Expressed Breast Milk Volume May Need to Be Considered in Studies of Lactational Programming of Obesity
Authors: Whitaker Kara M. , Gray Heather , Fontaine Patricia , Fields David , Demerath Ellen W. 
Published: November 2023

There is increased interest in “lactational programming” as a potential mechanism by which maternal and offspring obesity are linked; that is, there are bioactive hormonal and other factors in milk that vary by maternal nutritional status and can alter infant growth rate. However, existing studies often do not consider whether breast milk sample volume could play a role in these associations.ObjectivesWe tested whether the volume of milk collected from mothers for hormone assays was associated with maternal pre‐pregnancy body mass index (BMI) and gestational weight gain. Secondary analyses examined the associations of breast milk volume with infant weight status.MethodsParticipants were 54 exclusively breastfeeding mother‐infant dyads (average maternal age 30.9 ± 3.8 years, 91% White), enrolled in the ongoing Mothers and Infants LinKed for Healthy Growth (MILK) study. Maternal and infant anthropometrics and breast milk volume were assessed at one and three months postpartum. Pre‐pregnancy BMI and gestational weight gain were calculated using first prenatal visit weight, measured height, and delivery weight. Infant weight and length were measured and converted to WHO Z‐scores. Milk volume was assessed by having mothers provide a single complete breast expression using an electric breast pump (Medela, Inc.) exactly two hours after feeding their infant until volitionally satisfied. Linear regression models examined associations of maternal pre‐pregnancy BMI and gestational weight gain with milk volume, after controlling for age, parity, education, and race. Milk volume was also examined as a predictor of infant weight and length status.ResultsAverage pre‐pregnancy BMI was 25.6 ± 4.7 kg/m2 (53.7% normal weight, 25.9% overweight, 20.4% obese), and mean gestational weight gain was 14.1 ± 5.2 kg. Breast milk volume increased from 68.4 ± 32.0 ml at one month postpartum to 70.3 ± 35.1 ml at three months. An inverse association was observed between pre‐pregnancy BMI and milk volume at three months only (β=−3.13, p=0.006). Gestational weight gain was not associated with milk volume at one or three months. We also found that milk volume at one month was positively associated with weight for‐age Z‐scores at one (β= 0.01, p=0.015) and three months (β= 0.01, p=0.023). Milk volume was not associated with length‐for‐age or weight‐for‐length Z‐scores.ConclusionsPreliminary analyses from the MILK study showed that greater pre‐pregnancy BMI was associated with a reduction in milk volume from a standardized milk expression at three months postpartum. Milk volumes were also positively associated with weight‐for‐age Z‐scores at one and three months postpartum. If milk hormone concentrations are found to vary with milk volume produced, our results indicate the need to control for this factor when examining the role of breast milk hormones in the transmission of obesity risk from mother to offspring.Support or Funding InformationR01 HD080444 (National Institute for Child Health and Development)