The Distribution of Lead in Milk and the Fate of Milk Lead in the Gastrointestinal Tract of Suckling Rats

Abstract

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-

Lead

“…Lead is excreted in milk, probably mainly bound to casein (Beach and Henning 1988). In cows (Oskarsson et al 1992) and rats (Palminger and Oskarsson 1991) there is an exponential increase of lead levels in milk with increasing B-Pb.…”

Bioavailability of lead from various milk diets studied in a suckling rat model

“…They also suggested that the lower bioavailability of zinc and copper from cow's milk in contrast to human milk was influenced by the casein content of the milk (L6nnerdal et al 1985, L6nnerdal 1991). It has been shown that lead associates to the casein micelles in milk to a high extent (Beach & Henning 1988), hence the bioavailability of lead might be influenced by the casein content in the milk diet. The purpose of the present study was to investigate the bioavailability of lead from various milk diets and to study the impact of the casein content in milk.…”

Influence of the Destabilisation of the Maternal Digestive Microflora on That of the Newborn Rat

“…The animals were mildly anaesthe tized by intraperitoneal injection of chloral 2.5% at a concentration of 2.5 ml kg-'. To make milking more efficient a further intraperitoneal injection of 2 UI of ocytocin (Syntocinon Sandoz), was given [9], The milk was collected by suction through the nozzle of a 1-ml syringe connected to a vacuum pump.…”