The Importance of Source-Dependent Bioavailability in Determining the Transfer of Ingested Radionuclides to Ruminant-Derived Food Products

Abstract

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.

Dry matter intake - a generic approach to predict the transfer of radiocaesium to ruminants ?

“…Higher DMI's lead to a faster gut passage time [11] prompting the suggestion that absorption of radiocaesium in the gut may be reduced. However, in a previous study we found that the influence of DMI on radiocaesium absorption in sheep was minimal [2] and similar absorption values have been determined for radiocaesium for sheep in a large number of studies and also dairy cattle [12]. An increase in cellular metabolism with increasing food intake has previously been observed, the activity of Na+, K+-ATPase varying with food intake [4]; such an effect could influence the movement of Cs across cell membranes.…”

Modelling radiocaesium transfer and long-term changes in reindeer

“…Considerably lower absorption (20e30%) was suggested for reindeer on a lichen diet by Holleman et al (1971), but this is evidently too low to fit the activity concentrations observed in the experiments presented here. Higher factors for true absorption of radiocaesium from forage (mostly within the range 0.6e0.9) have been reported for sheep and cattle (various papers reviewed by Beresford et al, 2000).…”

The transfer of radionuclides from saltmarsh vegetation to sheep tissues and milk

“…The considerably higher activity concentration of refused feed compared to that of offered feed is likely to have been a consequence of the animals selecting against feed with especially high silt contents. The availability of 137 Cs associated with silt from the Ravenglass Estuary for absorption in the ruminant gut has previously been shown to be <10% of that for Cs incorporated into vegetation or ingested as CsCl (see review by Beresford et al, 2000). Apart from Cs there is only limited evidence to suggest that the availability for absorption in the ruminant gut of Am and Pu from saltmarsh vegetation and soil contaminated by marine discharges from the Sellafield reprocessing plant is low compared to 'recommended' values (see Ham et al, 1989;Harrison, 1991;Cooke et al, 1997).…”

“…However, the majority of radiocaesium associated with saltmarsh vegetation is due to adherent silt. The availability for absorption of radiocaesium from silt contaminated by Sellafield marine discharges is <10% (Beresford et al, 2000) compared to that for milk of circa 100% . Milk was, therefore, likely to contribute approximately 75% of the ingested 137 Cs available for absorption by the lambs of the study ewes (at 10 weeks old).…”

“…Ham et al, 1989;Howard, 1985;Howard and Lindley, 1985;Popplewell and Ham, 1989;Sumerling, 1981). There have also been a few experimental feeding trials to determine the transfer of radiocaesium and actinides from saltmarsh vegetation to sheep tissues , and the gastrointestinal absorption of radiocaesium and plutonium from saltmarsh silt fed to sheep Beresford et al, 1992Beresford et al, , 2000. This paper describes a controlled feeding trial to determine the transfer of a wider range of radionuclides than previously considered ( 60 Co, 95 Nb, 134 Cs, 137 Cs, 106 Ru,238 Pu,239,240 Pu and 241 Am) from saltmarsh vegetation harvested from the Ravenglass Estuary to the tissues of ewes and lambs.…”