When you eat too far below your maintenance level in a chronic way, then your body thinks it's starving and switches on some survival tactics. One of those is to conserve as much fat as possible and turn metabolism way down. When metabolism gets turned down, the basic functions of the body are compromised. The brain has less energy to spend, and consequently so do the other organs and glands and everything. Lean mass is catabolized for energy before stored fat, which is the opposite of how a healthy body functions. They may not be eating much, but they're not burning much, either. And their thyroids, among other things, are going to pay for it in a few years.
Note, this is completely different from intermittent fasting. This is eating under 1000 kcal for days and weeks and longer.
You lousy kids! Get off my savannah!