1 Sainte-Justine Hospital Research Centre, Montreal, Canada.
2 Department of Psychiatry and Addictology, University of Montreal, Montreal, Canada.
3 Department of Psychology, Concordia University, 7141 Sherbrooke Street West, Montreal, QC, H4B 1R6, Canada.
4 School of Public Health, University of Montreal, Montreal, Canada.
5 School of Epidemiology and Public Health, University of Ottawa, Ottawa, Canada.
6 School of Psychoeducation, University of Montreal, Montreal, Canada.
7 Department of Psychology, University Laval, Quebec, Canada.
8 Department of Psychology and Pediatrics, University of Montreal, Montreal, Canada.
9 Sainte-Justine Hospital Research Centre, Montreal, Canada. linda.booij@concordia.ca.
10 Department of Psychiatry and Addictology, University of Montreal, Montreal, Canada. linda.booij@concordia.ca.

Background: Adolescence is a critical period for the development of eating disorders, but data is lacking on the heterogeneity of their evolution during that time-period. Group-based trajectories can be used to understand how eating disorders emerge and evolve over time. The aim of this study was to identify groups of individuals with distinct levels of eating disorder symptoms between 12 and 20 years and the onset of different types of symptoms. We also studied sex differences in the evolution and course of eating disorder symptoms from early adolescence to adulthood.

Methods: Using archival data from the QLSCD cohort, trajectories of eating disorder symptomatology were estimated from ages 12 to 20 years using semiparametric models. These trajectories included overall eating disorder symptomatology as measured by the SCOFF (Sick, Control, One Stone, Fat, Food), sex, and symptom-specific trajectories.

Results: Two groups of adolescents following distinct trajectories of eating disorder symptoms were identified. The first trajectory group included 30.9% of youth with sharply rising levels between 12 and 15 years, followed by high levels of symptoms between 15 and 20 years. The second trajectory group included 69.1% of youth with low and stable levels of symptoms between 12 and 20 years. Sex-specific models indicated that the proportion of girls in the high trajectory group was 1.3 times higher than the proportion of boys (42.8% girls vs. 32.3% boys). Trajectories of SCOFF items were similar for loss-of-control eating, feeling overweight, and attributing importance to food. The weight loss item had a different developmental pattern, increasing between 12 and 15 years and then decreasing between 17 and 20 years.

Conclusions: The largest increase in eating disorder symptoms in adolescence is between the ages of 12 and 15 . Yet, most prevention programs start after 15 years of age. Our findings suggest that, unlike common practices, eating disorder prevention programs should aim to start before puberty.