Exposure to toxic elements via breast milk
Breast milk is the ideal nutrient for the newborn, but unfortunately also a route of excretion for some toxic substances. Very little attention has been paid to breast milk as a source of exposure to toxic elements. The dose-dependent excretion in breast milk and the uptake in the neonate of inorganic mercury, methylmercury and lead were studied in an experimental model for rats and mice. The transfer of mercury from plasma to milk was found to be higher in dams exposed to inorganic mercury than to methylmercury. In contrast, the uptake of mercury from milk was higher in the sucklings of dams exposed to methylmercury than to inorganic mercury. Pre- and postnatal exposure to methylmercury resulted in increased numbers and altered proportions of the thymocyte subpopulation and increased lymphocyte activities in the offspring of mice and also effects on the levels of noradrenaline and nerve growth factor in the developing brain of rats. Mercury in blood and breast milk in lactating women in Sweden was studied in relation to the exposure to mercury from fish and amalgam. Low levels were found; the mean levels were 0.6 ng g–1 in milk and 2.3 ng g–1 in blood. There was a statistically significant correlation between mercury levels in blood and milk, showing that milk levels were approximately 30% of the levels in blood. Inorganic mercury exposure from amalgam was reflected in blood and milk mercury levels. Recent exposure to methylmercury from consumption of fish was reflected in mercury levels in the blood but not in milk. A high lactational transfer of lead was found in rats and mice. A linear correlation was found in the dams between lead in plasma and milk and between lead in milk and tissues of sucklings. It was also found that the bioavailability of lead in milk diets is dependent on the casein content of milk. Thus, lead in human milk with a low casein content was absorbed more rapidly and to a higher extent in the sucklings than lead in rat milk with a high casein content. The excretion of lead in milk was also studied in cows after an episode of lead intoxication. A curvilinear relationship between lead in blood and milk was found, with a sharp increase in lead levels in milk at blood lead levels above 200–300 µg kg–1. Lead levels in human breast milk and blood were studied in Sweden. The mean levels of lead were 0.8 µg l–I in milk and 33 µg l–1 in blood. This can be compared with a reported mean value of 62 µg l–1 in milk from women living close to a smelter in Mexico. There was no correlation between lead levels in blood and milk in the Swedish study. However, significantly higher levels of lead in milk were found in women living close to a metal smelter as compared with women from a control area.