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New research from the Liggins Institute of University of Auckland has shed light on what affects stress hormone levels in breast milk.
Breast milk is recognised for its nutritional and immunity-boosting powers, but it also contains other less well-understood factors – including stress hormones.
As well as a major regulator of the body’s stress response, the stress hormone cortisol is an important influencer of mood and growth. Cortisol also directs energy towards making fat, not muscle, making it even more vital in early growth and development.
Researchers at the Liggins Institute analysed breast milk samples from 650 mothers when their babies were three to four months of age (12-16 weeks).
The findings revealed levels of cortisol, and the related hormone cortisone, are higher in the milk of mothers who delivered their babies by caesarean section, or who have no partner at home.
Emotional challenges
“These findings reinforce that there are real emotional and biological challenges that face mothers who have undergone a C-section delivery or who do not necessarily have the full support to cope fully with a new baby,” Shikha Pundir, a final year PhD student at the Institute said.
It is still unclear as to how stress hormones in breast milk affect babies’ growth and development, she said.
“We know that these stress hormones are not ‘bad’ in themselves. There is evidence a certain amount is needed to stimulate healthy development. Exactly how much has not yet been defined,” Ms Pundir said
Evidence from animal studies suggests that higher cortisol in mother’s milk affects babies’ temperament.
Studies in rodents showed that stress during motherhood emboldened offspring.
But primate studies showed that higher cortisol in milk made monkey babies more irritable and nervous, and a similar effect has been observed in human studies.
Environmental influence
“Our study shows that maternal biological and social environment can significantly modify milk composition. But more research is required to know if there are any long-term effects of these second-hand hormones on infant physiology,” Ms Pundir said.
Professor Cameron-Smith, who leads this project, said that these findings underline the importance of providing support to all mothers.
“Having a new baby and breastfeeding are often difficult and stressful times. If there is no effective intervention, then, this research shows that the mother may be transmitting to her baby some of this stress, which could potentially have a long-term adverse impact on the baby’s health. We need to be supporting all mothers,” he said.
This research also hinted at the complexity of stress hormones in milk.
Unlinked factors
Factors that were not linked to stress hormone levels included a mother’s age, BMI, weight gain during pregnancy, gestational diabetes, educational level, and number of other children.
Ms Pundir has a second study underway to investigate whether milk stress hormone levels fluctuate or remain stable over a 24-hour period.
She talked about her findings at an event in Auckland celebrating the Liggins Institute’s 15th anniversary on Wednesday, August 10, 2016.
Key points of Survey
Main finding: Higher cortisol and cortisone levels in the milk of mothers who delivered their babies by caesarean section versus vaginal delivery, and of mothers who had no partner at home versus those living with partners
There seems to be an optimal level of stress hormones in breast milk for babies’ healthy development
Elevated stress hormones are known to adversely affect babies’ physiological and psychological development, but more research is needed to understand exactly how, and the optimal range
Studies in rodents found that stress during motherhood emboldened offspring. But primate studies showed that higher cortisol in milk made monkey babies more irritable and nervous, and a similar effect has been observed in human studies.
