Worldwide patterns of linear growth faltering, based on data from many low- and middle-income countries (LMICs) (Victora and others 2010), indicate deterioration of child nutritional status, on average, from age 0 to 24 months; after this period, nutritional status levels off or slightly reverses (for example, Prentice and others 2013; Stein and others 2010). Analyses of the five countries in the Consortium of Health-Orientated Research in Transitioning Societies (COHORTS) study found that low birth weight or undernutrition at age two years (or both) were associated with shorter adult height, less schooling, and lower economic productivity (Victora and others 2008). The 2008 Lancet series on nutrition argued that height-for-age is the best nutritional predictor of adult human capital (Victora and others 2008).
These results influenced prioritization of global efforts to combat undernutrition in the first 1,000 days, from conception to age 24 months. More broadly, these 1,000 days are seen as a critical period for establishing the physical, cognitive, and socioemotional foundation for later life (Walker, Wachs, and others 2011) and are viewed as the period of greatest plasticity (Gluckman and others 2009). As reviewed by Halfon and others (2014), new approaches to life course development have integrated biological systems, drawing from genetics as well as epigenetics, with social and behavioral models. The approach in this chapter unites economic theory with health science.
We use the lifecycle approach to assess the benefit-cost ratios of interventions in nutrition and child development in LMICs, where undernutrition is a risk factor, with a focus on the first five years of life. Definitions of age groupings and age-specific terminology used in this volume can be found in chapter 1 (Bundy and others 2017).
Birth weight and linear growth in the first two years are associated with many beneficial outcomes later in life (Adair and others 2013). The 2013 Lancet nutrition series also acknowledged the need to address both undernutrition and increased obesity in LMICs (Black and others 2013), recognizing that there is a high prevalence of both conditions and that the conditions often are linked. The 2013 series connected the importance of prenatal nutrition and adolescent girls’ nutrition (Bhutta and others 2013). Women’s height affects risks for pregnancy complications (Toh-Adam, Srisupundit, and Tongsong 2012) and low birth weight (Black and others 2013); given associations of birth weight with subsequent undernutrition (Christian and others 2013), these findings bring the discussion full circle. Thus, the 1,000-day window could be made much longer—even going back to mothers’ childhoods.
Children’s early years are critically important for cognitive, language, and socioemotional development, and strong evidence indicates that the window of influence extends well beyond the first 1,000 days. Protective and risk factors for undernutrition are often similar to the factors influencing cognitive and socioemotional development (Walker, Wachs, and others 2011). For example, shared risk factors include intrauterine growth retardation, nutrient deficiencies, and social and economic conditions. Risks specific to poor cognitive development include inadequate learning opportunities and inadequate quality of caregiver-child interactions. Shared protective factors include breastfeeding and maternal education.
The overlapping risk factors, timing of peak vulnerabilities, and the possibility that early deficits have long-lasting impacts have motivated interest in interventions that integrate nutritional and other approaches to promote overall child development (Alderman and others 2014). Ideally, policies and programs must move from a focus on single issues to a wider-reaching, more integrated approach across the life course, which would allow for each child to develop as well as possible and mitigate the impact of constraints under which their development may be occurring (Fine and Kotelchuck 2010). Such integration, however, requires clearer understanding of individuals’ developmental timing and age-dependent responses to external factors (Wachs and others 2014). Cognitive functions, receptive and expressive language, and socioemotional skills develop at different ages (Grantham-McGregor and others 2007). Development in brain structure and function supporting acquisition of cognitive, language, and socioemotional skills is most rapid during early childhood, with continued development in later years for many skills.
The early years, beginning in utero and extending to age 36 months, are the best stage in which to prevent stunting. The debate continues as to whether children who become stunted before age 24 months can catch up later in their lives. Population averages from cross-sectional data show some limited catch-up in height-for-age z scores, though average height deficits widen beyond age two years into adulthood (Leroy and others 2014; Lundeen and others 2014). Longitudinal studies report considerable individual movements in both directions between stunted and nonstunted status after age 24 months that are associated with family and community characteristics, suggesting potential for catch-up or prevention of faltering (Crookston and others 2013; Lundeen and others 2013; Mani 2012; Prentice and others 2013; Schott and others 2013). Catch-up may, however, have some risks; for example, weight gain on small frames has been associated with subsequent obesity and adult chronic diseases (Monteiro and Victora 2005; Yajnik 2004, 2009).
As with malnutrition, cognitive delays can occur throughout infancy, childhood, and adolescence, understood in this volume as birth through age 19 years. Measurable differences in receptive language by socioeconomic groups are apparent in preschool children ages three to five years (Paxson and Schady 2007; Schady and others 2015); differences in cognitive ability have been observed even in the first two years (Fernald and others 2012). Early life stress—often toxic if extreme—can also have difficult-to-reverse lifetime consequences (Shonkoff and Garner 2012). Individual responsiveness to interventions implemented after initial developmental insults are widely debated (see chapter 8 in this volume, Watkins and others 2017).
More than 3 million children younger than age five years died in 2011; half of these deaths were associated with fetal growth restriction, suboptimal breastfeeding, stunting, wasting, and vitamin A and zinc deficiencies (Black and others 2013). Given that about 75 percent of child deaths before age five years occur in the first year, addressing catch-up growth beyond the 1,000-day window is driven less by concern for mortality risk and more by concerns relating to later-life consequences for survivors.
Some evidence indicates that skill accumulation is more plastic than physical growth; skills such as executive function—a component of cognitive function—and socioemotional development have time paths different from those of conventional cognitive abilities (Borghans and others 2008). Still, very little is known about time paths of effective interventions for addressing nutritional, cognitive, and socioemotional development, particularly in LMICs. Maximum gains relative to costs, particularly for cognitive and socioemotional developmental outcomes, are likely to require early investment, followed by appropriate nutritional and educational investments and continued support for effective parent-child interaction over childhood and adolescence. Determining which later-life interventions cost-effectively reduce consequences of early malnutrition or cognitive delay is important if efforts at prevention fall short, as they already have for hundreds of millions of children.
© 2017 International Bank for Reconstruction and Development / The World Bank.