# Bidirectional associations between smart device use and body mass index among children aged 3 to 5 years: a longitudinal study

## TL;DR

This study finds that smart device use and BMI in young children influence each other over time, with parent-child interactions playing a protective role.

## Contribution

The study reveals bidirectional links between smart device use and BMI in preschoolers and highlights the moderating role of mother-child interactions.

## Key findings

- Higher BMI at age 4 was linked to increased device use at age 5.
- Low-quality mother-child interactions strengthened the link between early device use and later BMI increases.
- High-quality interactions reduced the negative impact of device use on BMI.

## Abstract

The increase in smart device use, including smartphones and tablets, among young children has raised concerns about its impact on health, particularly on body mass index (BMI). However, the bidirectional associations between smart device use and BMI in preschoolers remain unclear. This study examined the longitudinal associations, considering the moderating effects of mother-child interactions and child sex.

Data were obtained from the Longitudinal Examination Across Prenatal and Postpartum Health in Taiwan, a cohort study conducted in Taipei, Taiwan. In total, 590 preschoolers were assessed at ages 3, 4, and 5 years. Smart device use, BMI z-scores, and mother-child interaction quality were evaluated using validated parent-reported questionnaires. The random-intercept cross-lagged panel model was used to investigate bidirectional associations, adjusting for stable confounders. Multiple-group models examined the moderating effects of mother-child interactions and child sex. Model estimates were reported as standardized coefficients.

Higher BMI z-scores at age 4 years were linked to increased device use at age 5 years (β = 0.36; 95% CI, 0.05–0.67). Multiple-group models revealed that among dyads with lower mother-child interactions, higher device use at age 3 years was associated with higher BMI at age 4 years (β = 0.40; 95% CI, 0.07 to 0.72), which was subsequently linked to greater device use at age 5 years (β = 0.50; 95% CI, 0.10 to 0.90). Additionally, higher device use at age 4 years was associated with higher BMI at age 5 years (β = 0.65; 95% CI, 0.31 to 1.00). A similar bidirectional pattern was observed among boys, while no significant cross-lagged associations were found among girls. In contrast, high-quality mother-child interactions revealed higher device use at age 4 years was associated with lower BMI at age 5 years, suggesting a protective role against prolonged device use and subsequent BMI increases.

Our study indicates bidirectional associations between smart device use and BMI among preschoolers, emphasizing the protective role of high-quality mother-child interactions. Interventions should focus on enhancing parent-child relationships, limiting device use, and promoting active engagement. Future studies should investigate the effect of media content and children’s self-regulation on these associations.

The online version contains supplementary material available at 10.1186/s12966-026-01883-3.