Identifying the Critical Window and Adiposity Trap in Spanish Children: Longitudinal Evolution and Transition Probabilities of BMI Z-Scores from a Clinical Registry (2021–2026)

Background: The prevalence of pediatric obesity in Spain is among the highest in Europe. Although national surveillance reports a 4.5% decrease in excess weight, these primarily cross-sectional data may underrepresent the metabolic complexity observed in clinical settings. We aim to characterize the evolution of abnormal adiposity and to evaluate the transition probabilities between body mass index (BMI) categories in a multi-regional Spanish cohort. Methods: We analyzed Real-World Data (RWD) from 37,465 children and adolescents (ages 0–18) treated at HM Hospitales “Observatorio de la Obesidad Infantil,” between 2021 and 2026. Longitudinal BMI z-score trajectories were evaluated using continuous-time Markov chain models to determine transition probabilities between weight categories and identify developmental windows of adiposity plasticity. Results: At baseline, 21.6% of the cohort presented overweight/obesity, with a higher frequency of overweight in females than males (13.0% and 11.2%, respectively; p < 0.01) and obesity similar between sexes (~10%). Markov modeling identified a “critical window” in children aged 2–5, who demonstrated a 75% probability of reverting to a normal weight. Conversely, children aged 6–11 entered an “adiposity trap,” characterized by a 60.0% obesity persistence rate and a 23.2% risk of progressing from overweight to obesity. A sharp initial drop in z-scores was observed immediately following the baseline clinical assessment across all groups. Conclusions: Our study reveals a significant window of opportunity for abnormal adiposity improvement between the ages of two and five, which contrasts with the adiposity inertia (diminished category fluidity) observed after age five.