[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.