Abstract
Vitamin D, which controls the concentrations of calcium and phosphate in the blood, is essential for keeping the bone density. However, severe vitamin D deficiency in a lot of Japanese population, especially in young women, has been reported since 2000s. In addition to dietary intake by food such as fish and mushroom, vitamin D can be produced in human skin by receiving solar ultraviolet-B (UV-B) radiation. However, recommended solar exposure time to produce required daily vitamin D amount has not been disclosed yet by any scientific society or institution, since the time varies greatly depending on latitudes, season, local time, and the amount of ozone and aerosol in the sky. We calculated surface solar UV radiation intensity E (λ) at specified location and time in 1 nm resolution between 280 and 400 nm by a radiative-transfer calculation model SMARTS2. We then calculated vitamin D-yield UV radiation amount IVitD (W/m2) by multiplying the above E (λ) by the relative effective significance SVitD (λ) of vitamin D synthesis by Commission internationale de l’éclairage (CIE) and by integrating in UV region. As a result, the amount of vitamin D synthesized in human body was estimated by multiplying the vitamin D synthesis factor for a unit UV-B radiation amount shown by a past literature. Finally, required solar exposure time was estimated in the case of 10 μg vitamin D as its requirement dose in human body. Similarly, when solar exposure time to reach 1 minimal erythemal dose (1 MED) of UV-B radiation amount IEry (W/m2) was estimated, that time was about 3 times longer than the time necessary for the synthesis of 10 μg vitamin D through exposing hands and faces (600 cm2 skin exposure in total) under wearing long-sleeve-shirts to sunlight. The information on the calculated results for vitamin D-yield UV radiation amount IVitD (W/m2), the required time for 10 μg vitamin D synthesis, and the time for 1 MED UV-B radiation amount IEry (W/m2), are opened to public in a near-real-time manner through the " Information on Vitamin D Synthesis / Erythermal UV" homepage (https://db.cger.nies.go.jp/dataset/uv_vitaminD/en/index.html), which is based on actual UV-B and UV-A measurements at 11 locations in Japan. We recommend that people expose their body to solar UV radiation for an appropriate time in daily life by utilizing the above information for healthy bone condition.
