Oral bioavailability of lead and arsenic from a NIST standard reference soil material

The oral bioavailability of soil contaminants is measured using in vitro or in vivo techniques. Current efforts in our laboratory are focused on the comparisons of in vitro methods for bioavailability estimation with the presently employed in vivo techniques, such as animal models. We present a comparison of two techniques for oral bioavailability estimation: in vitro dissolution and in vivo rat feeding using a standard reference soil. Lead (Pb) and arsenic (As) were chosen because of the range of concentration in this soil as well as the large historical database of bioavailability values for these metals. Metal solubility was measured using a sequential soil extraction in synthetic analogues of human saliva, gastric and intestinal fluids. The soluble metal was defined as the bioaccessible fraction. Oral bioavailability of Pb and As was measured in Sprague Dawley rats by determining metal levels in the major organs and urine, feces, and blood at 1-, 2-, and 3-day time points. Extractions to determine bioaccessibility yielded a gastric component of 76.1% and 69.4% for Pb and As, respectively, and intestinal components were 10.7% and 65.9%. The oral bioavailability of the standard reference soil was 0.7% and 37.8% for Pb and As, respectively. Bioaccessibility was greater than bioavailability for both metals in both gastrointestinal compartments. Although Pb had the highest soil concentration of the selected metals, it was the least bioavailable, while As was highly available in both the in vitro and in vivo method. These types of data allow for an in vitro-in vivo comparison of a soil whose metal concentrations have been certified and validated.