The importance of source-dependent bioavailability in determining the transfer of ingested radionuclides to ruminant-derived food products

The transfer of radioactivity to the milk and meat of farm
animals is likely be a major exposure pathway of human
populations, following an environmental release of
radioactivity. The importance of source-dependent bioavailability
in determining absorption from the ruminant
gastrointestinal tract of the radiologically significant
radionuclides (radiocesium, radiostrontium, radioiodine,
and plutonium) is reviewed. The requirements for and
suitability of in vitro methods of determining bioavailability
for absorption of these radionuclides is also assessed.
Radiocesium absorption varies over a 50-fold range, depending
upon dietary source. Source-dependent bioavailability is
therefore an important factor in determining the radiocesium
contamination of ruminant-derived food products, and
reliable in vitro techniques have been developed to rapidly
determine its bioavailability. In contrast, under conditions
of adequate calcium intake, the absorption of radiostrontium
will not be greatly influenced by the dietary source. Results
of in vitro extractions of radiostrontium could be misleading,
as they indicate differences in bioavailability that are
not observed in the animal. Absorption of radioiodine is
complete and independent of source. There is currently a
lack of data on plutonium absorption in ruminants.
However, on the basis of the limited data available, in
vitro incubations provide a measure of the relative
bioavailabilities of plutonium from different sources. The
applicability of the techniques reviewed to the study of other
radionuclides and heavy metals is discussed.