ABSTRACT. Milk can be a significant source of lead (Pb) for young mammals, including humans. Certain essential trace elements have previously been shown to be specifically associated with particular milk components and such associations often increase bioavailability. Thus, the first goal of this study was to determine the distribution of P b in cream, casein, and whey fractions of various milks under various conditions using '03Pb a s a tracer. In rat milk almost 90% of the P b was found to be associated with the casein micelles, regardless of: 1) whether the milk was labeled in vivo or in vitro; b) whether the milk was fresh or frozen; and c) the added concentration of P b (over the range 0.01-75 pg/ml). The remainder of the P b was approximately equally distributed between cream and whey. A virtually identical pattern of P b distribution was observed with bovine milk. P b added to infant formula also associated predominantly with casein micelles, although the P b content of this fraction was significantly less than with rat and bovine milks. The second goal of the study was to determine if P b remained associated with casein a s it traversed the gastrointestinal tract of infant rats. For this purpose, rat pups aged 15-16 days were gavaged with 2""b-labeled rat milk, and lumenal contents from the stomach and small intestine were collected 2 h later. Differential centrifugation of the homogenized lumenal contents showed that in the stomach the P b was associated primarily with the casein curd. By the time chyme reached the distal small intestine, P b was found predominantly in a fraction that was not precipitable by high-speed centrifugation (thus, not intact casein micelles), but was nondialyzable. We conclude that P b in milk is protein bound and remains this way a s it traverses the stomach and proximal small intestine of the infant rat. (Pediatr Res 23: 58-62,1988).The deleterious effects of both symptomatic and asymptomatic Pb poisoning are well documented for infants of humans and experimental animals (1-7). This high risk group demonstrates a greater intestinal absorption of Pb than do adults (8-1 1) and as a result may be more susceptible to even modest levels of environmental pollution. There is evidence in rodents that en-
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