Background: Poor diet quality and vitamin D deficiency among infants and young children 12-24 months of age is a significant public health problem across the world. This could result in suboptimal growth, development, and immune system functioning. Objective: There were three main objectives of this study: 1. to examine the patterns in dietary intakes and assess adequacy using different scales such as the infant and young child feeding indicators, nutrient intake adequacy in comparison with the recommended dietary allowances, estimated average requirements, and nutrient density among children and their mothers at start and end of a randomized controlled trial involving feeding of iron- and zinc- biofortified pearl millet-based recipes among 12-24 month children living in Mumbai, India. 2. to assess the characteristics associated with improvements in hematological and growth outcomes between the start and end of a randomized controlled feeding trial conducted among children aged 12-24 months living in Mumbai, India. 3. to determine the association between maternal vitamin D status and presence of Epstein-Barr viral load at antenatal care visits including enrollment (<30 weeks of gestation) and delivery and child vitamin D status and growth among participants of a longitudinal cohort study (Chulaimbo antenatal postnatal, CHAP) conducted in Kisumu district (malaria holoendemic), Kenya. Methods: Two different groups of participants were included in this analysis. First, in this analysis, only those infants 12-24 months of age were included who participated and completed a randomized controlled feeding trial involving consumption of foods prepared with either biofortified pearl millet (FeZnPM) or conventional pearl millet (CPM); in addition, pregnant women and their newborn infants who were part of the Chulaimbo antenatal postnatal (CHAP) cohort study in Kisumu district, Kenya, were included in this analysis. Multivariate linear regression models were used in all analyses combined with mixed models (with random slopes and random intercepts) and multinomial logistic regression models for identifying the predictors of improvements in hematological and anthropometric outcomes in the Indian cohort. Results: In the Indian cohort, the minimum dietary diversity, minimal meal frequency, and minimal acceptable diet was met by 36%, 38%, and 14% of the children, respectively. The intra-group changes in average intake per meal over the study duration of 9 months was significantly higher in the FeZnPM group compared to the CPM group for nutrients iron (p < .0001), zinc (p < .0001), vitamins B1 (p = .009), and B3 (p = .0006). Inter-individual differences in subscapular skinfold-for-age z-scores (SSZ) between baseline and endpoint of the study was a significant negative predictor of a change (∆) in serum ferritin from baseline to endpoint among males (ß (SE): -72.92 (11.82); p < 0.0001), among females ß (SE): -57.29 (9.48); p < 0.0001, and in the combined males and females cohort (ß (SE): -74.80 (8.71); p < 0.0001). Next, the timing of breastfeeding initiation was found to be a significant predictor of weight-for-length z-score (p = 0.003) in the combined cohort and weight-for-age z-score among males alone (p = 0.03). Lastly, immunological factors was observed to have a positive association with hematological (∆serum ferritin: ß (SE): 0.64 (0.21); p = 0.003) and growth outcomes (∆length-for-age z-score: ß (SE): 0.03 (0.01); p = 0.01). In the Kenyan cohort, nearly half (48%) of pregnant women were found to have active viral load at the time of enrolment into antenatal care (ANC). Maternal vitamin D concentrations at enrollment and during pregnancy were not significantly associated with either longitudinal maternal or neonatal EBV-related outcomes (p > 0.05). Maternal vitamin D concentration at delivery was associated with infant vitamin D concentrations at 6 months of age (p = 0.02). In addition, presence of EBV viral load at enrolment is associated with reduced neonatal birth weight and vitamin D concentration in cord blood. Conclusion: Infants may be lacking in minimum dietary diversity, minimum meal frequency and minimum acceptable diet. Dietary interventions focused on delivering additional meals and nutrients are important in the 12-24 month-age range. However, it may also be crucial to assess if the interventions translate to healthy growth and improvement, since we observed an inverse association between SSZ and change in serum ferritin concentrations. Additionally, reducing active infections, particularly during pregnancy, is vital for supporting optimal growth and health outcomes in infants. Further research is needed to validate these findings.