Bi-directional relationships between body mass index and height from three to seven years of age: an analysis of children in the United Kingdom Millennium Cohort Study

Adiposity and height are known to correlate in childhood but it is less clear whether height and weight gain occur in synergy. We investigate the bidirectional relationships between measures of height and body mass index (BMI) – an indicator of adiposity – and their rates of change. The sample comprises singleton children in the Millennium Cohort Study (N = 11,357). Child anthropometrics measured by trained interviewers at ages three, five and seven years (2003-2009) were transformed to standardised scores based on 1990 British Growth Reference data from which piecewise linear models for height and BMI were jointly fitted. At three years of age, zHeight was positively related to subsequent zBMI velocities, whereas zBMI at three years was positively related to zHeight velocity to age five but inversely related to zHeight velocity from five to seven years of age. Age three zBMI predicted zHeight velocity from three to five years more strongly than age three zHeight predicted zBMI velocity over the same period. The rate of change in zHeight was positively correlated with subsequent zBMI velocity and vice versa. This new evidence on the bidirectional relationships between height and BMI velocities sheds light on the early childhood origins of obesity in adulthood and the need to monitor growth as well as weight gain