19 March 1999

On The Breast And Feeling Mellow

A new study at the University of North Carolina at Chapel Hill links increased levels of the hormone oxytocin to lower blood pressure among mothers who nurse their babies. The findings help explain why many breast-feeding mothers report feeling mellow and relaxed after nursing.

"Oxytocin is most commonly known for its role in the letdown of milk during breast-feeding, but also has effects on brain areas involved in emotion and stress responses," said Dr. Kathleen Light, the study's senior investigator and professor of psychiatry at the UNC-CH School of Medicine.

The results -- reported for the first time March 17 at the American Psychosomatic Society Annual Meeting in Vancouver, British Columbia -- demonstrated that mothers with higher oxytocin levels in the blood had lower blood pressure before and after discussing recent life problems.

The UNC-CH study involved 26 mothers of infants; 14 breast-fed their babies and 12 bottle-fed them. Blood pressure and blood levels of oxytocin were measured before, during and after the stressful speech experience.

"A high oxytocin response appeared to help the mother limit the duration of the stress response," Light explained. "She showed a normal response during the stressful task, but less evidence of stress anticipating the event or recovering after it. Through their questionnaires, high oxytocin-responding mothers also seemed to have less hostile feelings and more positive mood and interpersonal interactions."

The lower blood pressure seen in the laboratory part of the study continued when the mothers had 24-hour monitoring at home. Blood pressure levels of high oxytocin reactors were 10 points lower than that of other women for one hour after feeding and six to nine points lower during sleep. They were also about four to five points lower during daytime ambulatory monitoring, Light said. The study suggests that mothers who nurse their infants are much more likely to be high oxytocin reactors than bottle-feeders, 50 percent vs. 8 percent.

"Parenting is such a stressful time, especially in the beginning," said Tara Smith, presenting study author and student assistant. "Biologists may know much more about the causes of increases in stress. But it is very exciting to find out more about what causes decreases in stress."

Along with Light and Smith, other UNC-CH collaborators were psychologists Josephine Johns and Julie Hofheimer and a team of students and research assistants, including Sunny Chung and Monica Adamian.