Objective: To determine which aspects of socio-economic status (SES) measured at birth and at ages 9 or 10 years, are associated with body composition at ages 9 or 10 years.
Design: Mixed longitudinal cohort.
Setting: Johannesburg-Soweto South Africa.
Participants: A sub-sample of the Birth to Twenty (Bt20) cohort (n = 281) with data on birth weight, height, weight, fat and lean tissue (whole body dual-energy x ray absorptiometry (DXA)), and birth and ages 9 or 10 years SES measures.
Main outcome measures: Linear regression was used to estimate the influence of birth and ages 9 or 10 years SES measures on three outcomes: fat mass index (FMI) (fat mass (kg)/height (m)4), lean mass index (LMI) (lean mass (kg)/height (m)2), and body mass index (BMI) at ages 9 or 10 years controlling for sex, age, birth weight and pubertal status.
Results: Compared to the lowest SES tertile, being in the highest birth SES tertile was associated with increased LMI at ages 9 or 10 years (beta = 0.43, SE = 0.21 for white and black children and beta = 0.50, SE = 0.23 for black children only), whereas children in the high SES tertile at ages 9 or 10 years had increased FMI (beta = 0.46, SE = 0.22 for white and black children, and beta = 0.65, SE = 0.23 for black children only). SES at birth and at ages 9 or 10 years accounted for 8% and 6% of the variance in FMI and BMI, respectively (black children).
Conclusions: These findings underline the importance of examining SES across childhood ages when assessing nutrition inequalities. Results emphasise the need to consider lean and fat mass as well as BMI when studying SES and body composition in children.