Responses to the 35% CO challenge in postpartum women

Lactation has been associated with suppression of some components of the neuroendocrine stress response. In humans, suppression of the hypothalamo-pituitary-adrenal (HPA) axis has been demonstrated in response to both a psychological and an exercise challenge, but appears to be limited to a short period of time following suckling. Information regarding other components of the stress response and to other challenges in humans is limited. We have evaluated the endocrine, autonomic and psychological response to a single breath of 35% CO(2) during lactation. The 35% CO(2) challenge is a safe and simple test that has been shown to stimulate the HPA axis, produce autonomic activation and emotional arousal. Eight breastfeeding and six bottle-feeding mothers, 6 weeks' postpartum, and eight control women were studied. Twenty minutes following the cessation of feeding, plasma cortisol levels were significantly reduced in the breastfeeding women (P = 0.002 compared with control and P = 0.003 compared with bottle-feeders). Despite this, cortisol, blood pressure, heart rate and psychological responses to the challenge were no different in the breastfeeding group compared to either the control or bottle feeding groups. These results confirm that suckling is associated with short-term suppression of cortisol, but this has no effect on the ability of the mother to mount a normal hormonal, autonomic and psychological response to the 35% CO(2) challenge.