BIOAVAILABILITY OF FOOD CADMIUM AS AN ISSUE IN RISK MANAGEMENT AND ASSESSMENT
Philip G. Reeves¹ and Rufus L. Chaney<sup>2

1</sup>U.S. Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, 2420 Second Avenue North, Grand Forks, ND 58203, USA
² U.S. Department of Agriculture, Agricultural Research Service, Animal Manure & By-Products Lab, Bldg. 007, BARC-West, Beltsville, MD 20705, USA

Bioavailability is defined as the degree to which a nutrient, toxin, drug, or other substances become available for body use after administration. When the substance is ingested orally, bioavailability of the substance generally includes both absorption and body utilization. Cadmium (Cd) is generally considered a toxic element where food is the major source of acquisition. 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) will reduce the rate of absorption of Cd from various food sources. Past work by others and more recent work in our laboratory has shown that marginal to low dietary concentrations of these mineral elements, for the most part, have the opposite effect. For example, we showed that the rates of absorption and whole-body retention of dietary Cd increased 7- to 10-fold when experimental animals were fed diets based on either sunflower kernels with a natural content of Cd and marginal concentrations of Zn, Fe, and/or Ca, or rice with concentrations of Cd similar to that grown in paddies contaminated with Cd/Zn and with low mineral content. The mechanism for this phenomenon is unknown, but we also found that marginal intakes of these nutrients increased the duodenal accumulation of Cd by as much as 100-fold when compared with distal parts of the intestine. Marginal intakes of Zn, Fe, and Ca reduced the turnover time of Cd in the duodenum and seemed to reduce the transit time of Cd down the GI tract. Two important conditions in these experiments were that the absorptive mechanisms in the gut were adapted to the diet by feeding it for five weeks, and that the Cd concentrations of the diets were similar to those found in some human diets. In addition, the marginal amounts of Zn, Fe, and Ca were such that weight loss or other overt signs of deficiency were not present. Although the concentrations of these essential minerals in the diet and the subsequent nutrient status of the consumer could be important factors that influence the extent of absorption and tissue accumulation of food-Cd, this concept receives little attention when decisions are made about the potential risk of food-Cd to humans. However, if the findings in future human studies prove to be similar to those in the rat model, then we feel that the bioavailability factor as affected by nutritional status will prove to be a very important part of the equation in assessing the risk of food-Cd in vulnerable populations.