"I think this is a major unrecognized epidemic in the United States," says Michael Holick, M.D., a researcher at Boston University medical center and the most high-profile member of the vitamin D research community. "It affects children and adults of all ages, all races, and both sexes. It's very significant." The way God drew it up, getting enough vitamin D ought to be a cinch, since the process is as unconscious as breathing. When you're outside in the sunlight, UVB rays from the sun activate an enzyme in your skin. Presto, vitamin D is created and goes to work in your body.
Unfortunately, in practice several things can interfere with the process. First is geography. The farther you are from the equator, the less direct the sunlight is, and the weaker the UV rays become. Above 42 north, for example — picture a line stretching roughly from Boston to northern California — it's difficult for many people to produce vitamin D during the winter. African-Americans, Latinos, and others with dark skin are at a further disadvantage, since their pigmentation limits the UV light they can absorb and slows vitamin D synthesis.
The final obstacle in vitamin D production is, or at least can be, the environment — the cause of the last big vitamin D crisis, in the early 1900s. As the industrial revolution kicked into high gear, more people moved to the cities and hunkered down in dark, dank tenements; meanwhile, pollution from bustling factories clouded the skies. The result was far fewer UV rays touching people's skin, and a lot more vitamin D deficiency. "At the turn of the last century," says Dr. Holick, "more than 80 percent of the kids in Boston had rickets." See: Why Sunshine is So Healthy