Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/1995
Publication Date: N/A
Citation: Interpretive Summary: It is important to know how one nutrient in the diet affects the absorption and utilization of another. For example, diets with high zinc contents will reduce the absorption of copper into the body. From results of previous studies, we had suggested that the theory used to explain this effect might not be true. This theory states that the presence of high zinc in the diet will cause the intestine to produce a small sticky protein (metallothionein, MT) that will bind up the copper and prevent its passage into the body. For this to be true, the copper must stick to the protein. We found that this did not occur. There are other dietary minerals besides zinc that will increase or stimulate production of MT. In the present study, we used both cadmium and zinc to stimulate the protein and see whether it reduced absorption. We found that copper absorption and copper status of rats fed high zinc and/or high cadmium were less than in those rats fed normal amounts of zinc and cadmium. However, this reduction was not related to the production of MT. Even though there was a hundred times more MT in the intestine of cadmium-fed rats than those not receiving cadmium, the copper concentration in the intestine was not changed. The results of this experiment suggest again that MT probably does not play a role in low copper status of rats fed high zinc or high cadmium diets. Our next project is to find out what really is causing the problem.
Technical Abstract: Feeding diets with high zinc or cadmium concentrations to animals compromises their copper status. Previous work suggested that metallothionein (MT), induced in the intestinal mucosa by zinc or cadmium, binds copper and inhibits its absorption. More recent studies showed that this mechanism may not be operative for zinc, and that intestinal MT adapts to long-term feeding of high-zinc diets. The present study was designed to determine the effects of feeding high-zinc and/or high-cadmium diets on the induction of intestinal MT and its subsequent effect on copper status. Six-week-old male rats were placed in a 2 x 2 x 3 factorial experiment with two concentrations of dietary zinc (60 and 350 mg/kg), two concentrations of dietary copper (3 and 9 mg/kg) and 3 concentrations of dietary cadmium (0, 1, and 5 mg/kg). After 3 weeks, the difference in MT concentration between rats fed high- and normal-zinc diets was only 1.5 times. However, rats fed the highest amount of cadmium had MT concentrations about 5 times higher than those not fed cadmium. In both zinc and cadmium groups the concentration of intestinal zinc and cadmium followed that of MT; however, copper concentrations were not changed. Although intestinal MT was not elevated appreciably in zinc-fed rats, the copper status of these rats fed 3 mg of copper/kg of diet was severely depressed. Rats fed 9 mg of copper/kg of diet were not affected. Copper status in rats fed high-cadmium and 3 mg of copper/kg was depressed even more than with high zinc. This study suggests that the effects of high dietary zinc or cadmium on copper status are not the result of induced intestinal MT binding of copper and preventing its passage into the circulation.