Early childhood adversity and body mass index in childhood and adolescence: linking registry data on adversities with school health records of 53,401 children from Copenhagen

Childhood adversity, including economic hardship, family illness, and parental separation, is associated with morbidity and mortality across the life course. Potential mechanisms include effects on health behaviors and physiological stress responses that may influence BMI in childhood and adolescence, which in turn is linked to adult morbidity and mortality. Prior evidence on associations between childhood adversity and child/adolescent BMI is mixed, potentially due to differences in adversity definitions, timing, and populations studied. This study leverages objective Danish registry data to investigate whether early childhood adversity (ages 0–5) is associated with BMI at school entry (6–7 years) and adolescence (12–15 years), considering the type, timing, and duration of adversity. By combining nationwide adversity data with school health measurements in an unselected large sample, the study aims to clarify if childhood BMI could serve as an early pathway linking adversity to later-life health.

Systematic reviews have reported mixed findings regarding adversity-BMI associations in youth. One review found no clear association between childhood maltreatment and obesity in children/adolescents, while two others suggested a positive association between adverse events and BMI/overweight/obesity. Heterogeneity in definitions and measurement of adversity, exposure and outcome time frames, and study populations likely contribute to inconsistency. The literature highlights the importance of considering the type of adversity, timing, and duration of exposure, and calls for longitudinal studies with large, unselected samples using objective measures of adversity and BMI.

Design and data sources: The study linked two Danish resources: (1) DANLIFE, a nationwide register-based cohort including childhood adversities for all Danish births from 1980–2015, and (2) the Copenhagen School Health Records Register (CSHRR), which contains objectively measured height and weight from universal school health examinations for children born 1930–1996. Sample: Included were children born 1980–1996 who were present in both datasets, survived and did not emigrate before age 6, and had at least one BMI measure at ages 6–7 and/or 12–15 years (n = 53,401). Approvals: Personal data processing was approved by the Faculty of Health and Medical Sciences, University of Copenhagen, on behalf of the Danish Data Protection Agency; ethical approval is not required for register-based studies in Denmark. Exposure: Early childhood adversity (ages 0–5) was characterized using group-based multi-trajectory modeling (Stata traj) on annual counts of adversities across three dimensions: (a) material deprivation (family poverty, parental long-term unemployment); (b) loss or threat of loss (parental/sibling somatic illness and death); (c) family dynamics (foster care placements, maternal separation, sibling psychiatric illness, parental psychiatric illness, alcohol and drug abuse). Zero-inflated Poisson models with cubic trajectory functions were fitted. Solutions with 4–6 groups were evaluated; the optimal number was chosen by interpretability and adequate group sizes. Each child was assigned to the most likely group; a post-hoc probability-weighted analysis was also performed. Outcomes: BMI was calculated from measured weight/height at school exams. Childhood BMI was assessed at ages 6–7 (prioritizing closest to 7 years). Adolescent BMI was assessed at ages 12–15 (prioritizing closest to 14 years). Sex-specific BMI z-scores were derived using internal LMS reference curves (VGAM lms.bcn) built from all CSHRR children born 1980–1996. Covariates: Maternal age at birth (<20, 20–30, >30 years), parental region of origin (Western vs non-Western), parental cardiometabolic illness in the three years before birth (yes/no; based on national patient and causes of death registers; including IHD, cerebrovascular disease, CHF, peripheral vascular disease, type 1 and type 2 diabetes), and birth year. Additional analyses further adjusted for parental education at birth (<10, 10–12, >12 years), size for gestational age (<10th, 10–90th, >90th percentile), and preterm birth (<37 weeks). Statistical analysis: Sex-stratified structural equation models estimated (a) direct associations between adversity groups and childhood BMI z-score; (b) direct associations with adolescent BMI z-score (adjusted for childhood BMI z-score); and (c) total associations with adolescent BMI z-score (direct plus indirect via childhood BMI). Main models adjusted for parental origin, maternal age, parental pre-birth cardiometabolic illness, and birth year. Additional models further adjusted for parental education, and for size for gestational age and preterm birth. A sensitivity analysis omitted adjustment for parental cardiometabolic illness. SEM was implemented in Stata 14 (sem, mlmv with FIML for missing data).

- Five adversity trajectory groups (0–5 years) were identified: low adversity 51.3%, moderate material deprivation 30.0%, high material deprivation 13.9%, loss or threat of loss 3.0%, high adversity 1.7%. - Boys: • Childhood (6–7y) BMI z-score: High material deprivation associated with slightly higher BMI (b = 0.08; 95% CI: 0.04, 0.12) vs low adversity. High adversity associated with slightly lower BMI (b = −0.12; 95% CI: −0.22, −0.02). Trends toward higher BMI for moderate material deprivation (b = 0.03; 95% CI: 0.00, 0.05) and loss/threat of loss (b = 0.07; 95% CI: −0.01, 0.14), the latter crossing zero. • Adolescence (12–15y) total effects: Moderate material deprivation b = 0.03 (0.00, 0.06); high material deprivation b = 0.10 (0.06, 0.14); loss/threat of loss b = 0.10 (0.02, 0.18); indicating slightly higher BMI z-scores vs low adversity. - Girls: • Childhood (6–7y) BMI z-score: High material deprivation associated with slightly higher BMI (b = 0.07; 95% CI: 0.03, 0.10) vs low adversity. • Adolescence (12–15y) direct effects: High material deprivation b = 0.07 (0.04, 0.10); moderate material deprivation b = 0.04 (0.01, 0.06); loss/threat of loss b = 0.15 (0.08, 0.21). • Adolescence total effects: Moderate material deprivation b = 0.04 (0.01, 0.07); high material deprivation b = 0.11 (0.07, 0.15); loss/threat of loss b = 0.18 (0.10, 0.26) vs low adversity. - Effect sizes were small; associations were generally stronger in adolescence than childhood, particularly for material deprivation and loss/threat of loss. - Boys in the high adversity group showed a small decrease in BMI in childhood; no strong associations were observed for the high adversity group otherwise. - Results were robust across adjustment sets (including models with parental education, gestational size, preterm birth) and in sensitivity analyses excluding parental cardiometabolic illness. - Clinical translation examples: a 0.10 BMI z-score difference at age 14 corresponds to ~0.8 kg (~1.5%) in boys; a 0.18 z-score difference corresponds to ~1.5 kg (~2.8%) in girls.

The study addressed whether early childhood adversity predicts BMI at school entry and in adolescence using objective, longitudinal data. Findings indicate only small associations overall, with patterns differing by adversity dimension. Material deprivation and loss/threat of loss were modestly associated with higher adolescent BMI in both sexes, with the loss/threat of loss association particularly pronounced among girls. High adversity characterized by family dynamics showed a small decrease in BMI in boys during childhood and no clear adolescent association, suggesting heterogeneity in physiological and behavioral responses to different adversity types. Although individual-level clinical relevance is limited due to small effect sizes, slight differences that emerge by adolescence may track into adulthood and could contribute to later-life morbidity at the population level. The results align with prior research showing mixed or weak associations between adversity and youth obesity, emphasizing the importance of adversity type, timing, and duration. Potential mechanisms include constrained resources influencing diet and activity in material deprivation, and psychosocial stress with impacts on mental health and support systems in loss/threat of loss. Sex-specific patterns were limited, with some indication of stronger effects in girls for loss/threat of loss and a boy-specific lower BMI in the high adversity (family dynamics) group, but overall no clear, consistent sex differences emerged.

In a large, population-based cohort with objective measurements, early childhood adversity was associated with only small differences in BMI by adolescence. Associations depended on adversity type, with material deprivation and loss/threat of loss linked to slightly higher BMI, and high family dynamics adversity linked to slightly lower BMI in boys in childhood. These findings suggest that childhood and adolescent BMI are unlikely to be major pathways linking early adversity to later-life morbidity. Preventive efforts to reduce childhood adversity remain important for overall health, recognizing that different adversity types may differentially affect weight and health. Future research should further disentangle mechanisms by adversity dimension and examine long-term trajectories into adulthood.

- Group-based trajectory assignment introduces uncertainty; small group sizes (e.g., high adversity 1.7%) may limit power and precision. - Lack of direct measures of household violence or maltreatment; proxies (e.g., foster care) were used, which may not fully capture such exposures. - Potential residual confounding (e.g., genetic, neighborhood factors) cannot be ruled out. - Variable intervals between childhood (6–7y) and adolescent (12–15y) BMI measurements (typically ~7 years but as short as ~4 years) may affect detection of adolescent associations. - Use of group-based models may mask within-group heterogeneity; posterior probability weighting post-hoc yielded similar, but in some cases slightly weaker, associations. - Generalizability is to Danish children in Copenhagen; context-specific factors may limit extrapolation to other settings.