High levels of perchlorate found in U.S. mothers’ milk
The first sampling of perchlorate in breast milk from nursing mothers in the United States reveals levels as high as 92 parts per billion (ppb), which expose infants to concentrations of the contaminant that are 20 times higher than the safe dose recently recommended by a National Academy of Sciences (NAS) committee. A potentially more disturbing finding is that some of the milk samples are so low in iodide—possibly because of the presence of perchlorate—that infants are receiving much less of this important trace nutrient than the international health experts recommend. This first-ever U.S. survey of perchlorate in breast milk, conducted by researchers at Texas Tech University in Lubbock, is reported in research posted to ES&T’s Research ASAP website (es048118t) and adds to the growing number of studies that suggest the contaminant is pervasive.
“Our findings are very surprising because we found that perchlorate in breast milk is widespread, doesn’t appear to come from drinking water, appears to suppress iodide in breast milk, and in some cases, the levels are really high,” says first author, Ph.D. student Andrea Kirk. The survey analyzed 36 samples from 17 states, drawing from participants contacted through Internet listserv groups for nursing mothers. According to corresponding author Purnendu “Sandy” DasGupta, the scientists found measurable levels of perchlorate in every sample they analyzed.
Perchlorate, whose presence has been linked to both anthropogenic and natural sources, can competitively inhibit iodide uptake to the thyroid gland. Breast tissue concentrates iodide in the same way, so perchlorate may also inhibit iodide uptake to breast milk, says committee member and thyroidologist David Copper of Johns Hopkins University. Other chemicals, such as nitrates and thiocyanates found in cigarette smoke and some vegetables, can also inhibit iodide uptake, but they are less potent than perchlorate.
Suppression of iodide is of concern because the thyroid uses it in the production of hormones that are essential to proper brain development. The Texas Tech scientists report that in 6 samples with 10 ppb or higher of perchlorate, iodide levels dropped as perchlorate concentrations increased. This supports the idea that perchlorate is blocking iodide uptake and possibly exacerbating already low levels of the nutrient in nursing moms.
These findings come a month after an NAS committee recommended a perchlorate reference dose of 0.7 micrograms (µg) per kilogram of a person’s body weight per day, an estimated value that is 23 times more than EPA’s draft proposal (Environ. Sci. Technol. 2005, 38, 96A–97A). Although the committee based its recommendations on adult human studies, the report does cite a single, industry-funded study of nursing mothers and their children in Chile who live in an area with high levels of naturally occurring perchlorate. (As of this writing, the work was still undergoing peer review.) This study suggests that perchlorate does not have an impact at high levels because “perchlorate in breast milk is high, but maternal thyroid hormones are normal and the newborns are normal and have normal thyroid hormones,” said Harvard thyroidologist and NAS committee member Robert Utiger at the report’s unveiling.
Those results seem to be at odds with the new research. However, Chilean mothers have higher levels of iodide in their diet and, consequently, in their breast milk, and they may be less affected by perchlorate than U.S. moms, who appear to be deficient in iodide, Cooper points out. Chilean values were above 300 ppb, whereas breast-milk iodide values in the U.S. survey were as little as 4.5 ppb, averaging around 63 ppb. “These iodide values seem quite low,” says Cooper.
The recommended adequate intake of iodide is 90–110 µg per day for infants up to 6 months of age, according to Boston University endocrinologist Elizabeth Pearce. Because infants consume 0.5–1 liter of breast milk per day, it must contain 110–220 ppb iodide to meet this target.
The highest concentrations of perchlorate reported in the U.S. breast milk are 20 times higher than the NAS committee’s estimate of a safe dose, according to Cooper’s calculations. This is not necessarily harmful to infants, say many committee members as well as government and academic scientists. The NAS reference dose is just an estimate and is accompanied by a great degree of uncertainty. But the experts say that the study raises a warning flag. It should be repeated and include measurements of mother and infant thyroid hormones.
The other unanswered mystery is the source of all the perchlorate. “We think this means that the perchlorate is coming from food,” says Kirk. Perchlorate has already been found in lettuce and milk at parts-per-billion levels in a nationwide Food and Drug Administration survey.
Recently, Texas Tech investigator Andrew Jackson found perchlorate in commercially grown wheat and alfalfa samples at concentrations that ranged into parts per million fresh weight. This is about an order of magnitude higher than the levels in lettuce and milk. These measurements represent bioconcentration factors of about 200 for wheat and 380 for alfalfa (J. Agric. Food Chem. 2005, 53, 369–373). Jackson and DasGupta have also found perchlorate in rainwater and snow that appears to form naturally in the atmosphere, but they believe that it is not likely to be a major source of the compound (Environ. Sci. Technol. 2005, 38, 120A).
