Marginal nutritional status of zinc, iron, and calcium increases cadmium retention in the duodenum and other organs of rats fed rice-based diets
Introduction
Cadmium (Cd) is generally considered a toxic element. Food is the major source of Cd with an estimated average intake of 70 to 140 μg/week for adults in the United States (Gartrell et al., 1986; Gunderson, 1995; Pennington et al., 1986). In the past, the concentration of Cd in food has been considered the overriding risk factor that determines body burden of this element. However, as studies have shown, there are numerous other factors that affect the intestinal absorption and organ retention of Cd. Perhaps the most important one is the interaction between Cd and other mineral nutrients that affect its absorption. It has been known for some time that feeding high concentrations of zinc (Zn), iron (Fe), and/or calcium (Ca) to animals reduces the rate of absorption of Cd from various food sources (Brzóska and Moniuszko-Jakoniuk, 1998; Evans et al., 1970; Ferguson et al., 1990; Fox et al., 1979; Fox (1983), Fox (1988); Kello and Kostial, 1977; Koo et al., 1978). It has also been shown that humans with low-Fe status (Åkesson et al., 2002; Flanagan et al., 1978; Shaikh and Smith, 1980) and rats with low-Ca intake (Kello et al., 1979) will have an enhanced absorption of Cd. Recently it was shown that the rates of absorption and whole-body retention of dietary Cd increased 7- to 10-fold when experimental animals were fed diets containing marginal concentrations of Zn, Fe, and/or Ca (Reeves and Chaney (2001), Reeves and Chaney (2002)). Although the concentrations of these essential minerals in the diet could be important factors that influence the extent of absorption and tissue accumulation of food-Cd, this has received little attention when decisions have been made about the potential risk of food-Cd to humans.
In previous studies, we found that a marginal nutrient status of Zn, Fe, and/or Ca in rats increased Cd absorption from foods, which seemed to be related to the transit time of Cd through the gut. We defined the term “marginal” as that concentration of Zn, Fe, or Ca in the diet that would be less than the requirement for the rat set by the National Research Council (1995) but not low enough to initiate frank deficiencies, such as severe weight loss. The fecal excretion of 109Cd from an oral dose of diet containing Cd was delayed in rats that were marginally deficient in Fe and Ca, compared with rats adequate in Fe and Ca (Reeves and Chaney (2001), Reeves and Chaney (2002)). When Zn also was marginal in the diet, the delay of Cd excretion was even more pronounced. Importantly, these diets contained Cd concentrations that could be found in natural foods (0.25–0.45 mg/kg), not toxicological concentrations as used in most studies of this nature. To determine how marginal mineral status might affect gut retention time of dietary Cd, we performed the present study. The general hypothesis was that diets marginal in Zn, Fe, and Ca would cause Cd to accumulate in the gut enterocytes and that Cd transit down the intestinal tract would be delayed, compared with that in rats fed adequate minerals. Cd retention by different segments of the small intestine and by the liver and kidneys was determined. We used 109Cd to track Cd redistribution over time. The dietary Cd concentration in this experiment was kept near that normally found in some foods that contain a natural abundance of Cd.
Section snippets
Materials and methods
This study was approved by the Animal Use Committee of the USDA-ARS, Grand Forks Human Nutrition Research Center and was in accordance with the guidelines of the National Institutes of Health on the experimental use of laboratory animals (National Research Council, 1996).
The experiment assessed two groups of 60 female rats each (strain SAS:VAF (SD); Charles River/Sasco, Wilmington, MA) beginning at 3 weeks of age. One group was fed a diet containing all nutrients in amounts that equaled or
Results
Although the rats consumed a marginal amount of dietary Zn, Fe, and Ca for a period of 40 days, the mineral concentrations were not low enough to produce an overt deficiency of Zn and Ca. The concentrations of Zn in serum, duodenum, and liver were not significantly affected by marginal intakes of dietary Zn; however, Zn was significantly (P<0.05) higher in the kidneys of rats fed the marginal diet (Table 2). The concentrations of Ca in serum, liver, or kidney were not significantly affected by
Experimental results
This experiment was conducted to extend a previous observation that the fecal excretion of 109Cd from an oral dose of diet containing Cd was delayed in rats that were marginally deficient in Zn, Fe, and Ca compared with rats that were not deficient (Reeves and Chaney, 2002). Fox et al. (1984) also had noted that Cd seemed to be taken up by the small intestine and remain there for a number of days, and the 109Cd in this pool could be either depleted by absorption or lost from the gut by
Acknowledgements
The authors appreciate the able assistance of Denice Schafer and staff for care of the animals, Jim Lindlauf for mixing and analyzing the diets, and the staff of the Mineral Analysis Laboratory, GFHNRC for analyzing the mineral content of dietary ingredients and animal tissues.
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