12 

 (Parisi and Vallee, 1970), transferrin (Charlwood, 1979), and 

 the amino acids cysteine and histidine (Morgan, 1981) . Plasma 

 Zn represents less than 1% of the total body content but 

 serves as a primary source of the element accessible to all 

 cells (Vallee and Falchuk, 1993). The exact mechanisms for 

 tissue uptake of Zn are not well known. Tissue Zn 

 concentration in most mammalian tissues has been reviewed 

 (Hambridge et al . , 1986), and tissue Zn levels were fairly 

 constant among species. The tissues most sensitive to excess 

 dietary Zn intake include liver, kidney, pancreas, small 

 intestine, and bone (Kincaid et al . , 1976a, b) . Concentration 

 changes of trace elements in tissues have been used as 

 indicators of Zn bioavailability in rats and sheep (Moncilovic 

 et al., 1975; Henry et al . , 1988). 



Bioavailability of Sources 



The two predominant Zn sources used by the animal feed 

 industry are ZnS0 4 (36% Zn) and ZnO (72% Zn) . It has been 

 suggested that the mineral source plays an important role in 

 the formation of unknown complexes inside the digestive tract 

 which in turn limit their absorption and further metabolism 

 (Hughes, 1984; Clydesdale, 1990), but there are many 

 contradicting studies as to the different effects and 

 bioavailabilities of different sources. Zinc as the metal, 

 sulfate, carbonate, oxide, and in several natural ores has 

 been shown to be relatively available when provided in 



