Premorbid body weight predicts weight loss in both anorexia nervosa and atypical anorexia nervosa: Further support for a single underlying disorder

Abstract

ObjectiveFor adolescents, DSM‐5 differentiates anorexia nervosa (AN) and atypical AN with the 5th BMI‐centile‐for‐age. We hypothesized that the diagnostic weight cut‐off yields (i) lower weight loss in atypical AN and (ii) discrepant premorbid BMI distributions between the two disorders. Prior studies demonstrate that premorbid BMI predicts admission BMI and weight loss in patients with AN. We explore these relationships in atypical AN.MethodBased on admission BMI‐centile < or ≥5th, participants included 411 female adolescent inpatients with AN and 49 with atypical AN from our registry study. Regression analysis and t‐tests statistically addressed our hypotheses and exploratory correlation analyses compared interrelationships between weight loss, admission BMI, and premorbid BMI in both disorders.ResultsWeight loss in atypical AN was 5.6 kg lower than in AN upon adjustment for admission age, admission height, premorbid weight and duration of illness. Premorbid BMI‐standard deviation scores differed by almost one between both disorders. Premorbid BMI and weight loss were strongly correlated in both AN and atypical AN.DiscussionWhereas the weight cut‐off induces discrepancies in premorbid weight and adjusted weight loss, AN and atypical AN overall share strong weight‐specific interrelationships that merit etiological consideration. Epidemiological and genetic associations between AN and low body weight may reflect a skewed premorbid BMI distribution. In combination with prior findings for similar psychological and medical characteristics in AN and atypical AN, our findings support a homogenous illness conceptualization. We propose that diagnostic subcategorization based on premorbid BMI, rather than admission BMI, may improve clinical validity.Public significanceBecause body weights of patients with AN must drop below the 5th BMI‐centile per DSM‐5, they will inherently require greater weight loss than their counterparts with atypical AN of the same sex, age, height and premorbid weight. Indeed, patients with atypical AN had a 5.6 kg lower weight loss after controlling for these variables. In comparison to the reference population, we found a lower and higher mean premorbid weight in patients with AN and atypical AN, respectively. Considering previous psychological and medical comparisons showing little differences between AN and atypical AN, we view a single disorder as the most parsimonious explanation. Etiological models need to particularly account for the strong relationship between weight loss and premorbid body weight.

Differentiating anorexia nervosa and atypical anorexia nervosa with absolute weight cut‐offs results in a skewed distribution for premorbid weight among youth

“…In the current study, we sought to provide further support for a homogenous conceptualization of AN and atypical AN by replicating and expanding upon Hebebrand and colleagues' parallel study (Hebebrand et al, 2024). Specifically, we assessed whether differentiating AN and atypical AN by clinical 'underweight' (i.e., BMI% < 5th) yields a skewed premorbid weight distribution in which youth with premorbid 'overweight/obesity' are disproportionally diagnosed with atypical AN versus AN.…”

“…Notably, our use of midpoint BMI% to calculate premorbid BMI% differed from Hebebrand et al (2024) methodology, which calculated premorbid BMI% with (a) self-or parent reported premorbid weight prior to initial weight loss (kg); (b) self-or parent reported age at onset of initial weight loss; and (c) measured admission height. We examined correlations between our respective methodologies; in a random 10% of participants, premorbid BMI% values were calculated using Hebebrand's method and compared to premorbid midpoint BMI% data.…”

“…For example, prior studies demonstrate significant correlations between premorbid BMI and presenting BMI and premorbid BMI and weight loss in hospitalized youth with restrictive eating disorders (Coners et al, 1999;Föcker et al, 2015;Peters et al, 2021). In their parallel study, Hebebrand et al (2024) found that premorbid BMI z-scores (BMIz) predicted admission BMIz and to a larger extent, total weight loss among hospitalized youth with AN and atypical AN. A skewed distribution for premorbid BMI was also demonstrated, with an overrepresentation of 'higher' premorbid BMIs in atypical AN and an underrepresentation of 'higher' premorbid BMIs in AN, suggesting a disproportionate weight loss requirement for AN among youth in larger bodies.…”

“…In contrast, youth with atypical AN demonstrate a greater prevalence of premorbid 'overweight/obesity' (i.e., BMI% ≥ 85th) (Lin et al, 2023;Matthews et al, 2023;Walsh et al, 2023). However, burgeoning research suggests that even in terms of weightrelated aspects, AN and atypical AN do not differ, with similar weight loss across diagnoses and an underlying regulatory mechanism in which premorbid BMI is associated with presenting weight (Coners et al, 1999;Föcker et al, 2015;Hebebrand et al, 2024;Peters et al, 2021). For example, prior studies demonstrate significant correlations between premorbid BMI and presenting BMI and premorbid BMI and weight loss in hospitalized youth with restrictive eating disorders (Coners et al, 1999;Föcker et al, 2015;Peters et al, 2021).…”

“…A skewed distribution for premorbid BMI was also demonstrated, with an overrepresentation of 'higher' premorbid BMIs in atypical AN and an underrepresentation of 'higher' premorbid BMIs in AN, suggesting a disproportionate weight loss requirement for AN among youth in larger bodies. Although Hebebrand's study precluded a well-powered analysis of clinically ascertained patients with atypical AN, the authors propose a homogenous illness conceptualization, with a common AN-like phenotype underlying both diagnostic groups (Hebebrand et al, 2024).…”