Underweight in the First 2 Years of Life and Growth in Later Childhood.

Importance: Few studies have examined the association between underweight in the first 2 years and growth in later childhood in high-income countries.

Objective: To evaluate the associations of underweight in the first 2 years of life with body mass index (calculated as weight in kilograms divided by height in meters squared) z score (zBMI), weight-for-age z score (WAZ), and height-for-age z score (HAZ) from ages 2 to 10 years.

Design, Setting, And Participants: This prospective cohort study was conducted between February 2008 to September 2020 in The Applied Research Group for Kids! practice-based research network in Toronto, Canada. Participants included healthy children aged 0 to 10 years. Data were analyzed from October 2020 to December 2021.

Exposures: Underweight (ie, zBMI less than -2, per the World Health Organization) in the first 2 years of life.

Main Outcomes And Measures: The primary outcome was zBMI from ages 2 to 10 years. Linear mixed-effects models were used to account for multiple growth measures over time.

Results: A total of 5803 children were included in the primary analysis. At baseline, the mean (SD) age was 4.07 (5.62) months, 2982 (52.2%) were boys, and 550 children (9.5%) were underweight. Underweight in the first 2 years was associated with lower zBMI (difference, -0.39 [95% CI, -0.48 to -0.31]) at 10 years and lower HAZ (difference, -0.24 [95% CI, -0.34 to -0.14]) at age 2 years. Stratified by sex, at age 10 years, girls and boys with underweight in the first 2 years both had lower zBMI (girls: difference, -0.47 [95% CI, -0.59 to -0.34]; boys: difference, -0.32 [95% CI, -0.44 to -0.20]). At age 10 years, children with underweight and a lower zBMI growth rate in the first 2 years had lower zBMI (difference, -0.64 [95% CI, -0.77 to -0.53) and HAZ (difference, -0.12 [-0.24 to -0.01]), while children with underweight and a higher zBMI growth rate in the first 2 years had similar zBMI (difference, -0.11 [95% CI, -0.22 to 0.001]) and higher HAZ (difference, 0.16 [95% CI, 0.05 to 0.27]) compared with children who did not have underweight in the first 2 years.

Conclusions And Relevance: In this prospective cohort study, children with underweight in the first 2 years of life had lower zBMI and HAZ in later childhood. These associations were attenuated among children with a higher growth rate in the first 2 years.