Except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north (in the United States, the shaded region in the map) or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency.
I saw the quoted text, but missed any explanation. I would love a clarification on whether they claim the skin is not getting exposed to sun or, even when exposed, does not make any vitamin D, which is how I read the above.
This is an honest question -- if it is the first, I am not too concerned. I grew up in a colder climate and, while living far in the shaded area today, routinely wear short sleeves and walk a lot outside from early spring to late fall. If it is the second, I would love to learn the underlying causes at least as a scientific curiosity (and start thinking about vitamin D supplements).
The sun is too low in winter and the atmosphere blocks the UVB rays our skin needs to produce Vitamin D. The same thing happens around sunrise and sunset even in summer. Here's a good explanation from HN's favorite site:
It is silly to go through the trouble of including a map, only to reinforce the imprecision of the parallel approximation. Obviously People in Denver get more sunlight than e.g. people in Nashville, given the altitude and weather patterns. More detailed maps don't limit themselves to drawing a straight line.
If you do need supplementation, then Vitamin D3 (specifically Cholecalciferol) is vastly better absorbed than Vitamin D2. If you want a specific recommendation, buy this: https://www.amazon.com/gp/product/B07234TTCC and take it twice per week (Sunday & Thursday for example), that'll average to about 1400 IU per day.
This could lead to a recommendation of 1000 IU for children <1 year on enriched formula and 1500 IU for breastfed children older than 6 months, 3000 IU for children >1 year of age, and around 8000 IU for young adults and thereafter.
Seems odd, considering cow milk contains virtually no vitamin D. It is often enriched with vitamin D, but that practice likely does not predate the evolution of lactose tolerance.
When it comes to nutrition, the majority of unsourced information on web forums is made-up nonsense. Perhaps not maliciously in bad-faith (i.e. maybe OP really has heard this before)... but usually someone repeating something they heard 3rd or 4th-hand without any real fact-checking.
Milk is high in vitamin D because it's intentionally added. Like iodine in table salt, or fluoride in drinking water. Obviously these things had no role in human evolution before they started in the 20th century.
Vitamin D is only absorbed in the presence of fat in the intestine. Milk is a source of fat. Of course it isn't the only source of fat, but the hypothesized explanation is more plausible than you might think.
I have observed 'seanmcdirmid to be a thoughtful commentator. It would be uncharitable to suggest that he thinks that USDA has regulated milk drinking for millennia.
Most "1st-hand" nutritional research is also wrong. That has been the case for decades, which explains the field's relative lack of progress. The reason is that most nutritional research is funded by large commercial food interests.
Oops. You are right, I was thinking of calcium, which you can get from green veggies as well. I get confused because we had to deal with them all at once when my wife was pregnant.
I know the subthread went in another direction, but: vitamin D increases the absorption of calcium. With minimal vitamin D synthesis from sun exposure in northern Europe, it makes sense that extra dietary calcium would be an evolutionary advantage. Hence, lactose tolerance. So OP's comment made sense even while confusing vitamin d and calcium.
Hah, that's a direction I hadn't considered. That is indeed exactly why vit D is added to milk in the first place. The milk industry claims it's a good source of calcium, but fails to mention it's generally a poorly absorbed source of calcium. Hence the addition of vit D.
Hard cheeses are generally very low in lactose, by the way, but I guess lactose intolerant societies are unlikely to discover this on their own.
Lactose intolerant societies generally have cheese and other forms of curd. Also, babies are generally lactose tolerant regardless, losing this tolerance later.
Lactose tolerance evolved in several different populations independently (including African populations), so I think Vitamin D can only be part of the story. Lactose tolerance gives you access to nutrients from milk, which has a huge impact.
You would need 1) the sun to actually be shining, 2) no cloud cover and 3) probably be naked. Most of us won't get enough vitamin D from being outside for an hour.
1. The key UV for vitamin D production by our skin are UVB rays, optimal between 10am and 2pm, except in the winter at 40+ latitude when the rays travel longer through the atmosphere due to the low solar angle. For instance in Washington state your skin cannot produce vitamin D from November to February.
2. UVB do pass through clouds. (But not through glass windows.)
3. Arms and face exposure are sufficient if you spend 10, 20, or 30 minutes a day depending on UV index of the day (the higher the UV index the less time you need to trigger vitamin D production) and on your skin color. There is some kind of "reservoir" effect that limits production of vitamin D to a maximum, so sun exposure time beyond the above does not have any benefit in terms of vitamin D production.
That's easier said than done for some. A couple of years ago we had an exceptionally bleak November here, with one city for example having only 12 minutes of sunshine in the whole month of November. (I'm not talking about places within the Arctic Circle, although relatively speaking they could almost seem that way for some, of course.)
The average where I live for Nov is about an hour of sunlight per day, but to catch that you'd have to spend all the light hours of day outside (and for sure, November sunshine isn't quite the same as summer sunshine).
It is more complicated than that. A dark cloudy day (no gaps to sky) will not have much UVB. A day with thin high clouds won't block much, and may make it worse (reflecting or scattering).
