Thursday, December 20, 2007

moving to the north



Humans carry a lot of cultural baggage when it comes to skin colour, but to scientists it is increasingly a simple matter of a vitamin.

Around the world, scientists have observed that the farther human communities live from the equator, the lighter their skin colour tends to be - the result of an evolutionary adaptation related to vitamin D synthesis.

At higher latitudes, such as those prevailing in Northern Europe, it's only possible for people to make vitamin D the natural way - through skin exposed to ultraviolet sunlight - during spring and summer. That means over time there was an evolutionary advantage for humans living in these areas to develop lower levels of melanin, the natural sunscreen that gives skin its dark pigmentation, but causes reduced production of the vitamin.

By developing lighter-coloured skin, humans in northern parts of the world were able to make more use of the available sunlight for vitamin D production and build up stores of the nutrient for the long period during winter when light is too feeble to make any of it.

Esteban Parra, an anthropologist at the University of Toronto, says there are a number of competing scientific hypotheses for differences in skin colour, but the vitamin D explanation, and the need to have protection against ultraviolet light in the tropics, appears to make the most sense.

The impact of having lower levels of melanin, or lighter skin colour, is huge, when it comes to the synthesis of vitamin D.

It would take someone with dark skin of African or South Asian ancestry about 60 minutes at the same time of day to make the amount of vitamin D that a person of European ancestry would make in about 10 minutes, estimates Reinhold Vieth, one of Canada's top vitamin D experts and a professor in the department of nutritional sciences at the University of Toronto.

He says a rough rule of thumb is that people can only make vitamin D when the sun gets high enough in the sky that their shadows are shorter than they are tall, explaining why Canadians can't make any during the winter.

Although light skin hue is an advantage in northern areas, the opposite is true in tropical and subtropical regions because of the more intense ultraviolet sunlight, which allows for year-round vitamin D production, but holds other health risks.

There, having higher levels of melanin offers protection against the damaging effects of ultraviolet light - sunburn and lowering of folate levels, an essential nutrient that is destroyed by exposure to strong sunshine.

With people of different racial backgrounds now living around the world, health implications are emerging from these evolutionary adaptations to sunlight.

Whites living in tropical or extremely sunny areas, such as Australia, are at elevated risk of skin cancer and need to take sensible precautions against sunburn.

But non-whites are at higher risk of not being able to make enough vitamin D if they live outside the tropics and subtropics.

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