Wednesday, August 25, 2004

When Skin Color Does Matter

Brad DeLong links to an article on Slate that suggests that contrary to expectations raised by the fortification of various food items, Vitamin D deficiency does continue to be a problem for darker-skinned individuals who reside in Northern Latitudes. In fact, it seems to still be a problem even for pale-skinned individuals, or at least, for those of them who avoid getting sufficient exposure to sunlight for whatever reason.

Amanda Schaffer writes in Slate about how a surprisingly large number of people who live her in the northlands far from the equator need more vitamin D. I am very surprised that this is controversial. When we Cro-Magnon types came out of Africa 60,000-100,000 years ago, none of us were white. Now practically all of us the bulk of whose ancestors stopped for long in northern Europe or northern China are remarkably pale indeed. I have heard no reason advanced for this other than that melanin in your skin blocks some vitamin D creation. If true, then there must have been a hell of a lot of selection pressure for low-melanin skin, which replies a hell of a large health cost to blocking even a small amount of sun-mediated vitamin D creation:
It has long been known that vitamin D is crucial for healthy bones. The presence of vitamin D in the small intestine aids in the absorption of dietary calcium—people with vitamin D deficiency are able to absorb only a third to half as much calcium as those with sufficient levels—and calcium is vital to the hardness of bone. The two diseases traditionally associated with severe vitamin D deficiency—rickets in children and osteomalacia in adults—are characterized by deformation or softening of bone. And chronic vitamin D deficiency is strongly linked to osteoporosis, a disease defined by loss of bone density and associated with increased risk of fractures.

The common assumption has been that with the fortification of milk, instituted in the United States in the 1930s, and casual exposure to sunshine, most people get all the vitamin D they need. But a small resurgence of rickets in the last few years, particularly among African-American children, has caught the health-care community off guard. As studies have probed the issue, it has become clear that vitamin D deficiency (usually defined as blood levels of less than 15 ng/mL [or nanograms/milliliter]) and insufficiency (less than 20 ng/mL,) are far more widespread than researchers had expected. The elderly, who often receive little sun, are at particular risk, as are African Americans and other dark-skinned people, since skin pigmentation, which protects against damage by UV rays, also interferes with vitamin D production. (Those with dark skin need to spend more time in the sun to produce the same amount of vitamin D.) Infants who are exclusively breast-fed are also at high risk since breast milk, for all its virtues, contains almost none of this vitamin.

Perhaps the biggest surprise, though, has been the prevalence of vitamin D deficiency among women of child-bearing age—particularly African-American women—and among healthy children and adolescents. Dr. Catherine Gordon, an adolescent-health specialist at Children's Hospital in Boston and an expert on vitamin D, told me that calling it a "hidden epidemic" would not be an overstatement. While severe cases early in life result in rickets, less-pronounced deficiencies may slip under the radar because they do not cause noticeable symptoms. Gordon and other doctors worry that for children and adolescents, insufficient vitamin D can prevent proper bone development and increase the risk of disorders such as osteoporosis later in life.

Vitamin D deficiency can easily go undetected in adults as well. In one study, published in 2003 in the Mayo Clinic Proceedings, researchers in Minneapolis tested vitamin D levels in patients suffering from chronic, non-specific, musculoskeletal pain: 93 percent of them turned out to be vitamin D deficient—a condition very likely (though not conclusively) related to their symptoms. And of the East Africans, African Americans, and Hispanics in the study, 100 percent were vitamin D deficient. As a result, the authors argue, all patients with such pain should have their vitamin D levels tested because osteomalacia may turn out to be the underlying cause.
I suspect that one factor that tends to aggravate the issue of Vitamin D insuffiency for people of African descent is the inability to tolerate lactose, which makes milk-drinking uncommon amongst those who aren't of European origin. As most cereal is consumed with milk, this automatically rules out two of the main food items which undergo Vitamin D enrichment. I myself am constitutionally lactose intolerant, but what many people don't realize is that one's level of tolerance can be raised a fair amount simply by undergoing long-term exposure; I'll never be one of those types who reaches for the milk bottle in the way the rest of mankind might reach for a carton of orange juice, but at least I'll be able to go some way to meeting my dietary requirements for calcium and Vitamin D without needing to live under a sweltering sky. Getting outdoors when the sun is out and about also helps, though here the vagaries of employment make this impractical for most adults.

The converse of all of the above is that there also still seems to be quite a bit of selection pressure for darker skin as one heads towards the tropics: in particular, pale-skinned people tend to have the folate bleached right out of them, which can lead to anemia, reduced sperm fertility and pregnancy complications. If one could somehow freeze both human technology and population movements where they are today and then wind the clock forward 40 generations or so, I have a feeling that the descendants of today's Canadians and American Southerners would seem to each other to be different "races", even if they were all of common European descent to begin with.