Interactions between metabolic and reproductive functions in the resumption of postpartum fecundity

Abstract

Lactation has long been recognized as a major determinant of interbirth intervals. The temporal pattern of nursing has been proposed as the mechanism behind lactational amenorrhea. We present a new model of the dynamic regulation of lactational amenorrhea that identifies maternal energy availability as the main determinant of ovarian resumption. Variation in the intensity of lactation remains a component of the model as a determinant of the absolute energetic cost of milk production. However, maternal energy supply determines net energy availability; a larger energy supply leaves a greater net energy surplus than a smaller energy supply (lactation costs being equal). We characterize the hormonal postpartum profile of 70 lactating Toba women of Argentina. We use C-peptide, which reflects maternal insulin production, as a measure of energy availability. Initially low, insulin production rises as the postpartum period progresses, reflecting the declining metabolic load of lactation. A short period of supernormal insulin production precedes menstrual resumption. The high levels of insulin may play a role in stimulating the resumption of ovarian activity, which in turn may help to resolve the transient period of insulin resistance. The dynamics of insulin sensitivity during lactation would aid in synchronizing the resumption of ovarian function with a reduction in the energy demands of milk production. This hypothesis is supported by the sustained weight gain experienced by lactating women during the months preceding the first postpartum menses. The link between fecundity and energy balance could serve as a mechanism for adjusting the duration of lactational amenorrhea to the relative metabolic load of lactation.

Figure 1

Changes in energy balance, as estimated using the change in body mass index (ΔBMI), in breastfeeding women as they approach their first postpartum menses (time 0).

Figure 2

Mean (± SE) values of individually normalized C-peptide values relative to month of menstrual resumption for all study subjects. Normalized C-peptide values are expressed as a percentage of the post-resumption mean valued for each subject (see text).

Figure 3

Mean (± SE) values of individually normalized cortisol values relative to month of menstrual resumption. Normalized cortisol values are expressed as a percentage of the post-resumption mean valued for each subject.