Social Isolation and Excessive Weight Gain in Infants: A Nationwide Cohort Study of 1.8 Million Infants

OBJECTIVE: Infant weight gain predicts childhood obesity, yet the effect of pandemic-related social isolation on first-year weight gain is unclear. We assessed its impact and implications for obesity risk. METHODS: We analysed a nationwide birth cohort of South Korean children born 2014-2021, excluding prematurity, low birth weight or neonatal intensive care admission. Anthropometrics at 9-12 months were used. Interrupted time-series regression compared pre-pandemic (January 2015-January 2020) and pandemic (February 2020-December 2021) periods, adjusting for confounders. Relative risks (RRs) were estimated using modified Poisson regression with robust standard errors. The primary outcome was accelerated weight gain, defined as a change in weight-for-age Z-score (ΔWAZ) > 1.0. The secondary outcome was rapid weight gain, ΔWAZ > 0.67, encompassing excessive (ΔWAZ > 0.67 to ≤ 1.0) and accelerated (ΔWAZ > 1.0) gain. RESULTS: Among 1 809 054 infants (51.2% male), accelerated gain rose immediately after social isolation began (RR, 1.173; 95% confidence interval, CI, 1.154-1.193) and remained elevated (RR, 1.093; 95% CI, 1.063-1.125). Excessive gain likewise increased immediately (RR, 1.173; 95% CI, 1.154-1.192) and persisted (RR, 1.040; 95% CI, 1.025-1.055). Birth weight was stable (mean 3.24 kg, SD 0.36), while breastfeeding rates declined, socioeconomic levels rose and perinatal abnormalities increased. Infants born during the pandemic gained an additional 244 g (95% CI, 214-275) by 9-12 months. CONCLUSIONS: Infants born during pandemic-related social isolation gained more weight in the first year than pre-pandemic cohorts. Policies that sustain breastfeeding, promote healthy feeding and active play, support caregiver mental health and ensure routine early growth monitoring may help mitigate these risks.