BIOAVAILABILITY OF LEAD FROM VARIOUS MILK DIETS STUDIED IN A SUCKLING RAT MODEL

The bioavailability of lead from various milk diets was studied in 14 day old suckling rats, Human milk, infant formula, cow's milk, rat milk and deionized water labeled with Pb-203 were given to rat pops by gastric intubation, Animals were killed after 2 or 6 h and the radioactivity in the tissues was measured, At 2 h after administration the lead bioavailability, defined as lead uptake in the body, excluding the gastrointestinal tract, was 47% from water, 42% from human milk, 40% from infant formula, 31% from cow's milk and 11% from rat milk, After 6 h the bioavailability of lead was about 50% from water and human milk, 45% from infant formula and cow's milk, and 36% from rat milk, The blood lead levels in the pups reflected the total body uptake and were also correlated to the brain lead levels, Thus, rat pups given lead in human milk had approximately twice as high lead levels in blood and brain than pups given lead in rat milk, The intestinal absorption of lead was dependent on the milk diet given to the sucklings, In duodenum, the highest uptake of lead was found in rats given water or human milk, whereas in rats given rat or cow's milk the highest uptake of lead was found in ileum, The distribution of lead in cream, whey and casein fractions of the milk diets after in vitro labeling with Pb-203 was also studied, The casein fraction in cow's and rat milk contained 90-96% of the total amount of lead in the diet, In infant formula and human milk, 77 and 56% lead was found in the casein fraction, respectively, The higher lead bioavailability observed in the suckling rat fed human milk than in those fed rat and cow's milk may partly be explained by a lower proportion of lead bound to casein in human milk.