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Nutrient Interactions and Toxicity

Metallothionein Induction Is Not Involved in Cadmium Accumulation in the Duodenum of Mice and Rats Fed Diets Containing High-Cadmium Rice or Sunflower Kernels and a Marginal Supply of Zinc, Iron, and Calcium ^1,2

Philip G. Reeves³, Rufus L. Chaney^*, Robert W. Simmons and M. George Cherian^**

U.S. Department of Agriculture, ARS, Grand Forks Human Nutrition Research Center, Grand Forks, ND 58203; ^* U.S. Department of Agriculture, ARS, Animal Manure & Byproducts Laboratory, Beltsville, MD 20705-2350; IWMI-SEA Regional Office, Kasetsart University, Bangkok 10903, Thailand; and ^** Department of Pathology, University of Western Ontario, London, ON N6A 5C1, Canada

³To whom correspondence should be addressed. E-mail: preeves{at}gfhnrc.ars.usda.gov.

Rats fed diets with cadmium (Cd) concentrations similar to that found in human diets, and nutritionally marginal with respect to iron (Fe), zinc (Zn), and calcium (Ca) retained 10 times more Cd in the duodenum than rats fed adequate mineral diets. In the current study, 2 experiments were performed to determine the role of intestinal metallothionein (MT) in the accumulation of duodenal Cd, and to determine whether endogenous rice grain Cd is as available as Cd exogenously incorporated into the grain. In Expt. 1, wild-type and MT-null mice were fed 40% rice diets containing marginal or adequate amounts of Fe, Zn, and Ca, and 240 µg Cd/kg. Duodenal Cd was 10 times higher in both wild-type and MT-null mice regardless of their mineral status. In Expt. 2, one group of rats was fed 40% rice diets in which Cd was incorporated into the rice during growth and maturation, and another group was fed 40% rice diets in which Cd was incorporated into the rice during cooking. Each group also was fed either marginal or adequate amounts of Zn, Fe, and Ca. After 5 wk, rats were given a single meal labeled with ¹⁰⁹Cd, and the amount of label retained after 7 d was determined by whole-body counting. Rats with marginal mineral status retained 10 times more ¹⁰⁹Cd than those with adequate status; however, there was no difference between rats fed endogenous or exogenous Cd rice. Although duodenal Cd concentration was 8 times higher in the marginally fed rats, MT concentration was unchanged. These 2 experiments indicate that MT induction is not involved in duodenal Cd accumulation in animals with marginal dietary status of Fe, Zn, and Ca. In addition, they support the hypothesis that marginal deficiencies of Fe, Zn, and Ca, commonly found in certain human populations subsisting on rice-based diets, play an important role in increasing the risk of dietary Cd exposure.

KEY WORDS: • cadmium • metallothionein • MT-null mice • rats • rice