C-peptide levels and the duration of lactational amenorrhea.

Changes in insulin sensitivity play an important role in directing metabolic energy toward milk production in lactating mammals. In rats, prolactin causes upregulation of insulin receptors in mammary tissue together with an increase in peripheral insulin resistance, leading to a preferential allocation of energy to support milk production (1, 2). In humans, the shift in energy allocation during lactation away from peripheral tissues and toward milk production is correlated with lower insulin levels (2–4).