations. These models will be tested using the standardised status data obtained in the population studies listed in Table 1 to predict the response of dif- ferent populations to changes in the dietary supply of vitamin D. This combination of approaches will provide much needed data on the current and potential role of sunshine in helping to meet vitamin D require- ments without increasing risk of skin damage. The modelling data will also be used in ODINto project the impact of such dietary changes on serum 25(OH)D distributions in European populations, while account ing for vitamin D arising from exposure to summer UVB sunlight.

 

References:

Holick MF (2008) Deficiency of sunlight and vitamin D.BritishMedical Journal (Clinical Research Ed.)336: 1318–19.

Matsuoka LY, Wortsman J, Hanifan N et al. (1988) Chronic sun-screen use decreases circulating concentrations of25-hydroxyvitamin D. A preliminary study.Archives of Derma-tology124: 1802–4.

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