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Only 1000 IU of vitamin D in 10 minutes at 36 degrees, nude, summer, noon – April 2013

The solar UV exposure time required for vitamin D3 synthesis in the human body estimated by numerical simulation and observation in Japan

Authors: Nakajima, Hideaki; Miyauchi, Masaatsu; Hirai, Chizuko
Affiliation: AA(National Institute for Environmental Studies, Tsukbua, Japan nakajima at nies.go.jp), AB(National Institute for Environmental Studies, Tsukbua, Japan nakajima at nies.go.jp), AC(Tokyo Kasei University, Tokyo, Japan)
Publication: EGU General Assembly 2013, held 7-12 April, 2013 in Vienna, Austria, id. EGU2013-3803
Publication Date: 04/2013
Bibliographic Code: 2013EGUGA..15.3803N

After the discovery of Antarctic ozone hole, the negative effect of exposure of human body to harmful solar ultraviolet (UV) radiation is widely known. However, there is positive effect of exposure to UV radiation, i.e., vitamin D synthesis. Although the importance of solar UV radiation for vitamin D3 synthesis in the human body is well known, the solar exposure time required to prevent vitamin D deficiency has not been well determined. This study attempted to identify the time of solar exposure required for vitamin D3 synthesis in the body by season, time of day, and geographic location (Sapporo, Tsukuba, and Naha, in Japan) using both numerical simulations and observations.

According to the numerical simulation for Tsukuba at noon in July under a cloudless sky, 2.3 min of solar exposure are required to produce 5.5 μg vitamin D3 per 600 cm2 skin. This quantity of vitamin D represents the recommended intake for an adult by the Ministry of Health, Labour and Welfare, and the 2010 Japanese Dietary Reference Intakes (DRIs).

In contrast, it took 49.5 min to produce the same amount of vitamin D3 at Sapporo in the northern part of Japan in December, at noon under a cloudless sky. The necessary exposure time varied considerably with the time of the day. For Tsukuba at noon in December, 14.5 min were required, but at 09:00 68.7 min were required and at 15:00 175.8 min were required for the same meteorological conditions. Naha receives high levels of UV radiation allowing vitamin D3 synthesis almost throughout the year. According to our results, we are further developing an index to quantify the necessary time of UV radiation exposure to produce required amount of vitamin D3 from a UV radiation data.

36 degrees North: July noon : 220 IU in 2.3 minutes = about 1000 IU in 10 minutes

Doubt that they considered reductions due to:

  • Age,
  • skin color
  • skin temperature (in winter),
  • air pollution/haze
  • body lying flat on the ground instead of at an angle of 36 degrees (19% less due to the cosine)

Reminder: It appears that vitamin D generated is not proportional to % skin area exposed

600 sq cm skin area

600 sq cm = 0.6 sq meter = 0.6 X 3.28*3.28 = 0.6 * 10.76 sq feet = 6.35 sq feet
Which is about the silhouette of person, so 600 sq cm is OK

See also VitaminDWiki