When potatoes are fried, they lose water through evaporation and gain oil. From 15% to 20% of their weight is made up of fats.
Fried potatoes contain 500 to 600kcal/100g (raw potatoes contain only 79kcal/100g). They also tend to contain abundant salt. All of this makes them rather undesirable from a dietary stand point.
The best oil for frying potatoes is olive oil since it is the most resistant to high temperatures. Still, care must be exercised not to overheat it. In other words, it must not smoke.
Fried potatoes are tasty, but not very healthful.