Fulvic and humic acids decrease the absorption of cadmium in the rat intestine.

Dissolved organic carbon (DOC) in drinking water is mainly composed of fulvic and humic acids, which may form complexes with metal ions. The influence of DOC on the intestinal absorption of Cd in the rat was investigated using an "isolated intestinal segment" technique in anaesthetised rats. The lumens of segments were exposed for 60 min to different concentrations of CdCl2 and DOC in intact animals. The fractional absorption (FA) was not dose dependent in the dosage interval 0.01-0.03 microgram Cd/kg. However, at 15 and 150 micrograms Cd/kg both FA and intracellular Cd distribution in the segments were dose dependent, which is in line with results from other studies performed on similar experimental models. In the presence of 1 and 10 mg DOC/l, FA of Cd was just half as high as FA in animals that received Cd alone (0.01 microgram/kg). Moreover, a higher percentage of Cd was associated with the metallothionein fraction in the intestinal segment of the DOC-dosed rats. An in vitro speciation experiment showed that only 0.2-7.9% of the Cd in the incubation solution was complexed to DOC. In deionized water, however, more than 99% of the Cd was complexed to DOC. This result indicates that the incubation solution contained substances that negatively affect complexation of Cd to DOC. Mechanisms other than complexation of Cd to DOC in the intestinal lumen may therefore be involved in the inhibitory effect of DOC on the absorption of Cd.