I am sorry but 'a real porker' is not eating 500-800 calories per day with any regularity, that's physically impossible, no matter how inactive. I eat 1800-2500 calories per day and I'm right at the bottom of the 'normal' bmi range (and no, I am NOT overestimating what I eat, it's fats who grossly underestimate what they eat). An overweight person is eating many thousands of calories on average per day by habit. Perhaps an overweight person may occasionally undereat a few days of the year but labelling this 'anorexia' is the equivalent of labelling someone an alcoholic because they got drunk once over the holidays. I can even buy that an overweight person might only eat 500-800 cals for a week. But in that case I would call it a crash diet or maybe fasting, not mental illness.