PROJECT SUMMARY Breast milk is a critical component of infant and young child nutrition in the first 1,000 days of life and has well- established benefits for mother and child. The complementary feeding period involves the transition from exclusive breastfeeding to partial/continued breastfeeding along with gradual introduction of complementary foods. During this time, the child is at increased risk for dietary and growth deficits, which are often due to poor complementary feeding practices and lack of evidence regarding maternal breast milk composition. Furthermore, the extent to which breast milk composition and quality can be modulated through maternal dietary interventions remains unclear. Interventions to date have mainly focused on breast milk composition during the first six months postpartum or analyzed single nutrients or nutrient groups. A comprehensive assessment of micro- and macronutrient contents in breast milk after 6 months postpartum, while breastfeeding is still ongoing, and its longitudinal change in response to a dietary intervention has, to our knowledge, never been conducted. To accomplish this, we will study breast milk samples from a subset of breastfeeding mothers who are participating in a randomized controlled feeding trial comparing the efficacy of biofortified versus conventional crop consumption in mothers and children in South India. We have collected and archived breast milk samples at five different timepoints across nine months during the child?s complementary feeding period. The objective of the proposed research is to increase our understanding of breast milk quality during the complementary feeding period. We aim to close the current evidence gap with two Specific Aims. (1) We will determine and describe the micro- and macronutrient composition of breast milk after 6 months postpartum; (2) We will also determine the impact of a nutrition intervention on breast milk composition with longitudinal samples collected during follow-up in a randomized controlled feeding trial. The proposed analysis will provide unique insights into the relationship between maternal nutritional status and breast milk composition in the complementary feeding period. Through this proposed research we aim to lay the foundation for future studies aiming to enhance breast milk quality to both improve the nutritional status of and lower the associated risk for adverse events and morbidity in children. 1