Women who breast-feed their infants produce lower levels of stress response hormones than do women who bottle-feed, according to research conducted by the National Institute of Mental Health (NIMH). The study is the first to explore the effects of lactation on hormonal stress responses in humans.

The NIMH researchers, led by Margaret Altemus, M.D., theorize that lactation-induced suppression of stress responses serves several purposes for both mother and baby. First, it may help to conserve energy needed for production of breast milk. Second, it may minimize the psychological stress associated with the demands of infant care, thus enhancing milk release. Third, it may improve immune function during the postpartum period.

During any stressful situation, the brain's neuroendocrine, or hormonal, systems are activated. In the hypothalamus, the brain's "control center" for the neuroendocrine system, various stress response hormones are released--including vasopressin and corticotropin-releasing hormone, or CRH. CRH and vasopressin have arousing effects in the brain and also travel to the pituitary gland, where they trigger the release of adrenocorticotropic hormone, or ACTH. ACTH, in turn, stimulates the adrenal glands to produce cortisol, which mobilizes energy for the body's response to stress.

Studies of rats have shown that lactation suppresses a variety of physiological responses to stress, including the release of several stress hormones. To determine whether the same changes take place in humans, Altemus and her colleagues studied twenty postpartum women--10 who were lactating and 10 who were not. Women in the study were between 7 and 18 weeks postpartum, and between 24 and 36 years of age.

The researchers used treadmill exercise--the same type of "stress test" given to cardiac patients--to elicit the hormonal stress response. Each woman performed treadmill exercise for 20 minutes, and blood hormone levels were taken before, during, and after the exercise was completed.

Before the treadmill test, levels of ACTH, cortisol, and vasopressin were similar in both the lactating and non-lactating groups. In response to the stress of exercise, all participants showed an increase in hormone levels; however, the increase was significantly less among women who were breast-feeding their babies compared to those who were bottle-feeding. Lactating women had lower levels of ACTH and cortisol and showed a trend toward a lower vasopressin response than did non-lactating women.

While the exact mechanisms responsible for the reduction of stress hormone responses during lactation remain to be determined, the findings could help to shed light on the biological underpinnings of stress and anxiety disorders.

According to Altemus, increased levels of the stress hormones vasopressin and CRH have been associated with obsessive- compulsive disorder (OCD), a type of anxiety disorder, in humans. Both hormones also promote fearful behaviors when administered to animals.

"Preliminary research has also shown that lactation appears to reduce the symptoms of anxiety disorders," said Altemus. "However, more work needs to be done to help us pinpoint which elements of lactation physiology are responsible for producing this anti-stress effect."

The study, "Suppression of Hypothalmic-Pituitary-Adrenal Axis Responses to Stress in Lactating Women," is published in the October issue of the Journal of Clinical Endocrinology and Metabolism. Other authors of the report are Patricia Deuster, Ph.D., Elise Galliven, C. Sue Carter, Ph.D., and Philip W. Gold, M.D.

Information provided by the NIMH.