Circadian Variation of Breast Milk Components and Implications for Care

Introduction

The extensive benefits of breast milk are well recognized. It is also well known that components of breast milk vary between foremilk and hindmilk, and over time. Less known is that a number of breast milk constituents vary over the course of a 24-hour day. In this brief review, we will explore the extent and potential biological implications of this circadian rhythm of breast milk components.

Components That Demonstrate Circadian Variation

It has been known for many years that the fat content of human milk displays a circadian variation in term samples, ¹ and more recently this has also been demonstrated in samples from mothers who delivered preterm,^2–4 with higher concentrations found in evening samples. Circadian variability in protein and carbohydrate concentrations are less robust, although the elevated nocturnal level of tryptophan ⁵ is a notable exception.

Some studies^6,7 demonstrated circadian variability in calcium, phosphorus, and magnesium levels in breast milk, whereas others^8,9 were unable to detect significant variation.

Several important micronutrients and hormones exhibit a diurnal variation in breast milk, including some vitamins, ¹⁰ iron, ¹¹ nucleotides, ¹² micro-RNA, ¹³ cortisol,^14,15 and melatonin.^16,17 Katzer et al. ¹⁷ also demonstrated diurnal variation in levels of superoxide dismutase and glutathione peroxidase-3 in preterm milk, but not from mothers who delivered at term.

In many cases of circadian rhythms of breast milk components, the individual compound appears to enter the breast milk by passive transfer, in that diurnal changes parallel those in the serum and levels in breast milk are the same or lower. In a few cases, this transfer appears to have an active component, when the pattern diverges from or levels exceed those in the serum.

Implications for Care

It is given that certain components of breast milk, such as macronutrients and minerals, are essential for the survival and growth of the newborn. It is less clear whether other components play a necessary function or are simply “innocent bystanders,” which are measurably present due to passive transfer, but have no inherent biological purpose, at least in breast milk. Many components, such as cortisol and melatonin, are not present in the formula, and it has not been demonstrated that the infant suffers from their absence. Even those that are essential and demonstrate circadian variation in breast milk, such as fat content, may or may not have biological impact if they are delivered at constant concentrations over the course of the day as would occur in formula feeding. Proof that there is value in delivering these substances at all (in the case of the hormones, for example) or with a circadian variation mirroring that in breast milk is difficult to achieve, and has not been done in the population of perhaps the greatest interest—preterm infants. Consequently, the discussion that follows is necessarily speculative and meant more to stimulate additional questions and research than to define optimal care practices. For the sake of this discussion, I will leave the question of whether the presence of certain compounds such as hormones and antioxidants is beneficial in milk fed to preterm infants (although it seems likely that many of them are), and focus only on whether circadian variation of certain substances has potential benefit to the preterm.

To address this question, it is worth reviewing the circadian environment of the fetus and contrast it with that of the newborn intensive care unit (NICU). In utero, the fetus receives a rich milieu of circadian stimuli, including the mother's activity, body temperature, and probably most importantly, transplacental hormones, including cortisol and melatonin. At birth, these prenatal zeitgebers are lost and a new set of circadian stimuli are presented, most notably daylight, but there is also a continuation of certain maternal signals, including those in the breast milk, if the infant remains in extended intimate contact with her. In the NICU, these signals are often lost, and any nascent circadian tendencies are suppressed. Lighting is often not cycled in spite of evidence of its benefit, ¹⁸ temperature variations are prevented by servo-controlled warming devices, breast milk is fed without consideration of the time of day it was expressed, and extended maternal contact is often restricted.

We might get some indication of the value of a circadian stimulus to the preterm infant by examining other age groups, where there is evidence of benefit when it is provided, or evidence of harm when normal circadian signals are disrupted. As already noted, providing either constant bright or constant dim lighting to high-risk premature infants in the NICU leads to less optimal outcomes than the use of cycled lighting. There is an extensive literature supporting this same effect in infants, children, and adults.^19,20

With respect to circadian stimuli contained in nutritional substances, data are less robust, but still credible. In term infants, formula adaptations that used higher levels of protein and nucleotides during the day with lower levels of tryptophan and carbohydrates, then reversed that pattern at night led to improved sleep latency and duration when compared with a standard formula.^21,22 The same group subsequently showed that adding tryptophan, adenosine, and uridine to an infant cereal fed at night to infants 8–16 months of age with pre-existing sleep disorders led to an improvement in sleep patterns. ²³ Again, evidence exists for a similar nutritional impact on circadian behavior throughout the lifespan.^24,25 Perhaps the study most relevant to preterm infants in the NICU, however, was done in Macaca mulatta nonhuman primates, in which it was shown that melatonin supplementation reduced the negative impact of mother–infant separation on the circadian system. ²⁶

Conclusion

Several components of breast milk show circadian variability. While this may involve passive transfer of some substances that do not act as zeitgebers, it is likely that at least some of these macronutrients, hormones, and micronutrients produce circadian stimuli that enhance the well-being of breast-fed infants. Future research is necessary to determine whether high-risk infants will benefit if expressed breast milk is given matching the same circadian phase as it was expressed.