Purpose: The aim was to examine duration of illness and body mass index as possible moderators of the relationship between eating disorder severity and functional impairment, as well as psychological distress as a possible mediator of this relationship.

Methods: The study included 159 patients diagnosed with bulimia nervosa, binge eating disorder or eating disorder not otherwise specified. Regression analysis was applied to assess the effect of the hypothesized moderators and mediators. Eating disorder severity was measured with the Eating Disorder Examination Questionnaire, functional impairment was measured with the Sheehan Disability Scale, and psychological distress was measured with the Symptom Check List-90-R. Duration of illness and body mass index were assessed clinically.

Results: Duration of illness significantly moderated the relationship between eating disorder severity and functional impairment; the relationship was strongest for patients with a shorter duration of illness. Psychological distress partly mediated the relationship between eating disorder severity and functional impairment. Duration of illness significantly moderated the relationship between psychological distress and functional impairment; the strongest relationship was seen for patients with a shorter duration of illness. Body mass index was not a significant moderator of the relationship between ED severity and functional impairment.

Conclusions: Overall, this study established a link between ED severity, psychological distress and functional impairment indicating that both eating disorder severity and psychological distress are more strongly related to impaired role functioning for patients with more recent onset of an eating disorder. More research in the complex relationship between ED severity and functional impairment is needed.