Hi Joziel, you seem very knowledgeable, how do you know all this, is it your job?
However, I don't understand how the above quote can be true, as someone who is anorexia still carries on losing weight.
I know all this from hard-won experience and a lot of personal suffering, ended by research and understanding. (And a lot of help from people who are experts in their field.)
As for how someone who is anorexic continues to lose weight, there is a point below which everyone will lose weight. After all, you don't see concentration camp victims gaining weight....
But unless you are literally trying to eat as little as possible (which is anorexia) and unless you are existing on something like 300 calories a day, you will gain weight if you are eating 1200-2000 or below maintenance long-term.
The body has many mechanisms (thyroid is one) to down-regulate metabolism at times when food is scarce. That's how our ancestors survived. You are telling your body that it is a period where food is scarce and so the body should be economical with energy expenditure - to survive. Sex hormones are one of the things which get down-regulated because reproduction is not a priority at times of scarcity.
Typical maintenance calories are something like 2000-2200 for most women and that's where you should be, most of the time. That will ensure hormones are optimal and the body is able to function optimally. If you want to lose weight, you need to go into a deficit of between 200-500 calories (depending on how resistant your body is) but for no longer than 12 weeks max. Then you need to reverse diet back up out of it again and go back to maintenance. If you didn't lose enough weight and want to lose more, you repeat the process after a period of time back at maintenance.
You can't stay long-term in a caloric deficit or your metabolism will slow down, you won't lose more weight and you will be stuck eating hardly anything. Then you cut calories again - and so on.... until you end up gaining weight whilst eating below maintenance.
You can find this info on loads of fitness Instagram accounts for women these days, by the way. As well as in that excellent Metabolism and Menopause podcast I mentioned.