The body is able to manufacture vitamin D from exposure to UV light. Dark skinned people will receive less sunlight than a light skinned person because much UV is blocked.
I once visited Baltimore, Maryland, and saw an unusual number of young African American men walking with bow legs and canes.
Papers on the internet suggest that Ricket's Disease is more prevalent among African Americans, even with 20th century nutrition.
In the 17th, 18th, and 19th centuries, did anyone comment on Ricket's impacting the quality of their slaves in the North (where there is less sunlight)?