25 February 2014

Catch up growth? Maybe, but what is catching up? Findings of a new paper

One of the rationales for the focus on 1000 days in undernutrition prevention is that this is when developmental velocity is largest in terms of fundamental systems (e.g. immunity, cognitive development) are being laid down, and any interruption of this cannot be retrieved, at least in full.  So when a new paper comes along that seems, on the face of it, to challenge that orthodoxy it causes a stir.  Such a paper was just published by Kalle Hirvonen in the Annals of Human Biology. 

The paper critiques current economic approaches to estimating the extent of catch up growth.  The paper finds considerable catch up growth in a 19 year cohort of children from Kagera region of Tanzania: the mean normalised height for age of the cohort in early childhood is -1.86 and in is -1.20 in adulthood.  Without catch up growth the author shows that as adults the children would have been 4.5-5 centimetres shorter.  The author argues that others may not find this result because they are doing the statistics wrong.  Many studies estimate the slope between adult height (normalised or otherwise) and the height for age of the child.  If the slope is one, then there is no catch up growth—early childhood height for age perfectly predicts adult height.  If the slope is zero (ie there is no relationship) then catch up growth is total.  Most importantly, Hirvonen shows that the slope estimate is mixing up two things—shorter children catching up or taller children falling back. 

I think this paper is a good contribution to the literature, but it misses an important point.  What is catching up?  In early childhood, height is a marker of other types of human development that cannot be so readily observed.  They are correlated with height at a particular point in time, not throughout childhood and adolescence to adulthood. 
In other words the main functional consequences of catch up in height are related to, well, height.  It is true that there is a premium to height in heavy manual work and for birth outcomes for women, so this is far from trivial, but catch up in growth does not mean a rehabilitation of the systems damaged in early childhood and marked visibly as a slowness of linear growth. 
In other words, there is partial catch up of a partial set of human development characteristics.   For me this finding does not weaken the 1000 days focus although it does remind us that this period is not the only one that is important for human development.

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