Scientists have long known that much of the lead entering the body during childhood is stored in the skeleton, where it can remain for decades. Most of this skeletal lead is unavailable to target organs (those to which lead is especially toxic) and produces little apparent toxicity during adulthood, although recent studies have found associations between bone lead stores and anemia, hypertension, and other diseases in men. But fractures and a number of conditions including pregnancy can release lead from bone back into the blood. Pregnant women with high skeletal lead concentrations run the risk of exposing their children to lead during critical periods of fetal development.
has been shown to reduce gastrointestinal absorption of lead and inhibit its
adverse effects on calcium-mediated cellular functions. In this issue, Shenggao
Han and colleagues from the Department of Preventive Medicine and Community
Health at the
In the study, groups of weanling female rats, five weeks of age, were given drinking water containing 250 milligrams per liter (mg/L) lead acetate for 5 weeks, followed by a 4-week period without lead exposure. Controls were given sodium acetate in drinking water. Both treatment groups and controls were fed diets containing 0.5% calcium. At 14 weeks, the females were mated with male rats of the same age, and the pregnant females were randomly assigned to two groups, one fed a 0.5% calcium diet (normal) and one fed a 0.1% calcium diet (low). Pregnant rats carried their litters to term, and the pups were evaluated for a variety of developmental end points.
Han and colleagues found that pups born to lead-exposed rats were reduced in birth weight and length after controlling for factors such as pup sex, litter size, and maternal weight gain during pregnancy. This is significant because it indicates that lead exposure occurring well before pregnancy can influence fetal development. The results following the rats' exposure at five weeks of age suggest that infants born to women with a history of lead exposure during childhood--the exposure period when the metal can cause the most enduring damage--may experience developmental problems in utero as a result.
However, dietary calcium was shown to provide benefits with respect to lead release from bones and fetal uptake. For example, maternal blood lead concentrations were lower in pregnant rats given the higher-calcium diet, which suggests that calcium has an inhibitory effect on lead release from bone. Fetal lead uptake was also reduced in pups born to rats with higher dietary calcium intakes. Dietary calcium did not appear to protect against lead-induced decreases in birth weight and length. Nevertheless, the results of this study provide evidence that the composition of the diet during pregnancy can influence the transfer to the fetus of an environmental toxicant from past maternal exposure. -Charles W. Schmidt