What's up with catch-up? No evidence of population-level catch-up growth in children under 5 years of age when using height-for-age difference (HAD) instead of height-for-age z-scores (HAZ)

Studies conducted in past decades showed little or no population-level linear catch-up growth (defined as reductions in absolute height deficit) in children after 2 y of age. Recent studies, however, have reported catch-up growth in groups of children using height-for-age z-scores (HAZ). We assessed whether population-level catch-up growth is found when height-for-age difference (HAD; child's height compared to standard in cm) is used instead of HAZ. Using data from Demographic and Health Surveys (DHS), Young Lives (YL), and the Consortium on Health-Oriented Research in Transitional Societies (COHORTS), we compared changes in HAD and HAZ in populations of children between 2 and 5 y. We show that HAZ is inappropriate to measure changes in linear growth as populations of children age because they are constructed using SDs from cross-sectional data, which increase with age. Using HAD, we find not only an absence of population-level catch-up growth, but a continued deterioration reflected in a decrease in mea...