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Complications with a low BMI (/ being underweight)o Low blood pressure
o Amenorrhoea
o Fainting/ Dizziness
o Osteoporosis
o Lack of Growth (if still a child/ teenager)
o Heart problems
o Lack of energy (due to low fat stores)
o Feeling cold (due to low fat stores)
o Circulation problems
Taken from the RYL Eating Disorder Guide, which is posted in a sticky at the top of the forum.
Loosing weight quickly can also produce the same problems, even if your BMI isn't low.