Studies of zinc metabolism in pregnant and lactating rats

Marginal zinc deficiency in pregnant rats impairs bone matrix formation and bone mineralization in their neonates

Zinc (Zn) deficiency during pregnancy may result in a variety of defects in the offspring. We evaluated the influence of marginal Zn deficiency during pregnancy on neonatal bone status. Nine-week-old male Sprague-Dawley rats were divided into two groups and fed AIN-93G-based experimental diets containing 35 mg Zn/kg (Zn adequately supplied, N) or 7 mg Zn/kg (low level of Zn, L) from 14-day preconception to 20 days of gestation, that is, 1 day before normal delivery. Neonates were delivered by cesarean section. Litter size and neonate weight were not different between the two groups. However, in the L-diet-fed dam group, bone matrix formation in isolated neonatal calvaria culture was clearly impaired and was not recovered by the addition of Zn into the culture media. Additionally, serum concentration of osteocalcin, as a bone formation parameter, was lower in neonates from the L-diet-fed dam group. Impaired bone mineralization was observed with a significantly lower content of phosphorus in neonate femurs from L-diet-fed dams compared with those from N-diet-fed dams. Moreover, Zn content in the femur and calvaria of neonates from the L-diet group was lower than that of the N-diet-fed group. In the marginally Zn-deficient dams, femoral Zn content, serum concentrations of Zn, and osteocalcin were reduced when compared with control dams. We conclude that maternal Zn deficiency causes impairment of bone matrix formation and bone mineralization in neonates, implying the importance of Zn intake during pregnancy for proper bone development of offspring.

Maternal zinc deficiency in rats affects growth and glucose metabolism in the offspring by inducing insulin resistance postnatally

Interactions among zinc (Zn), insulin, and glucose metabolism are complex. Maternal Zn deficiency affects maternal carbohydrate metabolism, but the mechanisms underlying changes in glucose homeostasis of offspring are not well understood. Rats consumed Zn-deficient (ZnD; 7 microg/g) or control (ZnC; 25 microg/g) diets ad libitum from 3 wk preconception to 21 d postparturition. Litters were culled to 7 pups/dam postnatally and pups were allowed to nurse their original mothers; after weaning, pups were fed nonpurified diet. Insulin and glucose tolerance tests were performed on the pups at wk 5 and 10. Although there was no difference in birth weight between groups, ZnD pups weighed significantly more than controls by d 10 (+5%) and 20 (+10%). Both blood glucose and serum insulin-like growth factor (IGF-1) concentrations at wk 3 were significantly higher in ZnD pups than in controls. Both male and female ZnD rats were less sensitive to insulin and glucose stimulation than controls at wk 5 and 10. At wk 15, serum leptin concentrations were higher in male ZnD rats than in controls. Phosphorylation of muscle Akt protein, an insulin receptor (IR) signaling intermediate, was lower in female ZnD rats than in controls at wk 15, but they did not differ in phosphorylation of IR. Maternal Zn deficiency resulted in greater serum IGF-1 concentrations and the excessive postnatal weight gain in their offspring as well as impaired subsequent glucose sensitivity. It was associated with gender-specific alterations in the serum leptin concentration and the insulin signaling pathway. These findings suggest that suboptimal maternal Zn status induces long-term changes in the offspring related to abnormal glucose tolerance.

Influence of low dietary calcium during pregnancy and lactation on zinc levels in maternal blood and bone in rats

The effect of low dietary calcium on maternal zinc nutritional status was studied. Two groups of 6 adult female Wistar rats were fed during pregnancy and lactation with experimental diets containing either 0.2 g (LCa) or 0.6 g (NCa) of calcium/100 g. Both diets contained/100 g: 20.0 g protein (potassium caseinate), 3.5 mg Zn, 0.6 g P. A third group (n = 6) was fed a "stock diet" (SG), containing/100 g: 24.8 g protein, 1.5 g Ca, 0.6 g P, 11.6 mg Zn. Maternal blood samples were drawn from the tail before mating (To), at delivery (D) and at weaning (W); dams were sacrificed at weaning and the right femur was excised. Determinations (atomic absorption spectrometry) were: Zinc in red blood cells (RBC), Zn and Ca in ashed femur. The results (mean +/- SD) were: RBCZn (microg/mL) at To: 8.65 +/- 1.80, which did not change in the SG or in the NCa groups, but increased significantly in the LCa group (p < 0.001) (D: 18.20 +/- 4.63; W: 26.70 +/- 6.02), regarding To. Femur Zn (microg/100 mg) showed an increase (p < 0.001) in the LCa group (30.2 +/- 2.1) regarding both SG (25.3 +/- 0.7) and NCa groups (24.1 +/- 0.7). Femur Ca (mg/100 mg) decreased (p < 0.05) in the LCa group (19.2 +/- 0.9) regarding both SG (24.0 +/- 0.5) and NCa groups (21.4 +/- 0.7) and leading to a significant increase in Zn/Ca ratio (p < 0.001) in the LCa group. Therefore, dietary calcium deficiency during pregnancy and lactation would produce an increase of Zn utilization, reflected in the increase of maternal blood Zn levels and in femur Zn content.

Determinants of maternal zinc status during pregnancy

Zinc deficiency in pregnant experimental animals limits fetal growth and, if severe, causes teratogenic anomalies. Although the data from human studies are not consistent, similar outcomes have been observed and were associated with poor maternal zinc status. This paper reviews humans studies of zinc status and pregnancy outcome, describes the physiologic adjustments in zinc utilization during pregnancy to meet fetal needs while maintaining maternal status, and identifies dietary and environmental conditions that may override those physiologic adjustments and put the health of the mother and fetus at risk. Adjustments in intestinal zinc absorption appear to be the primary means by which zinc retention is increased to meet fetal demands. However, transfer of sufficient zinc to the fetus is dependent on maintenance of normal maternal serum zinc concentrations. Conditions that could interfere with zinc absorption include intake of cereal-based diets that are high in phytate, high intakes of supplemental iron, or any gastrointestinal disease. Conditions that may alter maternal plasma zinc concentrations and the transport of zinc to the fetus include smoking, alcohol abuse, and an acute stress response to infection or trauma. Supplemental zinc may be prudent for women with poor gastrointestinal function or with any of these conditions during pregnancy.

Short-term zinc kinetics in pregnant rats fed marginal zinc diets

The effect of pregnancy and a chronic, marginal intake of zinc on zinc kinetics was studied in rats. Weanling female rats were fed either a zinc-adequate diet, containing 30 microg Zn/g, (30Zn) (n = 16) or a marginally zinc-deficient diet, containing 6 microg Zn/g, (6Zn) (n = 16). After 6 wk, half of each group was mated (30ZnPG, 6ZnPG). A third group of pregnant rats was pair-fed (PFPG) (n = 6) to the 6ZnPG group. On d 20 of gestation, or at the end of the 9-wk study, 65Zn was injected intravenously. The plasma 65Zn disappearance curve over the next 105 min was used to study the size and fractional turnover rates of two rapidly exchanging zinc metabolic pools (pool a and pool b). Plasma zinc concentrations on d 20 of gestation were significantly lower in the 6ZnPG group compared with the 30ZnPG and PFPG controls, (P < 0.05). The exchangeable pools were also smaller in the 6ZnPG group compared with the 30ZnPg and PFPG groups, (P < 0.02); this reduction was accompanied by a 60% greater fractional turnover rate of pool a, (P < 0.02). Pregnancy outcomes did not differ among the three groups. We conclude that there is an increase in the turnover rate of the exchangeable plasma zinc pool when dietary zinc intake is marginal during pregnancy. This response may help maintain a supply of zinc to the growing fetus when plasma zinc concentrations are reduced.

Zinc deficient diet consequences for pregnancy and offsprings of Wistar rats

Adult female Wistar rats (90 days old; weight 180 to 220 grams) were submitted to different zinc deficient diets (Zn; severe = 2.6 ppm; mild = 9.0 ppm and normal diet = 81.6 ppm), during 6 weeks. After this time they were coupled with normal male Wistar rats. No differences regarding fecundity and sterility were observed between the groups. During pregnancy, part of the animals from severe and mild Zn deficient groups received the same diet and the others received normal diet. The animals from the group receiving normal diet, were divided into other 3: the first received severe, the second mild Zn deficient diet and the third normal diet. During the study we observed that animals submitted to a Zn deficient diet (acute or chronic) had tendency to lower weight gain, lower weight of the offspring, lower serum levels of Zn in maternal and newborn (pool) blood. A significant reduction in the number of alive newborns was observed in the group of animals submitted to severe Zn deficiency. These data reinforces that Zn is a very important trace element overall during pregnancy.

Zinc, copper and selenium in reproduction

Of the nine biological trace elements, zinc, copper and selenium are important in reproduction in males and females. Zinc content is high in the adult testis, and the prostate has a higher concentration of zinc than any other organ of the body. Zinc deficiency first impairs angiotensin converting enzyme (ACE) activity, and this in turn leads to depletion of testosterone and inhibition of spermatogenesis. Defects in spermatozoa are frequently observed in the zinc-deficient rat. Zinc is thought to help to extend the functional life span of the ejaculated spermatozoa. Zinc deficiency in the female can lead to such problems as impaired synthesis/secretion of (FSH) and (LH), abnormal ovarian development, disruption of the estrous cycle, frequent abortion, a prolonged gestation period, teratogenicity, stillbirths, difficulty in parturition, pre-eclampsia, toxemia and low birth weights of infants. The level of testosterone in the male has been suggested to play a role in the severity of copper deficiency. Copper-deficient female rats are protected against mortality due to copper deficiency, and the protection has been suggested to be provided by estrogens, since estrogens alter the subcellular distribution of copper in the liver and increase plasma copper levels by inducing ceruloplasmin synthesis. The selenium content of male gonads increases during pubertal maturation. Selenium is localized in the mitochondrial capsule protein (MCP) of the midpiece. Maximal incorporation in MCP occurs at steps 7 and 12 of spermatogenesis and uptake decreases by step 15. Selenium deficiency in females results in infertility, abortions and retention of the placenta. The newborns from a selenium-deficient mother suffer from muscular weakness, but the concentration of selenium during pregnancy does not have any effect on the weight of the baby or length of pregnancy. The selenium requirements of a pregnant and lactating mother are increased as a result of selenium transport to the fetus via the placenta and to the infant via breast milk.

Assessment of zinc bioavailability: studies in rats on zinc absorption from wheat using radio- and stable isotopes

Absorption from wheat intrinsically and extrinsically labelled with 67Zn and extrinsically labelled with 65Zn was measured from 67Zn faecal excretion and 65Zn whole-body retention in rats. There were significant differences between the extrinsically- and intrinsically-labelled 67Zn (P < 0.001), but not between the extrinsically-labelled 65Zn and intrinsically-labelled 67Zn. The effect of chicken meat on the absorption of Zn from intrinsically-labelled wheat was also studied in the rat. Mean Zn absorption from wheat and chicken meat fed separately was 18.5 and 68.2% respectively, and from a mixture of the two containing the same level of Zn was 50.1%. The apparent absorption of Zn from the composite meal was significantly higher than predicted from the results of the foods on their own (P < 0.001).

Apparent zinc absorption by rats from foods labelled intrinsically and extrinsically with 67Zn

A variety of foods (peas (Pisum sativum), chicken meat, eggs, goat's milk, human milk) enriched with the stable isotope 67Zn were prepared by means of intrinsic- and extrinsic-labelling procedures. They were fed to rats and apparent absorption of 67Zn determined from faecal excretion measurements using thermal ionization mass spectrometry. There were significant differences in the absorption of the extrinsic and intrinsic label which differed in magnitude between the foods tested. The extrinsic 67Zn was less well absorbed in peas, chicken meat, eggs, and human milk than intrinsic 67Zn, but in goat's milk the extrinsic 67Zn was better absorbed than the intrinsic label. These results demonstrate that extrinsically-added stable Zn isotopes do not fully exchange with endogenous Zn in many foods, and illustrate the need for caution when using extrinsic labels for Zn bioavailability studies.

Placental zinc in normal and intra-uterine growth-retarded pregnancies

The zinc concentration of placental tissue and cord blood in sixteen mothers who gave birth to normal babies was measured. The blood volume of each placenta was estimated from measurements of haemoglobin concentration of placental homogenate and cord blood, and, by deduction, the Zn content of blood-free placental tissue was calculated. Results were compared with eleven mothers whose fetuses showed a low biparietal diameter velocity between 17 and 28 weeks gestation and with ten mothers who gave birth to intra-uterine growth-retarded (IUGR) babies. As expected, placental weight was significantly correlated with infant birth weight. Blood-free placental tissue contained about four times more Zn (approximately 10 micrograms Zn/g) than cord blood (approximately 2.5 micrograms Zn/ml). Concentrations of Zn in blood-free placental tissue were similar in all three groups, but the cord blood Zn of mothers producing IUGR babies was significantly lower than that of the other two groups. Results of the present study suggested that fetal growth retardation in the mothers studied could not be explained by differences in blood-free placental Zn concentration, but that there may be some association between lower cord blood Zn levels and intra-uterine growth retardation.

The effect of combined dietary iron, calcium and folic acid supplementation on apparent 65Zn absorption and zinc status in pregnant rats

In the present study the effect of combined iron, calcium and folic acid supplementation of the diet on 65Zn retention and zinc status was studied in the pregnant rat. Female Wistar rats were fed on a low- (8 micrograms/g) or high- (60 micrograms/g) Zn diet for 14 d and then mated overnight. After mating, half the rats were fed on the low- or high-Zn diet as before, whilst the other half were fed on similar diets supplemented with Fe, Ca and folic acid. The level of supplementation was chosen to reflect proportionately the possible increase in daily intakes of these nutrients by pregnant women. Rats which did not mate successfully were used as non-pregnant controls. On day 18 of pregnancy, each animal was given a meal of the appropriate diet labelled extrinsically with 65Zn, and on day 20 rats were killed. Carcass 65Zn retention was lower in pregnant and non-pregnant rats fed on the supplemented diets compared with those fed on the unsupplemented diets. Rats which consumed the supplemented diets throughout pregnancy had reduced plasma Zn concentrations but femur and fetal Zn concentrations were unaffected. Maternal femur Ca and fetal Fe concentrations were lower in the high-Zn groups compared with rats fed on low-Zn diets. It was concluded that the risk of inducing fetal Zn depletion as a consequence of Fe, Ca and folic acid supplementation during pregnancy appeared to be slight.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of alcoholic beverages on iron and zinc metabolism in the rat

1. Male Wistar rats (approximately 200 g) were given distilled water and a semi-synthetic control diet for 6 d. On day 7, 37 kBq 65Zn were administered intramuscularly and the rats were given distilled water, beer, cider, red wine, whisky or ethanol as their only source of fluid. The wine, whisky and ethanol were diluted so that each of the beverages contained a similar ethanol concentration (approximately 30 g/l). Food and fluid intake, growth rate and whole-body 65Zn were measured regularly over 11 d, after which animals were killed and blood haemoglobin (Hb) concentration, liver iron stores and the Zn concentration in testes determined. 2. There were no differences in body-weight gain or food intake between groups but fluid intake for the beer group was considerably higher than that for the other groups. 3. There was a significant effect of the type of alcoholic beverage consumed on whole-body 65Zn retention. Rats given whisky had a smaller daily loss of 65Zn than those given water, beer or cider. The ethanol group also showed a lower rate of 65Zn loss compared with the water group. The observed changes in whole-body 65Zn retention could be explained by an adverse influence of ethanol on Zn absorption from the diet. 4. Blood Hb and testes Zn concentration were similar in all groups but the type of liquid consumed influenced liver Fe levels. The cider group had the lowest liver Fe values and the ethanol group the highest values. 5. It is apparent from the present study that ethanol and alcoholic beverages affect Zn and Fe metabolism, but that the effects of ethanol are moderated by other components of the alcoholic beverages.

The effects of dietary folate and zinc on the outcome of pregnancy and early growth in rats

1. The objective of the present study was to determine the effects of two levels of folic acid and two levels of zinc in the diets of rats during pregnancy and lactation. It addressed, among other things, the question of whether an inhibitory effect of folic (pteroylmonoglutamic) acid on Zn absorption might result in a secondary Zn deficiency in either the dams or the pups. 2. A purified diet was given to four groups of female DNL (Norwegian) Hooded rats, before and during pregnancy and during lactation. It contained the four possible combinations of: no added folic acid or 100 micrograms added pteroylmonoglutamic acid/g, and 6.6 or 20.2 micrograms Zn/g. Pups and dams were killed on day 20 of gestation or on day 20 postpartum. Measurements of body-weights, food intakes, blood folate and tissue Zn levels were performed. 3. The group with low Zn and low folate intake had a satisfactory reproductive outcome, and there were only minor effects of the supplements on body-weights. 4. Additional folate greatly increased blood (erythrocyte and plasma) folate levels, but did not compromise tissue Zn concentrations. Zn supplementation also enhanced blood folate levels, for reasons which are not yet clear. 5. There was a moderate enhancing effect of the Zn supplement on Zn levels in the livers and kidneys of pregnant dams, and the kidneys of lactating dams. 6. If the conclusions can be extrapolated to humans, then the results provide some reassurance that a high folate intake from prenatal supplementation need not necessarily cause Zn depletion, and hence functional Zn deficiency in pregnant women and their offspring.

Fetal growth, glucose tolerance and plasma insulin concentration in rats given a marginal-zinc diet in the latter stages of pregnancy

1. Wistar rats were fed on a control semi-synthetic diet throughout pregnancy, or a control diet in the first 2 weeks and a marginal-zinc diet in the 3rd week of pregnancy. On day 20, after an overnight fast, half the animals in each group were given glucose by gavage and the 0-30 min rise in blood glucose measured in tail blood. After 60 min blood was taken by cardiac puncture for glucose and insulin assay. Maternal pancreases were removed and the Zn contents measured. Fetuses from each litter were combined for wet/dry weights, protein and DNA determinations. 2. Plasma insulin concentration was higher, and glucose concentration and pancreatic Zn content lower, in pregnant v. non-pregnant animals of similar age, fed on the same diet. Pancreatic Zn content was lowest in the marginal-Zn group of pregnant rats. Fetuses from mothers fed on the marginal-Zn diet during the last week of pregnancy were slightly heavier than controls and had a significantly higher protein: DNA ratio. The 0-30 min rise in blood glucose was significantly greater in the marginal-Zn animals. 3. In a second experiment, pregnant rats were given similar diets to those used in the first study, but the marginal-Zn diet was given for a shorter period (days 15-19 of pregnancy). On day 19 the rats were meal-fed and on day 20, after an overnight fast, an oral glucose dose was administered. Tail-blood was taken at timed intervals up to 60 min post-dosing for glucose assay.(ABSTRACT TRUNCATED AT 250 WORDS)