Longitudinal changes in human milk macronutrients across the first year of lactation in Indian mothers

Human milk is a dynamic biological system. Although early lactation has been extensively studied, longitudinal data beyond six months postpartum, particularly from low- and middle-income countries, remain limited. In India, continued breastfeeding beyond six months and into the second year is common, underscoring the need to understand temporal patterns of human milk composition. We aimed to characterize trajectories of human milk protein, carbohydrate, fat, and energy across the first year postpartum. In this community-based longitudinal study in semi-urban India, healthy mother-infant dyads were followed at 1, 3, 6, 9, and 12 months postpartum, with standardized anthropometry performed, feeding practices recorded, and milk samples collected at each visit. Human milk macronutrients were quantified using mid-infrared spectroscopy. Longitudinal trajectories were examined using linear mixed-effects models, adjusting for socioeconomic status, parity, maternal sum of skinfolds, and infant sex. Inflection points were derived from fitted quadratic models. At enrolment ( N = 95 dyads, postpartum days 35 ± 10), mothers had a mean age of 25.9 ± 4.7 years (BMI: 22.2 ± 4.1 kg/m²). Substantial inter-individual variability was observed across all macronutrients (401 milk samples). Inclusion of quadratic time terms significantly improved model fit for protein, carbohydrate, fat, and energy (all p < 0.001). Protein concentration declined in early lactation and stabilized around 183 (95% CI: 150, 212) days postpartum. Fat and energy reached nadirs at approximately 196 (160, 234) and 226 (198, 258) days, respectively, followed by modest increases toward late lactation. Carbohydrate showed a gradual decline with later stabilization [inflection point, 341 (272, 454) days]. Higher socioeconomic status and maternal adiposity were independently associated with higher milk fat and energy concentrations. Human milk macronutrient concentrations remained relatively stable during early infancy and showed non-linear changes later in lactation, around the period when complementary feeding was introduced. These findings highlight the dynamic nature of human milk composition across lactation and the importance of context-specific longitudinal data for understanding milk composition during the transition from predominant breastfeeding to complementary feeding. Such data can contribute to refinement of breastfeeding and complementary feeding guidance in diverse populations.