The first 1000 days of life are a critical period for growth and neurodevelopment, with lasting impacts on health. While strong associations exist between growth and neurodevelopment, using indicators like weight-for-length can be misleading as infants of similar weight can vary significantly in body composition indices (BCIs). Length and BMI, while often used, may not accurately reflect lean and fat compartments in infancy. This necessitates a more detailed characterization of BCIs at an early age. However, evidence on how BCI variations impact neurodevelopmental profiles in healthy infants, especially in low- and middle-income countries (LMICs), is limited. Most existing research focuses on preterm infants, children with specific pathologies, or populations from high-income countries. This study aimed to address this gap by investigating the associations between fat-free mass (FFM) and fat mass (FM) accretion during the first 1000 days of life and neurodevelopment at 24 months in healthy term-born infants from Karachi, Pakistan, a country with a high prevalence of childhood stunting, wasting, and underweight.

Existing literature robustly demonstrates the critical role of the first 1000 days in shaping growth and neurodevelopment, influencing later health risks. Studies across populations have shown strong associations between early childhood growth and neurodevelopment, with childhood stunting sometimes used as a proxy for neurodevelopmental risk. However, this approach is flawed as body composition varies considerably even among infants with similar weight and length. While length gains and BMI increases are linked to improved cognition and later obesity respectively, they may not accurately represent lean and fat mass during infancy. Thus, more detailed BCI characterization is crucial. While studies link BCIs to neurocognition in preterm infants, children with specific congenital pathologies, and high-income country populations, evidence from healthy infants in LMICs is scarce. This highlights the need for research investigating the impact of BCI variations on neurodevelopment in healthy infants from LMICs, specifically examining whether particular BCI phenotypes increase the risk of developmental delay.

This prospective, observational study, nested within the Multi-Center Body Composition Reference Study (MBCRS), was conducted at Aga Khan University Hospital in Karachi, Pakistan from October 2014 to November 2017. It involved 250 term-born infants with low-risk indicators for perinatal and postnatal morbidity. Repeated anthropometric and BCI measurements were taken at 3, 6, 9, 12, 18, and 24 months. BCIs (FFM, FM, fat%) were estimated using the deuterium dilution method (DDM), a stable isotope technique measuring total body water. Anthropometric measurements (weight, length, head circumference, etc.) were taken according to WHO protocols and converted to z-scores. Neurodevelopmental outcomes at 24 months were assessed using the INTERGROWTH-21st Project Neurodevelopment Assessment (INTER-NDA), covering cognitive, language, fine motor, gross motor, positive behavior, and negative behavior domains. The sample size (n=150) was determined based on the primary MBCRS outcomes, enabling detection of FM and FFM variations. Inclusion criteria included mothers aged over 18 with singleton pregnancies (37-41 gestational weeks) intending to exclusively breastfeed for the first 6 months. Exclusion criteria included mothers with less than secondary education, smokers, or infants with significant morbidity. Statistical analyses included correlations, t-tests, ANOVA, ROC curve analysis, logistic regression, and trajectory modeling using the LMS method in GAMLSS.

Of the 250 infants enrolled, 132 had complete INTER-NDA data. The mean age at INTER-NDA assessment was 25.76 months. Unadjusted analyses revealed that children with gross motor delays had significantly lower FFM at 18 months (8.01 ± 0.97 kg vs. 7.55 ± 0.20 kg). Children with positive and negative behavior problems showed significantly higher fat% at 24 months (20.62 ± 4.30% vs. 18.23 ± 5.46%, and 20.89 ± 4.24% vs. 18.54 ± 5.38%, respectively). However, after adjusting for covariates, none of these associations remained significant. Trajectory modeling of fat% revealed a critical period between 12 and 18 months, where a 1 SD increase in fat% z-score was associated with a 13.8-point increase in negative behavior scores. Comparisons between length, FFM, and fat% showed that length at 18 months was more strongly associated with gross motor delay than FFM at the same age. Fat% at 24 months was more strongly associated with positive and negative behavior problems than length at 18 months. Neither length, FFM, nor fat% showed significant associations with cognitive, language, or fine motor delays.

This study provides novel evidence from a low-risk LMIC population showing that early BCIs—higher FFM and lower fat%—are associated with enhanced behavioral and gross motor outcomes at 2 years. The findings align with those from high-income populations, suggesting that higher adiposity or insufficient FFM growth may negatively impact developmental trajectories. The association between FFM and positive neurodevelopmental outcomes might be attributed to the role of FFM in protein accretion and brain maturation processes. The variations in associations across different ages suggest specific body composition changes impact neurodevelopment during distinct sensitive periods. The stronger association of fat percentage with behavioral problems, particularly during a specific growth window, highlights the complex interplay between adiposity and behavior development. The study acknowledges limitations such as the relatively homogeneous BCI distribution and low prevalence of neurodevelopmental delay in the low-risk cohort, potentially leading to weak associations and type II error. Further research in more diverse populations is warranted to validate these findings.

This study demonstrates an association between early body composition indices (higher FFM and lower fat%) and improved behavioral and gross motor outcomes at two years in a low-risk LMIC population. Further research in more diverse LMIC populations is crucial to confirm these findings and to explore the underlying mechanisms driving these associations. Future research should consider larger sample sizes and explore potential causal relationships between BCIs and neurodevelopment.

The study's limitations include its single-center design, potentially limiting generalizability to the entire country. The low-risk, relatively homogeneous cohort (breastfed, low morbidity) might have restricted the observation of stronger associations and might lead to type II error. The study's power was primarily focused on detecting FM and FFM differences, not associations with neurodevelopment. The low prevalence of neurodevelopmental delays limits generalizability, potentially explaining weak associations and the lack of significance in some cases. A large number of covariates relative to the sample size might have inflated the risk of type II error. Future research is needed in more diverse, heterogeneous populations to address these limitations and confirm the findings.