When It Comes to Breastfeeding, Study Shows Timing Is an Important Consideration

Rutgers-led research suggests that feeding expressed milk at the right time may bolster infant sleep, immunity and development

Rich in nutrients, immune-boosting proteins and beneficial bacteria, breast milk has long been hailed as nature's perfect food for babies.

Now, a Rutgers University-led study suggests that breast milk is more than just nourishment. Breast milk also is a biological clock, sending time-sensitive signals to help guide a baby's development. With breast milk, timing might be an important consideration, especially when feeding expressed breast milk.

Researchers from Rutgers and the University of Puerto Rico have discovered that breast milk changes in composition depending on the time of day it's produced. Their findings, publishedinFrontiers in Nutrition,show that crucial hormones such as melatonin and cortisol, which are substances known for regulating sleep and stress, fluctuate in breast milk over a 24-hour period.

"Breast milk is a dynamic food," said Melissa Woortman, a recent doctoral degree graduate from the Department of Nutritional Sciences in the Rutgers School of Environmental and Biological Sciences and the study's lead author. "We found that the concentrations of bioactive components vary depending on the time of day, which means the timing of feeding expressed milk could be important."

The team collected 236 breast milk samples from 38 lactating mothers at four time points: 6 a.m., noon, 6 p.m., and midnight. They measured levels of melatonin, cortisol, oxytocin, and immune proteins, in addition to analyzing the breast milk microbiota. Melatonin peaked at midnight, helping signal sleep, while cortisol was highest in the early morning, supporting alertness and metabolism.

These hormonal rhythms were less pronounced in mothers with a higher body mass index, and the immune proteins were most abundant in milk for infants less than a month old. This suggests the bodies of lactating mothers adapt breast milk to meet the changing needs of growing babies, scientists said.

"Circadian rhythms in infants are still developing," said Maria Gloria Dominguez-Bello, the Henry Rutgers Professor of Microbiome and Health in the Department of Biochemistry and Microbiology in the Rutgers School of Environmental and Biological Sciences and the senior author of the study. "Breast milk may help guide that development, especially in the early months."

The researchers also found that the types of bacteria in breast milk shifted throughout the day, with skin-associated microbes more common at night and environmental bacteria more prevalent during the day. These microbial changes could influence how a baby's digestive system develops and how their immune system is trained.

Given these findings, the researchers suggest a simple practice. "Labeling expressed milk as 'morning,' 'afternoon,' or 'evening' and feeding it correspondingly could help align expressing and feeding times and preserve the natural hormonal and microbial composition of the milk, as well as circadian signals," Dominguez-Bello said.

In modern societies where mothers may not be able to breastfeed around the clock, Woortman added, "aligning feeding times with the time of milk expression is a simple, practical step that maximizes the benefits of breast milk when feeding expressed milk."

Other Rutgers researchers who contributed to the study included: Haipeng Sun and Jincheng Wang, postdoctoral associates in the Department of Biochemistry and Microbiology in the School of Environmental and Biological Sciences; Krystin Englehardt, formerly a Neonatal-Perinatal Medical Fellow in the Department of Pediatrics at Rutgers Robert Wood Johnson Medical School; and Lawrence Kleinman of the Institute for Health, Health Care Policy and Aging Research and the Department of Pediatrics at Rutgers Robert Wood Johnson Medical School.