Clinical and field studies of human lactation: methodological considerations

A variety of studies has been completed to assess selected methods that are presently being used or might potentially be used to evaluate lactation performance. During 64 test-weighings of infants before and after the consumption of a known amount of milk, the mean +/- SD "recovery" of milk ingestion was 94.9 +/- 13.2%. The weight of milk extracted by a mechanical pump was approximately 7% greater than the amount measured during test-weighings of infant of the same women within periods of 1 wk. To evaluate the possibility of performing abbreviated studies in field settings, the proportion of 24-h milk consumption received during 12 daytime hours was measured by test-weighings. Daytime consumption ranged from 46 to 58% of 24-h consumption (mean +/- SD = 52 +/- 3%). Attempts to predict the amount of milk consumption during 341 daytime studies from the age of infants and their frequency and duration of feedings met with limited success. Although each independent variable was significantly correlated with the amount consumed (multiple r = 0.69, p less than 0.001), the SE of the estimate (Sy.x) was relatively large. The effects of time of day and side of extraction on the volume and composition of extracted milk were determined during 24-h studies of seven women. There were significant changes in the concentration of fat (p less than 0.001) and nitrogen (p = 0.003) during the day and significant differences in the concentrations of fat (p = 0.04) and lactose (p = 0.04) and in the volume (p less than 0.001) of milk produced by each breast. The importance of these findings for the planning and interpretation of studies of human lactation is discussed.