Metabolism of Radiocarbon and Tritium in the Human Body
Tsuyoshi MASUDA, Kensaku Matsushita, Yasuhiro TAKO, Yuichi TAKAKU, Shun’ichi
HISAMATSU
Department of Radioecology
Abstract
In the radiation safety assessment for nuclear facilities including the first commercial spent nuclear fuelreprocessing plant in Rokkasho, Japan, the internal doses of the public due to ingested 14C and tritium havebeen estimated using the dose conversion factors based on the simple ICRP metabolic models in the humanbody. Although the biological half-life of tritium water (HTO) in the human body was examined in several
cases, actual data on the metabolism of organic 14C and organically bound tritium (OBT) in the diet are quite
limited. The objective of this research program is to establish experimentally the metabolic models of organic
14

C and OBT in the human body for more realistic dose estimation. To obtain metabolic parameter values of14C, which are also utilized for OBT, we used the stable isotope of 13C to label organic molecules on orally

administration experiments as a substitute for 14C.
Until FY 2014, 13C-labeled glucose, palmitic acid, and leucine were administered to volunteers, followedby measuring the 13C concentration in their breath, urine, feces, and hair. In FY2015, 13C-labeled linoleic acidand glutamic acid were administered, followed by collecting breath and hair samples as representative ofcarbon excretion via inorganic form and organic form, respectively. Following the measurement of 13C
concentration in breath samples in FY2015, hair samples were measured in FY2016. In 2016, 13C-labeled oleic
acid and glycine were also administered to volunteers, followed by collecting breath and hair samples. The
breath samples were measured for 13C, and the hair samples will be analyzed in FY2017. All processes of theexperiment were approved by the IES Review Board for Human Subject Experiments, and written informed
consents were obtained from all volunteers.
A model was developed, having five compartments consisting of a compartment for 13C in the digestive
tract (CDT), and two compartments each for 13C excreted inorganically (CBf and CBs) and organically (COf andCOs), and parameter values in the model were determined by using excretion data of 13C. Mean distribution
ratios to the inorganically excreted route of 13C in palmitic acid and glutamic acid were higher than those inlinoleic acid and leucine, respectively, suggesting that the former two are preferentially utilized for energy
production. However, un-recovered 13C was obtained in the model calculation for each substance, with anespecially large proportion of 0.380.16 for linoleic acid. We simulated carbon retention in the body afteringestion of the diet with nutrition compositions according to the Dietary Reference Intakes for Japanese(2015). The metabolic models for palmitic acid, linoleic acid, glutamic acid, leucine, and glucose was applied
to the nutrition components in the diet, while the components for which models have not yet been constructed
were represented by one of the substances mentioned above. We also assumed that the un-recovered 13C was
directly transferred to a total carbon compartment of 16 kg C. The 50-year cumulative body burden for 13C, as
an index of the committed dose of the radioisotope 14C, was found to be 51 g d, which is comparable with 58
g d by the ICRP metabolic model. Further accumulation of model parameters of different substances is requiredto obtain the value that is more realistic.

Fig.1

Structure of the metabolic model for ingested 13C.

Fig.2

Transition rate by carbon excretion

Compartment of 13C: CDT, digestive tract; CBf and CBs,pathway

fast and slow compartments for inorganic excretion,

f1dB, excretion via inorganic form path.

respectively; COf and COs, fast and slow compartments

f1(1-dB), excretion via organic form path.

for organic excretion, respectively.

1- f1, excretion not detectedd is distribution factor, and k is elimination rate

experimentally.

constant.