Ethnic and Social Health Inequalities in Body Mass Index Trajectories through Childhood and Adolescence: A Longitudinal Population-Based Study in Leicestershire UK

Background: Ethnic minorities and socioeconomically disadvantaged populations in the UK are at increased risk of obesity. We modelled longitudinal body mass index (BMI) trajectories through infancy, childhood, and adolescence to identify at-risk groups and modifiable risk factors. Methods: This cohort sampled 10,350 White and South Asian children born in Leicestershire, 1985-1997. We included 5,571 participants with [≥]3 BMI measurements between 0-18 years collected from healthcare records, questionnaires, and study visits. We used Group-Based Trajectory Modelling of BMI, separately by sex and ethnicity, and combined. We identified at-risk groups and modifiable risk factors using multinomial logistic regression, with inverse probability weighting to reduce selection bias. Results: We identified similar five BMI trajectories across sex and ethnicity: stable normal BMI (47%); persistent low BMI (30%); early overweight resolving (8%); childhood onset obesity (4%); and adolescent onset overweight (11%). Childhood onset obesity deviated from stable normal BMI at 2-4 years of age, adolescent onset overweight at 4-6 years. South Asians were at higher risk of childhood onset obesity (aOR: 1.66 [95%CI 1.08-2.53]) and adolescent onset overweight (1.29 [0.98-1.71]) than Whites. Children from deprived backgrounds (1.66 [0.92-2.82], most vs least deprived quintile) and those with less educated parents (1.67 [1.08-2.63], compulsory vs higher education) were at increased risk of childhood onset obesity. Smoking during pregnancy (1.50 [0.88-2.54]) and absence of breastfeeding (1.56 [1.07-2.29]) increased risk of childhood onset obesity. Physical activity decreased risk of childhood onset obesity (0.64 [0.44-0.93], [≥]4 vs 0-3 hours/week) and adolescent onset overweight (0.75 [0.59-0.94]). Conclusion: BMI trajectories diverge as early as age 2 years, revealing ethnic and social inequalities. Obesity strategies in the UK should intervene during critical windows in early life and prioritise South Asian children and those from socioeconomically deprived backgrounds.