Dark skinned people are more at risk for rickets? Huh? What about all those Africans?

Over in Comments, in the “sunscreen” thread, http://boards.straightdope.com/sdmb/showthread.php?threadid=68100

John W. Kennedy brought up “the rise of rickets” due to people not going out in the sun so much anymore. And while researching that, I stumbled across this remarkable factoid:


And I went, “Huh?”

And there’s also this:


The article is actually about the rise of rickets among babies who are exclusively breast-fed. The mothers themselves don’t go out in the sun much anymore, so they don’t have enough Vitamin D in their breast milk for their babies. However, it also has this in it:

From this, they (and the rest of the media) draw the conclusion that black skin means you’re more at risk for rickets. And I went, “Huh?”

You mean, for the last 50,000 years or so, all those Africans and Aborigines and Andaman Islanders have all been suffering from rickets? How could homo sapiens’ DNA overlook something obvious and important like that?

Or is it just a slanted, stupid interpretation of the results of a very limited study, combined with the media’s urgent need to provide catchy sound bites?

Couldn’t this be the explanation?

More breast-fed babies, more rickets. My WAG is that it’s got nothing to do with black skin. I can’t seem to find any actual studies anywhere, just the factoid.


Dang, I left out part of the Parents Place quote. :mad:

So if your skin pigment blocks 95% of the UV from reaching that deep layer of skin, you’re at risk for rickets?

Cite, please.

[sub]yes, I know, I’m demanding a cite from myself[/sub] :smiley:

Well, but aren’t these people dark-skinned in the first place because they come from places that get a lot of sun? I would assume that their level of pigmentation is just right for an equatorial country, where 5% of the UV rays is still a heck of a lot. It sounds like this is only a problem when people whose skin coloring is adapted to a sunny climate move to a less-sunny one.

Fretful Porcupine is right.

This is a major problem with people from the Indian Sub-continent living in Britain; especially for the veiled and covered women wearing traditional clothing.

I would guess that this is less of a problem in the USA because of the latitude and amount of sunshine, but here in Britain, there may be weeks (some would say months) without bright sunlight during the winter.

I think you’re missing a trick here Duck Duck. The darker your skin, the less vitamin D you produce. True. But, Walking in the sunshine for an hour or two, not wearing many clothes will produce all the vitamin D You need, no problem. There is a much bigger problem if you do this though and that is skin cancer. The more melanin in your skin, the more UV light is blocked, and the incidence of cancer is drastically reduced.

you might like to visit http://www.lalecheleague.org/releaserickets.html for more information on breast milk

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.

Thank you for 'splaining. I think I got it. Dark skin is designed to not let as much UV through, so people with dark skin who live in places where there’s too much UV bombarding down anyway get enough Vitamin D if they walk around semi-naked just long enough for the Vitamin D manufacturing process in the 5% of their skin that takes care of it to get underway. So people with dark skin who wear lots of clothes or who live in a place where there’s not much UV bombarding down (like England :smiley: ) are at risk for rickets.


Okay, I’ll get on with my life now. Thank you.