Background: Recent studies have found that postnatal catch-up growth might have a stronger impact than birth size on health in later life. Because growth is a continuing process, the challenge is to tease out the impact of body size at different critical phases. Ordinary least squares regression cannot differentiate the effects of birth size, growth, and current body size simultaneously, because growth is generally defined as the difference between birth size and current size, giving rise to perfect collinearity.
Methods: This paper aims to describe and justify the use of a novel approach, partial least squares, to estimate life course effects of body size z-scores on later blood pressure, using longitudinal data from a cohort of 960 Filipino boys. Body weight z-scores and changes in z-score weight were measured from birth to age 19 years, and systolic and diastolic blood pressures (BPs) were measured at age 19.
Results: In general, birth size had a very modest association with systolic BP. The early changes in z-scores between birth and the age of 2 years were positively associated with the outcome. Growth after age 8 had a larger effect than early growth, but the confidence intervals are generally large. For diastolic BP, the association was similar for early and later growth. Current body size had the strongest relationships with both outcomes.
Conclusion: By creating weighted composites of perfectly collinear variables as components, partial least squares estimates the life course effects of body size on later health according to the correlations between body size and health outcomes.