I don't think it has to do with BMI, per say. It has somewhat to do with weight, but mostly a lack of menustration is due to a lack of nutrients.
I think if you're on your way to recovery and are coming out of malnutrition, then your period will come back regardless of your weight. I may be wrong, but that's what makes sense to me.
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Too blunt your knife to slay this dreamer
-The Gathering, A Life All Mine