has been associated with attenuated hormonal responses to exercise stress
in humans. This study was designed to determine the effect of lactation
on hypothalamic-pituitary-adrenal axis, autonomic nervous system, and anxiety responses to psychological stress
The Trier Social Stress
Test was administered to 24 lactating women, 13 postpartum nonlactating women, and 14 healthy control women in the early follicular phase of the menstrual cycle. Lactating women were stressed at least 40 minutes after last feeding their infant.
, heart rate, diastolic blood pressure, systolic blood pressure, and subjective anxiety ratings were all significantly increased in response to the psychological stress
< .0001). There were no differences among the three groups in any of these responses to the stress
. However, postpartum nonlactating women did have a persistently higher systolic blood pressure and lower cardiac vagal tone
than the lactating women and control subjects. In addition, the typical negative correlation between cardiac vagal tone
and heart rate was consistently higher in lactating women than nonlactating postpartum women and controls, which suggests stronger vagal control of heart rate in lactating women. In addition, there was no change in oxytocin
in response to the stress
, and baseline oxytocin
levels did not differ among the three groups.
These results indicate that physiological and subjective responses to social stress
are not attenuated in lactating women tested at least one hour after feeding their infant. However, enhanced vagal control of cardiac reactivity was observed in lactating women. In addition, postpartum women who did not lactate showed evidence of increased sympathetic and decreased parasympathetic nervous system tone.