What would happen if I just ate lettuce everyday?

So I'm on a max 800 calorie diet and I'm trying to figure out if drinking lettuce and eating coke zero would keep me happy. I dont care about taste, I have forgotten about meat.
lettuce is around 8 calories per serving and I love soda. I am planning on losing 60 lbs very fast In 5 months
What would happen if I just ate lettuce everyday?
Post Opinion