> We say African Americans have higher rates of Vitamin D deficiency, but this is based on the assumption that their average levels of Vitamin D should equal those of European whites, not on an assessment of when they experience problems.

Couldn't you just survey average vitamin D levels in native West African populations, and compare it with African Americans?

Even if there was a difference in the requirements between native Africans and native Europeans, many African Americans also have varying levels of European heritage that would complicate the picture.