Those advocating 800 calories a day have either never tried it or are vastly underestimating the number of calories they eat. You do not need research or a doctor to tell you it is unhealthy. You know it is not healthy. You can feel it.
If I try it for over a few days I get headaches, nausea, difficulty concentrating, constantly freezing cold, lethargy, and a weird emptiness. It's not hunger, it's beyond that. It's difficult to describe. It's like your whole body is empty. It's an uncomfortable, kind of crampy sensation. The tiredness is something else. I live on caffeine pills imported from Canada where they are allowed to sell stronger pills. I can take 600mg of caffeine and then nap 30 minutes later. My entire day is spent thinking of food and waiting for bedtime but once I do get to bed I cannot sleep properly because of the weird empty feeling.