Once stunted always stunted? What’s up with catch-up growth?

Poor nutrition can cause young children to become stunted, that is, to be too short for their age. In fact, an estimated 159 million children under five worldwide are stunted due to chronic malnutrition. Linear growth retardation, resulting in stunting, begins in utero and continues into infancy and early childhood.

A question that has puzzled nutritionists for decades is whether delays in linear growth occurring during early childhood can be reversed. Research shows that there is little to no population-level catch-up growth for children who remain in the same deprived settings where the initial stunting occurred. This seems logical: the environment did not improve, and neither did the child’s height.

Recovery of at least some of the deficit is possible if the child’s environment is improved through nutrition and health interventions or through adoption, but this improvement needs to happen within the first few years of life. A common view within the global nutrition community is that stunting is largely irreversible after the child’s second birthday.

However, new buzz around the catch-up growth question emerged after recent studies suggested something totally different.

Before describing the new findings, let’s start with a definition of ‘recovery in linear growth’ −often referred to as catch-up growth− and defined as a reduction in a child’s height deficit (compared to the international height standard) as the child ages.  Malnourished children who are shorter than expected for their age can, in principle, ‘catch up’ by growing at a faster-than-expected rate for their age during a given period of time, thus overcoming their previous accumulated height deficit.

Findings from a recent set of studies shows, across a range of countries, suggest that there is indeed an indication of catch-up growth in children older than two years of age in settings where they did not receive interventions specifically aimed at improving linear growth. This assertion is creating quite a stir within the nutrition community.

Amidst this buzz, IFPRI researchers Jef LeroyMarie Ruel, and their co-authors decided to re-analyze data used in some of those studies to examine the validity of the surprising new claims.

As detailed in their recent A4NH-funded study published in BMC Pediatrics, Leroy and colleagues found no evidence of catch-up growth. In fact, they found that even in the groups of children who were previously identified as having experienced “catch-up” linear growth, the growth velocity was still not fast enough to catch up to the growth rate of their healthy peers, let alone surpass them.

The main difference between previous studies which cited catch-up growth and the recent IFPRI research lies in the measurement. Typically, chronic undernutrition is measured using a metric called ‘height-for-age Z-scores,’ or HAZ, which standardizes a child’s height based on what is expected for a child of that age and sex. This measure works for cross-country comparisons or to compare the nutritional status of different groups of children at different points in time.

 However, according to Leroy and his colleagues, HAZ is an incorrect measure for tracking changes in undernutrition as children age. HAZ is constructed using standard deviations from cross-sectional data, making it an inappropriate measure to evaluate changes in height as children grow.

To measure changes in height as children get older, Leroy and colleagues recommend using a different metric altogether, something called the absolute ‘height-for-age difference,’ or HAD, which measures the simple difference between a child’s actual height and how tall he or she should be based on the international height standard.

When re-analyzing the data from recent studies, Leroy and colleagues applied the HAD metric when measuring changes in children’s height, and the results were strikingly different. Not only did they find a complete absence of catch-up growth, but they also found a steady increase in the deficit in children’s height from birth through age five. In other words, the children were actually becoming more and more stunted with time and showed no sign of recovery.

This is not the first time inappropriate use of HAZ led to misconceptions within and beyond the nutrition community. For example, in earlier related work, Leroy and his coauthors showed how it led many to mistakenly conclude that linear growth retardation stops by age two, when, in fact, 30 percent of the total height deficit by age five is accumulated after a child’s second birthday.

While this most recent study concludes that there was no true evidence of catch-up growth in the data sets analyzed (and reported to have shown catch-up growth in previous analyses using HAZ), the authors do not claim that catch-up growth cannot happen under certain conditions. However, their findings indicate that population-level catch-up growth is highly unlikely among children who remain in the same impoverished environments that led to growth faltering in the first place.

Not only are changes in linear growth unlikely when environments do not improve, but unfortunately, a stunted start in life predicts a host of other harmful outcomes throughout the life cycle, including reduced cognitive development, school achievement, and economic productivity as well as increased mortality and risks of chronic diseases. Whether and to what extent interventions aimed at improving linear growth –and possibly achieving catch-up growth – can also successfully prevent these additional consequences of chronic malnutrition, remains unknown.

Enjoy the below video, in which IFPRI researcher Jef Leroy shares insights on this new study.

*To request a copy of this article, please contact: ifpri-library@cgiar.org.