Do you see the beautiful singer on the left? In her teens and twenties, she was on top of the world. She was half of one of the most popular musical groups of her time, selling millions of albums and earning millions of dollars.
Do you see the picture on the right? It’s the same singer, at age 31. The puffed-out hair and baggy clothes were her attempt to disguise the fatally underweight condition she had acquired through years of chronic under-eating. She was trying to recover, but it was too late. She was dead within a year.
The answer to your question is that if you only eat 50 calories every three days, you will die. You will die a slow, and probably painful death of multiple organ failure and opportunistic disease.
Edit: First point: I deliberately did not say who the singer was because I felt it would be a more powerful answer without naming her. There have been enough comments criticizing this choice, though, that I will add her name. It is Karen Carpenter. (When I said she had been on “top of the world”, that was a bit of a hint.) Second point; I know that it is possible to interpret the question differently, but I checked the history of posts by the person who asked, and based on that history I decided that the intention behind the question was most likely the more extreme interpretation.