Longitudinal Changes in Human Milk Macronutrients Across the First Year of Lactation in Indian Mothers

Background 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, prolonged exclusive breastfeeding and continued breastfeeding beyond two years are 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. Methods 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 the socioeconomic status, parity, maternal sum of skinfolds, and infant sex. Inflection points were derived from fitted quadratic models. Results 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. Conclusions Human milk macronutrient composition remains relatively stable during exclusive or predominant breastfeeding, and undergoes measurable shifts around the time complementary feeding is introduced, reflecting dynamic adaptation of lactational biology. Maternal energy reserves appear to influence milk lipid and energy content, with potential implications for infant growth and metabolic health. Context-specific longitudinal data such as these are essential to inform breastfeeding policy and maternal nutrition strategies.