Lactose in blood in nonpregnant, pregnant, and lactating women

Lactose synthesized in the mammary gland can pass into the bloodstream by either a paracellular or transcellular pathway. In nonpregnant, nonlactating women, the concentration of lactose in the blood plasma was 1.5 +/- 0.1 micromole (mean +/- SEM) in 9 women and undetectable in another 11 women. During pregnancy, this concentration was 3.7 +/- 0.4 micromole at 10-21 weeks of gestation, with an increase to 8.7 +/- 1.8 micromole by 38-40 weeks of gestation. At the initiation of lactation, the concentration of lactose peaked 3-5 days after birth, with a mean peak concentration of 75 +/- 18 micromole, and then decreased to 30 +/- 8 micromole when lactation was well established at 6 weeks after birth. These findings suggest that the mammary glands are synthetically active by the beginning of the second trimester of pregnancy and reach maximum synthetic capacity soon after birth. Measurement of concentrations of lactose in the blood plasma during pregnancy and lactation may allow an assessment of the successful initiation of both lactogenesis I and II