Maternal, Placental and Cord Zinc Components in Healthy Women with Different Levels of Serum Zinc

Abstract

Serum zinc is known to decrease during pregnancy, but the range of individual values at the end of normal gestation may be considerably large, with uncertain physiological significance in terms of maternal zinc status and maternal-fetal transfer of zinc. In this study we compared several maternal and cord blood indices of zinc status, placental zinc, placental metallothionein and the relationship between maternal, cord and placental components, in 40 healthy pregnant women at delivery with different levels of serum zinc. Subjects were divided into three groups according to serum zinc using as cutoff points the lower and upper quartile values [LZn, < 7.6 µmol/l (n = 10); MZn, 7.6–10.7 µmol/l (n = 20), and HZn, > 10.7 µmol/l (n = 10)]. Habitual zinc intakes were similar in all groups (average 11.5 mg/day). Considering all women, maternal serum and erythrocyte zinc correlated significantly (r = 0.40; p = 0.021). Maternal erythrocyte zinc was higher (p < 0.01) in HZn compared to the other groups. Maternal and cord values of serum zinc correlated significantly (r = 0.43; p = 0.006). Cord serum zinc values were very similar in LZn and MZn but were higher (p < 0.01) in HZn. Cord erythrocyte zinc levels were similar in all groups and about 20–25% of the maternal values. Cord erythrocyte metallothionein levels were also similar in all groups and similar to maternal values. Comparing the percentage distribution of zinc in maternal and cord serum fractions, major differences were observed in HZn, with zinc in albumin fraction being 70% in cord compared to 56% in maternal serum. There was a higher (p < 0.01) percentage of zinc bound to α₂-macroglobulin fraction in maternal serum of HZn compared to maternal serum of the other groups. Maternal and cord zinc in the albumin fractions correlated significantly (r = 0.48; p = 0.002) particularly in HZn (r = 0.71; p < 0.021). Placental zinc correlated negatively (r = ––0.34; p = 0.035) with zinc in the maternal α₂-macroglobulin fraction, but did not relate to placental metallothionein, which had similar levels in all groups (average 0.28 nmol/g wet tissue). Placental zinc levels were lower (p < 0.01) in HZn. Our results indicate that high levels of maternal serum zinc in healthy women at delivery may be related to maternal tissue zinc redistribution that could favor diffusional components of maternal-fetal transfer of zinc.

Maternal Zinc Intakes and Homeostatic Adjustments during Pregnancy and Lactation

“…However, the increase in maternal and cord serum zinc was small (≈5%), and their serum zinc concentrations were lower than the average values reported for American women [9]. Cord serum zinc concentrations also were lower than values reported in other populations with higher zinc intakes [38,43]. These data suggest that zinc supplementation has a small effect on maternal plasma and cord blood zinc concentrations, and there was no effect on infant birth weight or other body-size measurements [44].…”

“…Cord blood plasma/serum zinc concentration is consistently higher than corresponding levels in maternal blood with a mean maternal/cord ratio of about 0.7 [7,33,38]. However, cord blood erythrocyte zinc is about one-third or less than maternal concentrations with an average maternal/cord ratio of about 3 [7,33,38].…”

“…However, cord blood erythrocyte zinc is about one-third or less than maternal concentrations with an average maternal/cord ratio of about 3 [7,33,38]. In vitro studies of human placentae and animal models [39,40,41,42] show that placental zinc uptake and release involves an active and highly regulated process.…”

“…In vitro studies of human placentae and animal models [39,40,41,42] show that placental zinc uptake and release involves an active and highly regulated process. Thus, cord serum zinc concentrations remain relatively constant at about 14 μmol/L over a wide range of maternal serum zinc concentrations (6.0–15.6 μmol/L) [38]. …”

“…Human placental metallothionein has been measured in several studies using different methods [33,38,50,51,52,53]. For example, metallothionein levels were measured in maternal and cord erythrocytes and placental tissue from Peruvian and Brazilian women with habitual zinc intakes of 8 and 11.5 mg/day, respectively (Figure 1).…”