BIOAVAILABILITY OF CADMIUM IN FOOD AND WATER - A CASE-STUDY ON THE DERIVATION OF RELATIVE BIOAVAILABILITY FACTORS FOR INORGANICS AND THEIR RELEVANCE TO THE REFERENCE DOSE

Published studies in which rats were exposed to CdCl2 in standard chow or drinking water were analyzed to compare the relative bioavailabili ty of cadmium from the two media. Relative bioavailability was assesse d from estimates of the rate of accumulation of cadmium in kidney cort ex or liver. Data were grouped into tiers based on study design and re porting of data: Tier 1, identical experimental protocols and dosage c an be estimated; Tier 2, very similar or identical protocols and dosag e can be estimated; Tier 3, protocols may differ and dosage can be est imated; and Tier 4, protocols may differ and dosages cannot be estimat ed (but concentration of cadmium in food or water is reported). Tiers were nested, such that Tier 4 contained all relevant studies; Tier 3 i ncluded data sets from Tiers 1 and 2; and Tier 2 included the data set from Tier 1. Data within Tiers 1, 2, and 3 were subjected to a linear regression analysis with dosage as the independent variable and tissu e accumulation rate as the dependent variable to determine whether bio availability of cadmium was significantly different based on medium of administration. The results of this analysis show the following: (I) In rats receiving food and drinking water ad libitum, the bioavailabil ity of cadmium in drinking water is not significantly different (P > 0 .05) from the bioavailability of cadmium in food when dosages are less than 4 mg/kg body wt/day. (2) Cadmium decreases food and water consum ption; therefore, assessments of relative bioavailability should be ma de based on actual dosage rather than exposure levels. (3) Diet compos ition and status of the gastrointestinal tract are probably a more imp ortant determinant of the bioavailability of cadmium than is the expos ure medium. (4) Studies of the effect of total diet composition on bio availability of cadmium may be more relevant than are studies of the e ffect of the exposure medium. It is concluded from this analysis that the bioavailability of cadmium in food is not different from that in w ater when diet is provided ad libitum. Therefore, we recommend that di stinct RfDs for cadmium in food and drinking water should not be based on the assumption that the bioavailability of cadmium in drinking wat er is greater than that of cadmium in food. (c) 1994 Academic Press, I nc.