Previous work by geneticist Rick Kittles at the University of Chicago suggests that upwards of 75 percent of African-Americans are deficient in vitamin D. Kittles says that African-Americans living north of the 37th parallel — just about anyplace north of central California, Texas, Tennessee or North Carolina — will have difficulty through most of the year absorbing enough sunlight to make vitamin D, because of the low angle of the rays reaching the Earth's surface.