Use of the true absorption coefficient as a measure of bioavailability of radiocaesium in ruminants

Abstract

Limitations of existing methods to describe the bioavailability of dietary radionuclides to ruminants (the transfer coefficient and apparent absorption coefficient) have led to the alternative suggestion of using the true absorption coefficient (A(t)). Various approaches to estimating A(t) for radiocaesium, involving the intravenous administration of a second isotope, are presented and discussed with reference to results from studies in which a range of radiocaesium sources were examined in sheep. Although estimates of A(t) differed between the sources, they were reasonably consistent between measurement techniques. Those methods which involved the estimation of endogenous faecal excretion of radiocaesium could be used with previously contaminated animals and did not require continuous administrations of radiocaesium isotopes, but gave unreliable results for sources of low bioavailability. Methods based on estimating the turnover rate of dietary radiocaesium through blood plasma were sufficiently sensitive to measure A(t) for the range of sources studied. However, they require previously uncontaminated animals and continuous administration of both isotopes for approximately 7 days. Bioavailability is more effectively measured as A(t) than as the transfer or apparent absorption coefficients since A(t) does not incorporate factors relating to the metabolism of radiocaesium in the tissues of the animal. The results of these studies show that differences in transfer coefficients between sheep and cattle and between sheep of differing ages are not due to variation in absorption across the gut. The potential for applying these approaches to other radioactive elements is discussed.

Variation in the metabolism of radiocaesium between individual sheep

“…The review by Howard [10] summarised reported mean F f values for ewes ingesting herbage contaminated by root uptake to be 0.3 -0.35 day · kg -1 . The availability of radiocaesium incorporated into herbage by root uptake has been shown to be similar to that of ionic radiocaesium in a number of studies [9,11]. However, there was a threefold range in F f values determined for individual sheep in the current study (0.19 -0.56 day · kg -1 ).…”

“…Gastrointestinal absorption was determined as the true absorption coefficient (A t ) [9], where: (2) where EF, the endogenous faecal output of 137 Cs is given by: [16] b Where A t is the true absorption coefficient [9]; comparisons involving A t are for 12 animals (all other comparisons n = 22)…”

Radiocaesium variability within sheep flocks: determination of 137 Cs intake in free-ranging sheep

“…30% difference in the estimated values of 137 Cs intake and subsequent transfer coefficient to muscle. However, the mean of the seven A a values for contaminated herbage and ionic radiocaesium (CsCl) reported for sheep in the review of Mayes et al [21] is also 0.6, supporting the value selected here as the 'best estimate'.…”

“…Whilst we consider that this is the most appropriate estimate of A a available for the conditions of the current study, reported values of A a range from 0.45 (grass hay fed as minor component of the diet) to 0.73 (CsCl) [21]. A choice of A a at the extremes of the reported range would result in ca.…”

“…For instance, ingested soil can contribute considerably to the radiocaesium intake of grazing animals [23]. The bioavailability of radiocaesium from a number of different soils has been shown to be considerably lower than of radiocaesium incorporated into vegetation via root uptake [21,24]. Consequently, when soil ingestion represents a large proportion of the dietary radiocaesium intake, a lower A a value would be expected.…”