The Independent and Interactive Effects of BMI and Weight Suppression on Symptom Trajectories in Anorexia Nervosa and Bulimia Nervosa
Authors:   , Chen Joanna Y. , Lowe Michael R. 
Published: September 2023

Eating disorders have historically been conceptualized primarily in psychological and cognitive terms, and weight-related variables had received less attention in the literature. However, in the past two decades, weight history variables have shown to be robust correlates and predictors of eating disorder symptoms and trajectories. Specifically, weight suppression, or the difference between one's past highest weight and current weight, and current body mass index (BMI) have been found to be independently and interactively associated with degree of eating pathology in anorexia nervosa (AN) and bulimia nervosa (BN). However, most of these studies were cross-sectional in nature and only included single diagnostic groups. This study extended this body of research by examining the independent and interactive effects of current body mass and history of weight suppression on eating disorder symptoms, depression and anxiety symptoms, and weight change over time. Cross-sectional analyses examined how these weight variables related to symptoms at baseline, and longitudinal analyses examined how they moderated symptom outcome and weight change trajectories through discharge and follow-up. Different methods of computing body mass (BMI vs. zBMI) and weight suppression (traditional vs. developmental weight suppression; TWS vs. DWS) were used to compare the traditional and developmental approaches. Participants in this study were patients with AN and BN at two residential treatment facilities who completed self-report assessments at admission, discharge, and 6-month follow-up. The results showed that at baseline, higher BMI and zBMI were associated with worse global eating disorder symptoms and depression, in both AN and BN, while weight suppression was not independently associated with any symptom measures. Longitudinally, in AN, higher baseline BMI was associated with steeper decreases and rebounds in eating disorder behavioral and global symptoms and depression, while zBMI did not moderate any outcome trajectory. In BN, higher baseline BMI was associated with steeper decreases and rebounds in global eating disorder symptoms and depression, while higher zBMI was associated with steeper decreases and rebounds of eating disorder behavioral and global symptoms, depression, and anxiety. Weight suppression did not independently moderate any outcome trajectory, though the zBMI*DWS interaction moderated all outcomes of interest in BN. Specifically, lower zBMI and DWS were associated with better sustainment of eating disorder symptom improvements following discharge, while higher zBMI and DWS were associated with greater rebounds of depression and anxiety following discharge. Further, across AN and BN, lower BMI and higher TWS were each independently associated with more rapid weight gain, and lower zBMI and higher DWS were independently and interactively associated with more rapid weight gain. This study offers novel perspectives into how current body mass and history of weight suppression independently, interactively, and differentially moderate treatment outcome trajectories in AN and BN. Results of this study continue to suggest the relevance of both body mass and weight suppression to eating disorder symptom and outcome. The findings indicate that the effect of one weight variable often depends on the value of the other, and that their individual and joint influences may only apply to a subset of eating disorder outcomes.