Suntanning and cutaneous synthesis of vitamin D3

Skin tanning is the melanization of the epidermis induced by excessive sunlight exposure. Since melanin absorbs preferentially the wavelengths around 300 nm and the cutaneous synthesis of vitamin D3 is stimulated by the same wavelengths (290 to 320 nm, ultraviolet light B [UVB]), we investigated the effect of tanning on vitamin D3 formation. Vitamin D3 and 25 hydroxyvitamin D (25-OH-D) serum levels were measured during midwinter (untanned state) in seven healthy subjects. Blood was obtained immediately before whole body exposure to UVB in a phototherapy unit, and again 24 hours later. The study was repeated in the same subjects during midsummer (tanned state) using the same UVB dose. Serum vitamin D3 increased in the untanned state from 1.7 +/- 0.4 ng/ml (mean +/- SE) to 11 +/- 1.5 ng/ml following UVB (p less than 0.001). In the tanned state, basal serum vitamin D3 was significantly higher: 9.6 +/- 2.8 ng/ml (p less than 0.04 basal untanned versus basal tanned), and exhibited minimal rise after UVB to 14.3 +/- 4.1 ng/ml (p greater than 0.1 for tanned basal versus post UVB tanned). Tanning was also associated with significantly higher serum 25-OH-D levels: 22.5 +/- 2.9 ng/ml (untanned) versus 36.9 +/- 4.7 ng/ml (tanned) (p less than 0.02). Thus excessive solar exposure produces, besides erythema and tanning, the resetting of the vitamin D3 synthetic mechanism with blunting of the response to UVB.