LAUSR

Abstract Objectives Underweight in children is defined by the World Health Organization (WHO) as a body mass index z-score (zBMI) < -2. The objective was to evaluate the relationship between underweight in the first two years of life and zBMI, weight-for-age z score (WAZ), and height-for-age-z score (HAZ) from 2 to 10 years of age. Secondly, to explore whether sex and zBMI growth trajectory in the first two years of life modify these associations. Methods A prospective cohort study was conducted through The Applied Research Group for Kids (TARGet Kids! ) practice-based research network. Healthy children 0–10 years of age in Canada were included. Linear mixed models were used in the primary and secondary analysis to take into account multiple growth measures over time. Results 5,803 children were included in the primary analysis. At baseline the mean age was 4.07 months, and 52.2% were male. In adjusted models, underweight in the first two years was associated with lower zBMI, lower WAZ, and higher odds of underweight at 2, 5, 10 years of age (all p values < 0.01). There was evidence that underweight in the first two years was associated with a lower HAZ at 2 years of age (P < 0.01) but not 5 (p = 0.08) and 10 years (p = 0.80). At 10 years of age, females with underweight in the first two years had 0.47 lower zBMI compared to females who did not have underweight, while males with underweight in the first two years had 0.32 lower zBMI compared to males who did not have underweight (all p values < 0.01). Children with underweight and low growth trajectory in the first two years had lower zBMI, WAZ, HAZ, and higher odds of underweight at 2, 5, 10 years of age compared to children who did not have underweight (all p values < 0.05). Children who had underweight and high growth trajectory in the first two years had similar zBMI (p = 0.06), WAZ (p = 0.67), higher HAZ and lower odds of overweight at 10 years of age (P < 0.05), compared to children who did not have underweight. Conclusions Children with underweight in the first two years of life had lower zBMI, WAZ, and HAZ in later childhood. These relationships were stronger for females and low growth trajectory in the first two years and were attenuated among children with underweight and high growth trajectory in the first two years of life. Funding Sources Canadian Institute of Health Research (CIHR).