1.

Individual purposely takes in too little nourishment, resulting in body weight that is very low and below that of other people of similar age and gender.

2.

Individual is very fearful of gaining weight, or repeatedly seeks to prevent weight gain despite low body weight.

3.

Individual has a distorted body perception, places inappropriate emphasis on weight or shape in judgments of herself or himself, or fails to appreciate the serious implications of her or his low weight.

(Information from: APA, 2013.)