Maternal versus infant factors related to breast milk intake and residual milk volume: The DARLING study

Factors related to milk volume and sources of variance in intake were examined using data at 3 months from the longitudinal Davis Area Research on Lactation, Infant Nutrition and Growth study. Milk intake of 73 exclusively breast-fed infants was measured by 4-day test-weighing, correcting for insensible water loss. Total volume extracted was assessed by a combination of test-weighing and expression of breast contents during a subsequent 24-hour period. Residual milk volume was calculated as volume extracted minus intake. Intake, volume extracted, and residual milk volume averaged 818, 914, and 109 g/d, respectively. The interindividual coefficient of variation in intake (16.3%) was greater than within-individual day-to-day variation (8.9%). In comparing those with low (<650 g/d) vs average intake, there was no significant difference in residual milk volume between groups (85 vs 111 g/d), indicating that infants with low intakes left as much milk unconsumed as those with higher intakes. There were no differences between groups in maternal age, parity, or anthropometric indices. Infants with low intakes were smaller and spent less time nursing, even though their mothers reportedly nursed on demand. In linear regressions, none of the maternal variables was significantly correlated with intake, volume extracted, or residual milk volume. Infant birth weight, weight at 3 months, and total time nursing were positively associated with intake. The results suggest that infant demand is the main determinant of lactation performance in populations such as this.