Hey, Duck Duck - the link between dark skin and rickets is nothing new. It was figured out by the end of the 19th century, prompting parents to force cod liver oil (very rich in vitamin D) into children for a couple generations.
Vitamin D is manufactured by the body on exposure to sunlight. In tropical/equitorial latitudes there is a lot of sunlight beating down, taking an almost perpedicular course through the atmosphere. In fact, there is so much sunlight that OVERexposure is a problem. There is a theory that dark skin evolved not only as a protection against skin cancer but also to prevent the body from making too much vitamin D, which in large (megadose) amounts can be toxic to kidneys and liver.
As folks moved north and south of the equator, lack of vitamin D became a problem. There is less sunlight for at least half the year (maybe more, if you’re in an area that gets a lot of cloud cover), and the sunlight that reaches you takes a more slanted course through the atmosphere, so it is not as intense. Those with lighter skin produced more vitamin D, which was advantageous enough that they left more offspring and certain groups of people began to bleach.
There are, however, exceptions. Tierra del Fuegians, for instance, used to live on the very tip of South America but were relatively dark skinned. They also ran around with little clothing prior to European contact and ate a lot of seafood - which is usually a good alternative source of the vitamin - so their dark skin didn’t cause problems. And some Middle Eastern folks near the equator are lighter than their neighbors, but tend to wear a lot of clothing which would have a simillar effect to dark skin in regards to vitamin D production.
The problem of rickets in children who seldom went out of doors was recognized during the industrial revolution. Child miners, for instance, commonly suffered from it. Children in city tenements, some of which had no windows, who worked in sweatshops 12 hours a day, many of which had no windows, also were at high risk of the condition. As I said, back in the 1800’s doctors had noticed that dark skinned children were at particularly high risk.
All of this is curable by a little sunshine. Light Caucasians require about 15 minutes of direct sunlight on bare skin to produce enough vitamin D for health. Vitamin D is also storable in the body, so a summer of sun can easy tide white folks over a winter. Darker folks require more sunlight. A dark Caucasian may need 1/2 hour, a very very dark skinned person of African descent may require 2 hours of sunlight at typical United States latitudes.
Of course, we also have the folks advising everyone to wear sunblock when going outside - and sunblock slows down or even stops vitamin D production. Probably, we need to inject a little common sense here. Get enough sunlight to prevent rickets, then put on the sunblock. All adjusted by skin type, because this is one instance where skin color does matter.
