Humans have evolved a nifty ability to make Vitamin D in our skins, but the process requires an intensity of ultraviolet light that doesn't exist here in winter outside of a tanning booth.
It may look sunny outside in November and December, but the angle of the sun is too low, and the intensity too weak, to make any Vitamin D, said Pennsylvania State University anthropology professor Nina Jablonski, author of the book Skin: A Natural History. Even if you go skiing in the Poconos and come back with a burn, you haven't gotten enough of the right wavelength to make Vitamin D, she said.
That's why some doctors disagree with a recent government panel's statements that most Americans get enough Vitamin D from a combination of sunshine and fortified foods.
Since then, a Stanford study of 6,000 people showed that more than half of those who avoided sun were deficient. Among dark-skinned people, deficiency was common even among those who didn't avoid sunshine.
Pediatrics professor Bruce Hollis has sampled blood levels of Vitamin D and found that nomadic people in Africa have about four or five times as much as a typical African American person living in the United States.
Some scientists have proposed that white skin evolved to solve the problem of Vitamin D production, but it's just a partial solution. Light skin just extends the portion of the year when it's possible to make Vitamin D from sunlight, said Hollis, who works at the Medical University of South Carolina.
The government panel was commissioned just to determine whether Americans got enough to keep our bones healthy, but Vitamin D has dozens of other jobs. It's essential for our hearts, brains, and nervous systems to function, Hollis said. "It affects everything."
