[Retric]

People most in need of vitamin A don’t realize or can’t pay a premium for something else.

Ignorance is a big part of why foods get fortified. Scurvy is making something of a comeback among wealthy kids eating an unhealthy diet because high temperatures destroys Vitamin C so you need to eat un/minimally cooked foods or have it added back in. https://www.timesofisrael.com/scurvy-makes-a-shocking-comeba...

[triceratops]

I think the unhealthy diet is the fundamental reason, not cooking food at high temps.

"The amount of vitamin C retained from raw potatoes varied according to the method of cooking, e.g. about 80% for potatoes boiled in skin and about 30% for hash-browned potatoes."

Source: https://www.sciencedirect.com/science/article/abs/pii/S03155...

[Retric]

Both of those are at the low end of cooking time * temperature. Mashed potatoes provide essentially zero vitamin C same with many potato soups. https://fdc.nal.usda.gov/fdc-app.html#/food-details/170699/n...

365 calories of raw potatoes have 155% of your daily vitamin C requirement. A 365 calories serving of French fries has ~9% of your daily vitamin C requirements. Of course the cooking oil is part of that difference, but few cooked foods have much vitamin C. Worse many things like rice, bread, or beer provide none.

So you need some aspect of your diet to be a plentiful supply or take a supplement.