I am 80 pounds overweight and plan on eating nothing but negative calorie foods until I get down to my ideal weight. What would happen if I were to continue on this diet regimen?

You need protein and fiber for food to properly digest.

You would most likely get constipated and possibly impacted. Also, you would suffer from some terrible nutritional deficits.

I am 80 pounds overweight and plan on eating nothing but negative calorie foods until I get down to my ideal weight. What would happen if I were to continue on this diet regimen?
I've tried something similar, and it has worked wonders.
I eat primarily fruits and vegetables (like 90%).
I eat nuts and drink soymilk for protein. Very occasionally I eat lean chicken.

I always have lots of energy and have lost a lot of weight and have maintained it for two years.
The great thing about fruit - you can eat as much of it as you want!

But yeah, you need to find ways to get protein.