Question: how many calories will you lose if you don't eat for a day?
Answer: You will likely lose weight on any diet if you eat less than 910 calories a day. But losing 10 pounds in 3 days is both unlikely and unhealthy. To lose just 1 pound of body fat, you need to reduce your daily calories by about 500 a day for a whole week. That's giving up 3,500 calories over the course of 7 days.