Vitamin D production requires ultraviolet B spectrum light (280-315 nm wavelength). Cholesterol from our diet and that produced in our liver is modified then transported to the skin where it is stereochemically altered by UVB light and heat, returned to the liver and further modified to 25-hydroxy cholecalciferol (D3). This is the molecule we measure in our blood.
The shortest day of the year marks the winter solstice which falls on December 21st of a solar calendar year. This year the sun is furthest from North America at 12:04 pm on December 21st. Most cultures celebrate the solstice between December 24th and January first. These celebrations often mark the birth of the new solar cycle. They often signify the beginning of true winter (the coldest part of the year).
Not only are we further from the sun at this point in the year but the angle of the sun from the horizon is more acute or smaller. In the case of Michigan this angle is less than 25 degrees from horizontal. We see this as the sun crossing the sky very close to the horizon through the course of the day.
At this sharp angle the path of the suns rays must travel through more ozone to reach the earth’s surface. Subsequently, no UVB reaches the earth’s surface. Cloud cover and smog can also reduce penetration of ultraviolet light throughout the year, reducing or preventing vitamin D production.
At a winter solstice, solar angle of greater than 35 degrees, without cloud cover, enough UVB reaches the earth’s surface to produce vitamin D even on the shortest day of the year. This occurs at a latitude of about 30 degrees. So if you live in one of these cities or south of these, potential year round sun is available to make D.
Temperature also plays a role in this equation because warmth of the skin is also necessary to facilitate production and allow comfortable exposure of 50 percent or more of the body surface (shorts and T-shirts or less cover). The jet stream affects the temperature at different latitudes and longitudes, as does altitude and the proximity to large bodies of water, humidity, and ground cover. Remember warmth lags behind solar radiation. The warmest time of day is between 3-5 pm, the warmest time of year is about 3-5 weeks after summer solstice. Conversely the coldest time of year is 3-5 weeks past winter solstice.
Suffice it to say that you want to be in a sunny place 30 degrees or less latitude, receiving equatorial winds, near a body of water, with moderate humidity and little cloud cover. These conditions will allow at least fifty percent skin exposure in the afternoon hours of the winter months. Voila, you’re making vitamin D. Remember sunscreen blocks vitamin D production so don’t use it when you are trying to make vitamin D.
Now you have the recipe for finding a winter vacation spot. If you want something to read while you are making vitamin D this winter get The Vitamin D Cure and send us a post card.