All nutritions from Potatos?

Potato is one of vegetables eaten a lots in all over the world.
However, it does not contain every nutrition that human body needs for its activity.
So does other vegetables. One species of vegetables can not be like a multi-vitamin tablet.
I thought that this concept is obvious, but it seems not.

In the movie,"Super size me", there is a interview to a high school student at the cafeteria.
She grabed fries and said, "This is my vegetable!".
If this is the only case that somebody said, I would not care so much, but
I had another chanse to hear the same opinion.
It was in the making film of the movie "School of Rock" starred by Jack Black.
When he grabbed a burger and fries, he said the same thing to the camera man.

It could be only in my family in Japan, but when I was a kid,
my mother told me that necessary nutritions for human body in a day are equivalent to
ones from 30 verious ingridients.
It is very difficult to use and cook 30 ingridients in a meal.

I guess there is no or little education about nutritions in junior high and high schools.

For a dinner, only cheese pizza? C'mon!