Differential associations between body composition indices and neurodevelopment during early life in term-born infants: findings from the Pakistan cohort: Multi-Center Body Composition Reference Study

Early life (first 1000 days) is a critical window for growth and neurodevelopment with long-term health consequences. Traditional anthropometric markers (length, BMI) are imperfect proxies for underlying body composition indices (BCIs), as infants with similar size can differ in fat mass (FM) and fat-free mass (FFM). While growth and neurodevelopment are associated, evidence on how specific BCIs relate to neurodevelopment in healthy term infants—especially in low- and middle-income countries (LMICs)—is limited. Most prior research involves preterm infants, children with congenital conditions, or high-income country cohorts. This study aimed to (1) examine associations between FFM and FM accretion during the first 1000 days and neurodevelopmental outcomes at 24 months, and (2) compare associations between length, FFM accretion and fat% with developmental delay, in a term-born, low-risk infant cohort from Karachi, Pakistan, a setting with high burdens of stunting and undernutrition.

Evidence shows strong links between early growth and neurodevelopment, but anthropometric measures like BMI and length may not accurately reflect lean and fat compartments. Prior studies have associated length gain with improved cognition and BMI gain with later obesity. Most BCI–neurocognition research involves preterm infants or children with specific pathologies and originates from high-income countries, limiting generalizability to LMIC settings. Studies in HICs suggest higher adiposity and/or inadequate FFM accretion may relate to poorer cognition or behavior, though findings vary and can be weak in well-nourished populations. Potential mechanisms include FFM reflecting protein accretion and growth factor activity necessary for brain maturation and myelination; shared social determinants (e.g., parenting, socioeconomic status) may also influence associations. Sensitive periods of growth may differentially impact neurodevelopmental domains.

Design and setting: Prospective, observational study conducted at Aga Khan University Hospital, Karachi, Pakistan (October 2014–November 2017), nested within the Multi-Center Body Composition Reference Study (MBCRS). Participants: Mother–infant dyads enrolled at birth. Inclusion: mothers >18 years, singleton pregnancies, 37–41 weeks’ gestation, intention to exclusively breastfeed for 6 months. Exclusion: <secondary education, smokers, or significant morbidity. Sample size: Target n=150 based on MBCRS to detect FM and FFM <1 SD from a US reference with 90% power; overall 250 enrolled, 132 completed INTER-NDA at 24 months. Measures: Anthropometry (weight, length, head circumference, mid-upper arm circumference, triceps and subscapular skinfolds) taken by two assessors per WHO MGRS protocols and converted to age- and sex-adjusted z-scores. Body composition indices (FM, FFM, fat%) assessed via deuterium dilution method: oral deuterium dose, saliva collection, isotope ratio mass spectrometry to derive total body water, from which FM, FFM and fat% were estimated. Neurodevelopment at 24 months assessed with the INTERGROWTH-21st Neurodevelopment Assessment (INTER-NDA), covering cognition, language, fine and gross motor, and positive and negative behavior; standardized protocols with established inter-rater and test-retest reliability and international standards for defining delay. Data collection ages for BCIs: 3, 6, 9, 12, 18, and 24 months. Statistical analysis: Distribution checks (visual, Kolmogorov–Smirnov). Six analytic approaches: (i) Pearson correlations between BCIs at each age and INTER-NDA scores at 24 months; (ii) independent-samples t-tests comparing BCIs between children with any delay vs no delay, and ANOVA for severe vs mild-to-moderate vs none; (iii) ROC curves and AUCs for significant BCIs predicting any delay; (iv) associations between BCIs and potential confounders via correlations and group tests (covariates listed in Table S2, including anthropometric z-scores); (v) binary logistic regression adjusting for covariates for factors associated with any delay; (vi) fat% trajectory modeling using LMS in GAMLSS (R), and multiple linear regression to identify critical periods when changes in fat% z-scores associated with INTER-NDA outcomes. Effect sizes quantified with Cohen’s d and 95% CIs.

Sample and outcomes: Of 250 enrolled, 132 (52.8%) had complete INTER-NDA data at ~25.8 months; 52% male. Mean anthropometric z-scores across ages were within ±1. Prevalence at 24 months: cognitive delay 4.5% (severe 1.5%), language delay 5.3% (severe 3.0%), gross motor delay 6.1%; no fine motor delays. Behavior problems: positive 28.0% (severe 12.9%), negative 21.2% (severe 4.5%); no sex differences. Correlations: Higher FFM at 6, 9, and 24 months associated with higher positive behavior scores at 24 months (r≈0.19–0.21, p<0.05). Higher FFM at 3 months associated with higher gross motor (r=0.24, p<0.01) and lower language scores (r=−0.18, p<0.05). Higher fat% at 24 months associated with lower positive behavior scores (r=−0.21, p<0.05). Unadjusted group comparisons: Gross motor delay associated with lower FFM at 18 months (t=3.51, p=0.001; means approximately 7.55 vs 8.01 kg). Children with positive and negative behavior problems had higher fat% at 24 months: 20.62% vs 18.23% (t=−2.28, p=0.02) and 20.89% vs 18.54% (t=−2.25, p=0.03), respectively. Some associations for lower FFM at 6 and 24 months with behavior problems approached but did not reach significance (p≈0.05–0.08). ROC: Fat% at 24 months predicted behavior problems with modest discrimination: positive behavior AUC=0.65 (95% CI 0.54–0.75; p=0.01); negative behavior AUC=0.64 (95% CI 0.53–0.76; p=0.03). Adjusted analyses: After adjusting for covariates in logistic regression, none of the BCIs remained significantly associated with delays in any domain. Trajectories: Fat% increased from 19.6% at 3 months to 23.7% at 6 months, then declined to 19.1% at 24 months. The 12–18 month period was a critical window: a 1 SD increase in fat% z-score between 12 and 18 months was associated with a 13.8-point increase in negative behavior scores. Comparisons with length: Lower length at 18 months showed a stronger association with gross motor delay than lower FFM at the same age (Cohen’s d≈0.80 vs 0.50). Higher fat% at 24 months was more strongly associated with positive behavior problems than lower length at 18 months (d≈0.52 vs 0.50) and was significantly associated with negative behavior problems (t=2.25, p<0.03), whereas length was not. No associations were found between length, FFM, or fat% and cognitive, language, or fine motor delays.

This study in healthy, term-born, predominantly breast-fed Pakistani infants indicates that aspects of body composition, particularly FFM and adiposity, relate to neurodevelopmental outcomes at 24 months, with domain- and age-specific patterns. Early higher FFM correlated with better behavior scores and with better gross motor (but lower language) scores when measured at 3 months, while higher adiposity at 24 months related to poorer behavior. A critical period of fat accretion between 12 and 18 months was linked to worse negative behavior scores. While unadjusted analyses showed several associations, these did not persist after adjustment for multiple covariates, suggesting confounding and/or limited power. The findings question the universal use of length as a proxy for neurodevelopmental risk by showing nuanced, domain-specific relationships where fat% at 24 months related to behavior more strongly than length. Results align with prior HIC and preterm evidence that inadequate FFM gains or higher adiposity may adversely influence neurodevelopment and behavior, potentially via mechanisms involving protein accretion, growth factors, brain myelination, and shared social determinants. Identifying sensitive periods highlights opportunities for targeted nutritional strategies, but interventions must balance optimizing neurodevelopment with minimizing metabolic risk.

In a low-risk LMIC cohort, higher early-life FFM and lower adiposity were associated with more favorable behavioral and gross motor outcomes at 2 years, with a critical adiposity-related window identified between 12 and 18 months for negative behavior. Although adjusted analyses did not retain significance, the patterns suggest that body composition may influence early neurodevelopment in domain- and age-specific ways, and that length alone may not capture neurodevelopmental risk. Future research should include larger, more heterogeneous LMIC cohorts (including higher-risk infants), be powered to detect associations with developmental outcomes, refine understanding of sensitive periods, and assess causal pathways to guide balanced nutritional interventions that support neurodevelopment while avoiding excess adiposity.

Single-center, normative, low-risk cohort may not represent broader Pakistani or LMIC populations; relative homogeneity in body composition and low prevalence of developmental delays limited variability. The study was powered for detecting body composition differences, not associations with neurodevelopment, leading to small numbers in delay groups and potential type II error. Multiple covariate adjustments relative to sample size further reduced power. Lack of internationally standardized thresholds for pediatric body composition precluded high/low BCI group comparisons. Observational design limits causal inference.