Daily and within-feed variation of macro- and trace-element concentrations in human milk and implications for sampling

•Concentration of micronutrients in human milk vary pre- and post-feed.•Concentration of micronutrients in human milk vary throughout the day.•Characteristics of sample collection influence calculation of micronutrient intake.•Changes in micronutrient concentration should be considered when calculating intake. Daily variations of macro- and trace-elements in human milk (HM) are not well characterised and sampling protocols are highly variable between studies. To investigate diurnal and within-feed variation of HM macro- and trace-elements using pre- and post- feed concentrations and to compare infant intake estimates using limited samples with measured 24-hour intake. HM Samples were collected pre- and post- every feed in a 24-hour period from 11 mother-infant dyads. Test-weighing was used to determine the volume of HM consumed in each feed. For macro- and trace-elements within-feed and daily variation was measured. Intake estimated from a morning pre-feed sample was compared to the measured milk intake calculated from every feed over 24-hours. Macro- and trace-elements concentrations were measured using ICP-MS. Linear mixed modelling was used for statistical analysis. Average intake of HM was 737 ± 63 mL for infants aged 1–6 months and 508 ± 50 for infants aged 6–12 months. Pre- and post-feed HM variation was found for phosphorus, calcium, manganese, iron, copper, zinc, selenium, molybdenum, and iodine (p