Background: Little is known about circadian variations of the fat content in expressed human milk by mothers of preterm infants.
Objective: To test the hypothesis that the fat content of expressed preterm human milk is higher in samples expressed in the evening (i.e. after 3 daily meals) than in the morning (after a night-long fast).
Methods: We collected samples of expressed human milk obtained from 39 mothers of hospitalized growing preterm infants aged 7-14 days, with a gestational age at birth ranging from 26 to 33 weeks, who routinely expressed all their milk every 3 hours, during the day time, just before bed time, and as soon as they woke up, using a commercial breast pump (Medela AG, Baar, Switzerland). One sample was obtained from the first morning expression (between 0600 and 0900) and the second from the evening expression (between 2100 and 2400). The entire quantity of expressed milk was collected, mixed and measured in a capillary tube after centrifugation at 9000 rpm for 5 minutes. Creamatocrits (CMT) were performed in duplicates. Each sample was read independently by 2 investigators who were not aware of the origin and time of sampling and the results were averaged. Results are expressed as mean +/- SD, and analyses were by paired t-test and regression analysis.
Results: CMT was significantly higher in evening than in morning samples (7.9 +/- 2.9% vs. 6.6 +/- 2.8%, P = 0.005). Neither CMT nor the morning-evening difference in CMT correlated with gestational age or birthweight. The morning CMT correlated significantly with the evening CMT (P < 0.001).
Conclusions: There are significant circadian variations in the CMT of expressed preterm human milk. We speculate that if higher caloric content of expressed human milk is needed in a specific preterm infant, evening sample, if available, should be used preferentially.