Abstract
Human milk (HM) is the optimal source of nutrition for infants, conferring health benefits for both infants and mothers. While exclusive breastfeeding is recommended by the World Health Organization and New Zealand Ministry of Health for the first 6 months(1, 2), complementary foods are typically introduced at around 6 months with breastfeeding often continuing into late infancy. HM composition during the complementary feeding period remains under-researched in New Zealand, with most studies classifying HM expressed beyond two weeks postpartum as “mature milk,” potentially overlooking changes in nutrient content, as HM composition is known to change over the course of lactation. Understanding HM composition in relation to maternal and infant characteristics, and how energy is derived from milk sources and complementary foods across feeding practices, is essential to accurately assess nutrient intake and inform evidence-based feeding guidelines during this critical stage. This research aimed to determine the nutrient composition of human milk between 7–10 months postpartum and its contribution to infant energy intake. The cross-sectional study included 625 parent-infant dyads from the multi-centred (Dunedin and Auckland) First Foods New Zealand (FFNZ) study(3). Infants were 7–10 months of age, and participants were recruited in 2020-2022. This secondary analysis examines a subsample of 121 mothers who provided HM samples. Demographic and anthropometric measures, 24-hour dietary recalls, and HM volume (measured using the dose-to-mother stable isotope technique), were collected as part of the FFNZ study. Macronutrients (fat, lactose, protein), micronutrients (vitamin A, C, E, folate, sodium, magnesium, phosphorus, potassium, calcium) and trace elements (iron, copper, selenium, zinc, iodine) were analysed using validated methods. Twelve of the eighteen nutrients were within ranges reported in the literature. Average HM vitamin C (2.39 mg/100mL), iron (133.9 µg/L), selenium (10.21 µg/L), and iodine (62.82 µg/L) concentrations were lower than values reported in the literature, whereas vitamin E (0.27 mg/100mL) and magnesium (34.30 mg/L) were higher. No clear differences in HM micronutrient concentrations were observed across maternal (e.g., age, BMI, parity) or infant characteristics (e.g., age, BMI z-score, sex). Significant differences (p<0.001) were observed between breastfed (n=330) and formula-fed (n=217) infants in the energy contribution from milk sources (mean+SD: 2,068+561 kJ/day and 2,251+601 kJ/day, respectively) and total energy (i.e., milk sources and complementary foods), of 3,239+525 kJ/day and 3,534+609 kJ/day, respectively, including after adjusting for infant age and sex. These findings indicate that HM nutrient concentrations in New Zealand mothers between 7–10 months postpartum are broadly consistent with international literature, though lower iodine, iron, and selenium concentrations may place infants at risk of inadequate intake. Future work should integrate HM composition with measured intakes to determine nutrient adequacy. This will clarify HM’s contribution in late infancy and ensure feeding guidelines adequately address potential nutrient gaps.