A winter acne increase has been explained as a fall in skin moisture levels resulting in a compensatory increase in sebaceous gland production or sebum containing more lipids. Another suggestion is that lower UV levels may affect the balance between skin flora and the skin’s immunological response, increasing the risk of acne breaking out.25

Seasonal UV exposure might have some bearing on vitamin D and psoriasis. Vitamin D is involved in regulating keratinocyte differentiation as well as supporting integrin proteins in cell surfaces providing intracellular support. These can be disrupted in psoriatic skin at the dermal and epidermal junction.26

A small observational study has found that psoriatic patients had significantly lower vitamin D serum levels compared to ‘healthy’ control patients, with 38.9 per cent (n=66) of psoriasis patients being vitamin D deficient (<20 ng/mL) or insufficient (20–30 ng/mL). People with bullous diseases were found to have the lowest vitamin D mean values.26

The seasonal variation of vitamin D blood levels was noted, but the study points out that vitamin D levels are dependent on a number of factors, not just the amount of UV exposure.