14 September 1998

Heavy Milk Danger For Infants

The results of a new international study suggest that mothers who have a long-term history of lead exposure may pass on high levels of lead through breast feeding.

The joint study, by Australian and American scientists, was published in Environmental Health Perspectives (October issue), the monthly journal of the National Institute of Environmental Health Sciences.

"Breast feeding is still an important option for healthy mothers," says Dr. Brian Gulson, a toxicologist with Macquarie University in Sydney, and chief investigator for the project. "Breast-fed infants are only at risk if the mother has been previously exposed to high concentrations of lead from external sources such as leaded paint, pottery, or, in many other countries, industrial sources and leaded gasoline."

Previous research has demonstrated that lead is stored in bones, but scientists could not quantify its release into the blood and other bodily fluids. To answer that question, the researchers compared second-generation Australian women with women who had migrated to Australia from Eastern Europe.

Lead found in Eastern Europe has a slightly different 'signature' from lead found in Australia, so scientists were able to differentiate between skeletal lead deposits that had accumulated from the immigrants' earlier exposures while in their native country, and more recent exposures. Hence, any circulating blood lead that matched the 'Eastern European' profile could only be derived from skeletal stores.

Earlier data from the same study had shown that as much as 40 to 70 per cent of the blood lead in pregnant women can come from lead that has accumulated in the bones.

Additional analyses revealed a significant increase in the mobilization of maternal skeletal lead during lactation. "Based on these observations, we wondered whether the infants born to these mothers might be at greater risk from breast feeding than from formula," says Dr Gulson.

Because the study participants' blood lead concentrations were relatively low - most had values less than 5 micrograms lead per deciliter (100 milliliters) of blood - the lead concentrations in their breast milk were also low - 0.7 parts per billion on average. These concentrations were not significantly different from those found in various kinds of infant formulae.

In spite of these low concentrations, the researchers found a strong correlation between the amount of lead in the mothers' milk samples and the corresponding blood lead levels of the infants. In fact, calculations based on the first 60 to 90 days postpartum indicated that 36 to 80 per cent of the infants' total blood lead was coming from breast milk or formula.

"Our data indicate that lead from urban air and water where these infants reside contributed negligibly to blood lead, while soil and dust were not considered to be relevant sources because these infants had not reached the stage of crawling and ubiquitous hand-to-mouth activity," said Gulson. "Therefore, we concluded that dietary lead was the primary source contributing to the body burden observed in these subjects, with a major part of the dietary lead ultimately derived from the mothers' bones."

The researchers also found a statistical link between the lead concentration in the maternal cord blood and lead levels in the first breast milk samples, a finding that could have clinical significance. Bill Jameson, a project officer for the study, says "If this relationship can be verified through further investigations, then one could obtain an accurate estimate of an infant's lead exposure by simply testing the mother's blood lead."