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Comment by BurningFrog | Hacker News Reader

This is one reason arctic people eat so much (vitamin D rich) fish. The other is of course that a plant based diet is impractical when plants are only alive half the year.

White skin itself is an evolutionary adaption for getting enough vitamin D outside of tropical Africa. The extreme "melanin-anemic" blond & blue eyes look of Scandinavia is thought to be because it has a uniquely warm weather for the latitude, which make it possible to grow (vitamin D poor) grains there. That supports by far the biggest population density that far north, but also induces extreme whiteness to soak up every possible sun ray.

Conversely, if you have dark skin and live in the non tropics, you absolutely need to monitor and enhance your vitamin D intake. Sadly, doctors don't seem to bring this up with dark skinned patients, at least from what I hear from my (OK, not that many...) black friends.

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I don't have the source handy but there was a geographic point made about Scandinavia that went something like this:

- Scandinavia is very far north and therefore gets less sun in the winter (obviously)

- Apparently the soil of Scandinavia is well suited to crops that are high in Vitamin D

The soil being good for Vitamin D crops (plus lightening of skin/hair and fish diets etc) was a part of why people were able to live in such far northern climates even if they didn't always have access to fish.

> - Apparently the soil of Scandinavia is well suited to crops that are high in Vitamin D

IIUC it's the other way around. soil/climate allows for foods low in Vitamin D -> Vitamin D needed from other sources -> evolutionary pressure for getting it from the sun -> less melanin in skin

I can't find any crops that are high in vitamin D.

Some fungi have decent vitamin D, but it's certainly not a major "crop" in Scandinavia.

I'm probably missing something obvious but... Should white skin not reflect sunlight more, and dark skin absorb it more? I agree that northerners have more need to absorb sunrays and hence make more vitamin D, so shouldn't they be black?

Melanin is deposited in the epidermis, vitamin D conversion happens deeper, in the blood vessels of the dermis.

Vitamin D synthesis happens underneath the skin, and requires UV B radiation. The dark skin is an adaptation to avoid UV damage from the normal UV levels you get at the less extreme latitudes by filtering out most of it.

Yeah, it's confusing.

I think this is how it works: The melanin is not transparent, so it stops the light from penetrating beyond the very surface, and so a smaller volume of skin cells get access to sunlight.

To go a little deeper: Sunlight does two major things in our skin. It creates vitamin D, and destroys folate. Both are essential, so the various skin colors we have reflect the balance evolution has struck in various environments.

IIRC from high school biology, the melanin in the upper skin layer absorbs the sunlight thus preventing it from going any deeper into the skin.

Nope. Dark skin is an adaptation to exposure to lots of UV rays: https://en.wikipedia.org/wiki/Melanin#Evolutionary_origins

Incidentally you can test this yourself: if you go out in the summer you will tan i.e. your skin will darken. This is adaptation to lower the risk of sunburn and skin cancer.

Melanin in dark skin doesn't absorb as much UV radiation. It makes sense for people who live under the sun with relatively little protection to be dark skinned but it wouldn't make sense for people who are rarely exposed to the sun to be resistant to it.

> Should white skin not reflect sunlight more, and dark skin absorb it more?

"White" and "dark" refer to the skin's effects on visible light. Visible light is different wavelengths from UV light; vitamin D production in the skin depends on UV light. "Dark" skin blocks UV light more than "white" skin does.

Evolution makes adaptations to what it already has, unlike a sentient designer who might create an optimal design then look for ways to implement it.

If you could design a human from scratch, it might be most efficient to give northerners a D synthesizing pigment that's opaque in the necessary wavelength, resulting in the situation you describe.

Instead we have protective pigments near the surface, synthesizing structures underneath, and a need to balance D synthesis against UV damage.

It's not obvious; this confused me for a long time. The skin we call "white" is fairly transparent to UV, and the skin we call "black" is much less so. Skin is a mixture of transparency and reflectivity at different wavelengths and different depths below the surface. As such it's extraordinarily difficult to model it so it looks right, as any CGI engineer will tell you.

The dark (or not so dark) layer is quite superficial, not far from surface. But the vit D production happens deeper in. For sun rays to get there, that layer must be light-colored.

Sure, some photons do get reflected, but the benefit is from those photons that get deeper into the skin.

Ummm, plants are alive longer than “half the year“. Humans have also preserved foods for out of season consumption for millennia. Natives in California had plenty of plant-based foods in winter months.

"California" being the operative word, there. Oslo, Norway has a 60 degree latitude. Sacramento has a 35.8 degree latitude.

Most humans live closer to 27 degrees north of the equator. I mentioned California natives because that’s what I’m most familiar with. Plant preservation is a thing in northern latitudes too..

Natives in California can still be harvesting in November and can see the ground all year round except in the mountains. Natives at the far north might be done with harvesting by late September or early October and might not see the ground again until April. Not even remotely comparable in terms of the severity of the issue and the impact to survival.

Natives in the far north still preserve food for consumption during winter months.

Sure, the plants are alive as roots, seeds etc under the snow, but they have nothing edible to offer, so for the purposes of nutrition, they might as well be dead.

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