Background: Family Based Behavioral Treatments (FBBT) are reported to have a favorable impact on the short and mid-term evolution on the body mass index (BMI) of adolescents suffering from obesity. This study investigated the long-term BMI z-score evolution, as well as variables associated with favorable or unfavorable evolution in adolescents who beneficiated from FBBT group therapy treatment for obesity.

Methods: This was a prospective study including adolescents who participated in FBBT group therapy for obesity sessions (n = 131). All adolescents were invited for a study's clinical interview 4 years after the therapy, during which their weight and height were measured, and they answered a questionnaire on some life habits. Anthropometric measurements at the time of therapy as well as socio-demographic data were retrospectively extracted from the electronic medical record.

Results: Seventy-six subjects (57% of the sample) accepted to participate in the study. At the study's clinical interview (mean 5.5 years after FBBT), 52.6% of the adolescents showed a favorable evolution of their weight status defined as a decrease (>-0.2) or stabilization (between - 0.2 and + 0.2) of their BMI z-score. 32% were engaged in a daily physical activity and 40.6% monitored their weight at least once a week or more. Only these 2 variables were associated with a favorable evolution (p = 0.009 and p = 0.001, respectively). Less than half of the sample (45.9%) have maintained a medical weight-management follow up, of which 67% had a BMI equal or more than 30.

Conclusions: Long-term evolution of the BMI z-score was favorable for most of the adolescents, emphasizing the potential benefits of FBBT treatment on the long term for adolescents suffering from obesity. This study highlighted the difficulty for long-term weight management follow up in this population at risk of numerous medical comorbidities, confirming the need to improve adherence to weight management treatment at this age of transition of care. Prospective observational study registered.