This is part of our ongoing The Best Kept Secrets to Healthy Aging spotlight. Each day, we will be posting some of the great information that’s packed into our book, The Best Kept Secrets to Healthy Aging.
The Sun, the Skin and Vitamin D
Skin production of vitamin D is determined by length of exposure to sunlight, latitude, season, amount of skin exposed, height of the sun in the sky, the amount of air pollution, and degree of skin pigmentation. The reason vitamin D is considered to be a vitamin is that there is virtually no way most of the world’s population living in non-equatorial areas of the globe can be exposed to sufficient sunlight year-round to be able to make enough vitamin D in their skin to satisfy their metabolic needs. As the sun must be high enough up in the sky so your shadow is shorter than you are in order to make Vitamin D, additional vitamin D from fortified foods or dietary supplements is needed by almost everyone from birth throughout life.
The amount of vitamin D that is formed in the skin is proportional to sunlight exposure – greatest at solar noon (when the sun is as high as it will be that day). During the summer, at most latitudes, that means between 11 AM and 3 PM (local time) during the months with the longest day length. Likewise, the amount of vitamin D that is partially activated by its conversion to 25OHD3 by the liver is determined by the amount of vitamin D made by the skin and absorbed from the diet. Consistent with this concept, darker-skinned ethnic groups produce less vitamin D and therefore less 25OHD3 than do lighter-skinned individuals in response to the same degree of sun exposure. Similarly, effective sunscreen also prevents vitamin D synthesis in the skin, as do clothes. Fortunately, all adults are able to absorb supplemental vitamin D unless a disorder in fat digestion or absorption is present or inhibitors of fat digestion are present.
Tomorrow’s topic: How Much Vitamin D Do I Need?