This ^^^. This is how anorexics living on 300-500 calories per day can live for like a decade. Your body starts shutting down otherwise essential bodily functions just to keep you alive, thinking you'll eventually start eating again and it can kick them back into gear.  I survived years on 500-800 calories a day. When I went into recovery, I started eating 1500 calories a day and gained weight like crazy for almost a whole year until I got to like 10lbs below my normal healthy weight. Now, I lose weight on anything less than 1800 a day. I could lose 15lbs and be below that weight when my metabolism changed on 1800 now though(although I'd technically be back in anorexic bmi then, but just barely), because my body would maintain a similar bmr because I wouldn't be restricting calories so severely for a long time.