The survey in the article that assessed vitamin D deficiency was a bit odd:

>Because physical exams are performed in mobile vans in NHANES, data could not be collected in northern latitudes during the winter; instead data were collected in northern latitudes during summer and in southern latitudes in winter. To address this season-latitude aspect of the NHANES design, we stratified the sample into two seasonal subpopulations (winter/lower latitude and summer/higher latitude) before examining vitamin D status.

Yeah, I'm not surprised that the rates for vitamin D deficiency were low.

>Less than 1% of the winter/lower latitude subpopulation had vitamin D deficiency (25-OHD <17.5 nmol/L). However, the prevalence of vitamin D insufficiency in this group ranged from 1%–5% with 25-OHD <25 nmol/L /.../, even though the median latitude for this subsample (32°N) was considerably lower than the latitude at which vitamin D is not synthesized during winter months (∽42°N).

and the more northern latitude in summer:

>With the exception of elderly women, prevalence rates of vitamin D insufficiency were lower in the summer/higher latitude subpopulation (<1%–3% with 25-OHD <25 nmol/L)

Now imagine if you lived in northern Europe around the 60th parallel, where the sun doesn't get high enough in winter to produce vitamin D.

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qurren 4 hours ago | parent | next [-]

Not to mention, northern latitudes get more sun that average in summer, and most northern countries have more reasonable working hours so people actually do go outside.

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HPsquared 2 hours ago | parent [-]

More hours of sun, but it's less intense.

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calgoo 33 minutes ago | parent | next [-]

Sure but i hide a lot more from the intense sun in spain then i do for the softer sun in sweden. In general i think it balances out quite well.

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codethief an hour ago | parent | prev [-]

The sun in northern countries in summer is less intense?

	▲	xdavidliu a minute ago \| parent \| next [-]
		less intense than in countries near equator, not less intense than winter
	▲	pdonis 42 minutes ago \| parent \| prev \| next [-]
		Yes, because it's lower in the sky so more of the UV in it gets absorbed by the atmosphere before reaching the ground. For example, in Florida in the summer, the sun is close to directly overhead at noon. In Scandinavia in the summer, the sun is only about halfway up the sky from the horizon at noon.
	▲	NoPicklez 36 minutes ago \| parent \| prev [-]
		Come to Australia during the summer and watch how quickly you burn outside without sunscreen.

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galleywest200 2 hours ago | parent | prev [-]

Could this be why evidence suggests that redheads synthesize Vitamin D more efficiently? Red hair is more prevalent at higher latitudes (I think).

Can redheads produce Vitamin D in these darker conditions while others cannot do so effectively?

https://www.sciencealert.com/evolution-favored-genes-linked-...

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annzabelle 2 hours ago | parent [-]

Pale skin in general helps synthesize Vitamin D. The less melanin you have the more you absorb, this is why lighter skin happens at northern latitudes. Darker skinned people are more likely to need to supplement it.

	▲	bakul 2 hours ago \| parent \| next [-]
		Darker skinned people in northern latitudes. Made worse for vegetarians like Ramanujan in England.
	▲	galleywest200 2 hours ago \| parent \| prev [-]
		Makes sense, thanks for the reply and clarification!