Catch-up Growth or Regression to the Mean? Recovery from Stunting Revisited

Abstract

An important question for policy is the extent to which catch‐up growth can ease the impact of early stunting. Martorell et al. (1992) showed that stunted Guatemalan infants remain stunted into adulthood, whereas Adair (1999) found appreciable catch‐up growth in Filipino children from 2–12 years. Both groups defined catch‐up as an inverse correlation between early height and subsequent growth, but Martorell based the correlation on height, whereas Adair used height z scores. The statistical phenomenon of regression to the mean is much like catch‐up growth, an inverse correlation between initial height and later height gain. The objective of this study was to reexamine the relationship between stunting and later catch‐up growth in the context of regression to the mean. The design was a theoretical analysis showing that catch‐up growth is more evident based on height z scores than on height, validated using data on 495 stunted South African children seen at 2 and 5 years of age. The correlation between height at 2 and height change from 2 to 5 was small based on height (−0.11) but large and highly significant based on height z score (−0.58), providing strong evidence of catch‐up growth. We argue that catch‐up growth should be estimated using height z score not height and that catch‐up is present only when the change in z score exceeds that predicted by regression to the mean. This leads to a compact definition of catch‐up growth: if z₁ and z₂ are the initial and final (mean) height z scores, and r is the correlation between them, then catch‐up growth for groups or individuals is given by (z₂ − rz₁). Am. J. Hum. Biol. 17:412–417, 2005.