Abstract
Isn't it time that we realize that breast milk is critical for normal human development beyond its nutritional advantages, its anti-infectious protective qualities, or its enhancement of the maternal–infant attachment process. Maturation of the fetus, to a degree, relies on the development of functional intrauterine circadian rhythms that reflect the varying maternal nutritive, metabolic, non-nutritive hormonal, and physiological cues set by the mother's own circadian rhythms. At birth, the newborn has not as yet developed its own circadian rhythms. Thus, after delivery the infant relies on rhythmic maternal cues that are mediated through the breast milk's varying composition that in turn reflects the mother's own rhythmic patterns.1,2
These fluctuations of breast milk content across the 24 hours day, particularly the differences between the daytime as opposed to the night-time components, and the infant's ingestion of such milk with the varying content is termed “chrononutrition,” that is, the conveying of time-of-day information through appropriately timed feedings.3,4
It should be noted that these differences in milk composition over the day are in addition to the well-known differences in the content of colostrum and mature milk, let alone the differences of milk from mothers who delivered a preterm infant as to what is the content of the milk from a mother who delivered at the end of the 9th month. To remind the readers; colostrum and transitional milk as opposed to mature milk is lower in fat and lactose content but richer in oligosaccharides, total protein level, and has higher levels of secretory IgA, lactoferrin, whereas preterm milk is richer in protein, sodium, and calcium as opposed to mature milk. 5
What has not been fully appreciated to date is the fact of now well-documented intra-daily variations in the levels of the components of human milk, both nutritive and non-nutritive. Total fat content is higher at night. Activity promoting neuroactive amino acids (tyrosine, methionine phenylalanine aspartic acid, and glycine) are at peak level in day milk, whereas tryptophan peaks at night. Most striking is the diurnal variation of milk cortisol level with its peak level in the day. In contrast, there is a near absent level of melatonin in day milk and an elevated level in the night-time sleep period milk. Similarly leptin levels are highest between 10 PM and 4 AM. 1
Previous surveys of breastfeeding mothers in the United Stated have noted that 85% of those mothers who are breastfeeding feed their infants previously expressed milk, whereas 25% bottle feed expressed milk on a daily basis and 6% exclusively feed expressed milk. 6 Thus, it is clear that a significant proportion of the human milk being fed to infants is “mistimed” and potentially has a negative effect on the development of the infants' own circadian rhythms.
To date, there is, as yet, sparse information on the clinical consequences of feeding of such mistimed milk and thus the article by Booker et al. in this month's issue of Breastfeeding Medicine reporting on a study of the impact of feeding mistimed expressed milk on sleep patterns is most welcome. Hopefully, this article and others 7 will serve as a stimulus for more extensive studies, particularly relating to the consequences of mistimed feedings with their inappropriate and discordant cortisol and melatonin levels. Simply put, exposing infants to these levels is inappropriate as it ensues from current practices of ignoring the dramatic changes of the levels milk components over the 24 hours of the day/night cycle.
What can be done, however, until we have more evidence-based scientific data. In my opinion, there are most simple and “low cost’ practices and procedures that should be routinely instituted. First and foremost mothers should be encouraged as much as possible to nurse, especially at night and to minimize bottle feeding of mistimed expressed breast milk. To minimize the mistiming, mothers should label all expressed milk with the time of day it was expressed and in turn feed such milk at the same comparable time slot to the infant. Feeding daytime expressed milk with its elevated cortisol level at night or melatonin-free milk at night to infant should be avoided as it may have ultimately negative consequences, but surely matching the feeding and expressing time as much as possible should be the default procedural approach. Breast milk banks should similarly request donor mothers to label the time of expression and in turn the banks should pool the donated milk into separate day and night batches.
It is clear that the old adage that “there is a time and place for everything” is still valid, most especially when the right “place” for the infant is at the mother's breast and when the breast is not available then feeding appropriately “timed” milk is just the correct thing to do.