One interesting aspect of Vitamin D that is not regularly discussed is the photo-isomerization of Vitamin D into inactive isomers in the skin.
I think of it similarly to Beta-Carotene, in that consuming the "natural source" has no known toxic amount, while consuming processed vitamin A can be toxic in large doses.
Increasing sun exposure will not raise blood vitamin D metabolite concentration indefinitely, and we don't have good studies that accurately measure the amounts of Vitamin D and its photo-isomers and their concentration in relation to large doses of UV exposure.
Therefore, through sun exposure, people may be storing large doses of Vitamin D isomers in their skin, keeping their levels much higher than typically prescribed oral supplementation levels.
When I did my master's thesis on Vitamin D supplementation the conclusion was that vitamin D testing is wildly inaccurate, and we needed to have studies based on a target level - ideally one based around sub-saharan African community levels.
As for supplementation, sunlight has been estimated at providing up to 20,000IU per day, which I think (not medical advice due to the lack of study), from a mechanistic point of view, suggests an upper limit to daily supplementation.
You may have a point about processing, but I think by talking about "most people" you have invalidated any future points you may be trying to make.
For example:
"single natural ingredient"
not every cow is only fed with grass, and what about that grass, has it been treated, etc...
also
Neu5Gc
Mammal meat contains it, Humans have lost the enzyme (perhaps over that time since "the dawn of man"), it causes inflammation.
Looking at chimpanzee diets, I don't think our common ancestor was regularly eating burgers. More likely insects and leaves...which do not contain Neu5Gc.
As a self-proclaimed "healthy person", I'm not regularly eating either of these, but unless I know where the meat comes from, I'm likely sticking to the non-inflammatory burger.
I think of it similarly to Beta-Carotene, in that consuming the "natural source" has no known toxic amount, while consuming processed vitamin A can be toxic in large doses.
Increasing sun exposure will not raise blood vitamin D metabolite concentration indefinitely, and we don't have good studies that accurately measure the amounts of Vitamin D and its photo-isomers and their concentration in relation to large doses of UV exposure.
Therefore, through sun exposure, people may be storing large doses of Vitamin D isomers in their skin, keeping their levels much higher than typically prescribed oral supplementation levels.