A study showed some people with type 2 diabetes were cured of the disease after going on a 600-calorie-a-day diet for 8 weeks. So I'm thinking maybe semi-fasting like this for a long period each year could be a good re-set for your metabolism. It makes sense from a paleo perspective as our ancestors likely had to go without much food periodically during the year. 600 calories is not much though. I would probably last a week to ten days and then cave in. How about you?
EDIT BY PATRIK -- Hopefully, this question is just a silly, theoretical exercise: MY OPINION: DO NOT DO THIS. THIS SOUNDS LIKE A GREAT WAY TO DAMAGE YOURSELF PERMANENTLY. SEE PERSONMAN'S ANSWER BELOW.