I think one of the main healthy ingredient lost to processing is fiber. It is good for moving food through your digestive system, plus it moderates harmful glucose spikes which is very unhealthy.

Potatoes have a lot of fiber in the skin, but fries remove all that.

(I've also been told, eating potatoes are like eating a plate of sugar)

Because they're usually cut up, soaked in a salt bath, coated, then deep fried in a trans fat.

This takes a complex carbohydrate, strips it of most of its fiber content, adds sodium and then adds trans fats through the deep frying process.

I mean, they typically have their skin removed then are soaked in some kind of saline bath. At least half if not more of the fiber in a potato is remove when you skin it. Then oil based frying adds a massive amount of calories.

If you need calories to survive potatoes are a good option, once you've met those minimum requirements they are a pretty terrible food in the sense it's really easy to over consume them.

often dipped in batter/corn starch