Catch-up growth in height and cognitive function: Why definitions matter.

There is substantial evidence that early growth retardation, indicated by stunting, is associated with poorer cognitive function among children. There are, however, contradictory findings on the extent to which subsequent 'catch-up' growth among stunted children is associated with similar 'catch-up' cognitive functioning. In this paper we show that the apparent contradictions in the literature may be a result of differences in the definition of catch up used in the different studies. We explore two variations in definition: the age from which catch-up growth is measured, and the extent of growth required to be classified as 'caught up'. Using cohort data from South Africa with repeated measures of length in early childhood, we first show that varying the starting age from which catch up is measured from 1y to 2y greatly affects the conclusions drawn with respect to cognitive outcomes, as the prevalence of stunting tends to peak around 2y. Second, we show how the results differ when we vary what counts as catch up, and here we explore five definitions ranging from most lenient to strictest. The strictest definition requires children to have caught up sufficiently that their height-for-age falls within the 'normal' range at follow-up; very few children catch up to this extent. For all definitions of catch-up, except the strictest, we find that children who are stunted at 2 years of age who subsequently experience catch-up growth, on average, do worse on cognitive tests than children who were never stunted, and almost as poorly as children who remain stunted (with the coefficient ranging from -1.584; p < 0.01 to -1.753; p < 0.01). This suggests the timing of investments in early childhood is key, with intervention in the first two years to prevent deprivation that affects both linear growth and cognitive function.