You might be interested in the British history of Vitamin D supplementation. It all started with kids in the cities getting rickets because the pollution (smog) was that bad that they never got to see further than a metre or two during the worst of it. The way to get around was by taking the tram as that had rails to guide it through the 'pea soupers'.

So they put the kids on trains and took them off to the seaside.

But then...

The railway also allowed milk to be brought into the cities. So they added vitamin D to milk. That was how the rickets was solved. In time milk became free at school, usually it was warm by morning break, which was when it would get consumed, from mini-milk bottles, that would get reused.

I am only piecing together this history, no definitive source, unless you include my elderly neighbour. However, food history is fascinating, once you get away from celebrated brands to the unsung heroes of the vegetable aisle.

What I can't work out is why the children were so vulnerable to rickets when the adults weren't. Workers weren't being sent out to the countryside or beach to get some sun, just the kids. Rickets doesn't affect adults with grown bones, in theory, the adults should have had really painful joints and osteoporosis, but maybe this was not understood at the time.

In time the clean air zones were setup and the smog was banished to a certain extent, by which time it became uncommon to fortify milk with vitamin D. Finally we had Margaret Thatcher, famously the 'milk snatcher', for stopping free school milk.

In the UK we do get vitamin D randomly added to processed foods (what else?) and this is a scattergun approach to fortifying the population. If you don't eat processed foods then you are not going to get any of that processed food fortification goodness.

Then there are the animal corpse sources, as in oily fish and whatnot. If you eat any diet except for whole-food-plant-based vegan, then you are going to get vitamin D either through dead animal or fortification. Vegetarians just have to eat maaassivve blocks of cheese, which they will, with a few eggs and some breakfast cereal to get their vitamin D needs roughly covered. Junk-food vegans should get some vitamin D goodness from fortifications too, particularly if they consume things like 'oat milk' (as if oats have mammary glands). Pure junk food, a.k.a. 'Standard American Diet', should also be pretty good for vitamin D.

So this only really leaves the whole-food, plant-based, everything-cooked-from-scratch vegan diet as lacking, at least as far as the winter months is concerned. Was this a problem historically? I don't think so. Since people used to work the fields, they had plenty of vitamin D to carry over for winter.

Before we had 'modern day racism' in the UK, we had a situation where the aristocracy had white skin and everyone else had leathery brown skin, from working outside. White skin was proof that you didn't have to work the fields and therefore, you were higher status. Racism pre-dated racism, if you get my drift, it was mere class-based xenophobia back then. To be 'truly white' you had to have no tan.

Since meat was hard to come by, peasants were 95% vegan by default, yet working the fields, so vitamin D deficiency was not a problem, for the 1% aristocracy (since they had their oily fish, red meat and dairy) or for the 99% that had to spend lots of time outdoors.

I am not sure where you are coming from regarding the Gulf Stream and cereals. The Fertile Crescent was where farming began for Europe, with wheat not actually growing in the UK and other grains (barley) being the chosen grain. It was only with the Norman Conquest that wheat made it to the UK.

When the Romans made it to the UK they were perplexed at what they found. There were two tribes, the nomadic cattle types and the hill fort living grain growers that were not nomadic. The hill forts got in the way of the migration routes between pastures. The Romans were disgusted by the milk drinking since nobody would do that in Rome, where everyone was lactose intolerant, unlike the Celts.

> What I can't work out is why the children were so vulnerable to rickets when the adults weren't

Presumably, children need regular and consistent amounts due to bone growth. Once past puberty, less mineralization of calcium and phosphate happens, which is one of the processes in the body that requires vitamin D.