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
fuel reprocessing plant in Rokkasho, Japan, the internal doses of the pubic due to ingested 14C and tritium
have been estimated using the dose conversion factors based on the simple ICRP metabolic models in thehuman body. Although the biological half-life of tritium water (HTO) in the human body was examined inseveral cases, actual data on the metabolism of organic 14C and organically bound tritium (OBT) in diet arequite limited. The objectives of this research program are to establish experimentally the metabolic modelsof organic 14C and OBT in the human body for more realistic dose estimation. To obtain metabolic parametervalues of 14C, which are also utilized for OBT, we used the stable isotope of 13C to label organic molecules
on oral administration experiments as a substitute for 14C.
Until FY 2016, various 13C-labeled nutrients were administered to volunteers, followed by measuring
the 13C concentration in their breath and hair as representatives of inorganic and organic excreta, respectively.
In FY2017, hair samples from volunteers administered with 13C-labeled oleic acid or glycine were measured,and 13C-labeled phenylalanine and glucose 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 inFY2018. All processes of the experiment were approved by the IES Review Board for Human SubjectExperiments, and written informed consents were obtained from all volunteers.
The recovery of 13C of administered nutrients was 83-97% so far excluding unsaturated fatty acids forwhich recovery was 59-60%. Although the undetected proportion of ~10% may be attributed to a
non-absorbing fraction in the gastrointestinal tract, the large undetected proportion of unsaturated fatty acids
was hard to explain by only a non-absorbing fraction in the tract. We assumed that the undetected fraction
was accumulated in adipose tissue, which is reported to have a long mean residence time of carbon based on
analyzing 14C that originated from nuclear weapon testing. A tentative metabolism model was constructedbased on our results and the reported mean residence time. The 50-year cumulative body burden for 14C, as
an index of the committed dose of the radioisotope 14C, in unsaturated fatty acids was estimated to be larger
than that of the other nutrients by the model.
Fig.1

Structure of the metabolic model for ingested 13C.
Compartments of 13C: CDT, digestive tract; CBf and CBs,
fast and slow compartments for inorganic excretion,
respectively; COf and COs, fast and slow compartments fororganic excretion, respectively; CND, compartment for no
detected component of administered 13C. d is distribution
factor and k is elimination rate constant.

Table 1

Parameters for carbon in fatty acids

Parameter

Oleic acid

Linoleic acid*

Palmitic acid*

kDT

11±4

3.8±1.1

2.6±0.3dB

0.61±0.04

0.61±0.07

0.90±0.05

dBf

0.57±0.07

0.48±0.07

0.56±0.08

kBf

3.2±0.720±3

3.2±0.3kBs

0.11±0.03

0.081±0.015

0.096±0.029

dO

0.018±0.001

0.019±0.005

0.034±0.008

dOf

0.53±0.17

0.41±0.05

0.66±0.20

kOf

0.52±0.09

2.1±1.7

0.44±0.13

kOs

0.022±0.016

0.043±0.017

0.044±0.002*

Mean value±standard deviation. , Determined using data from previous research.

Table 2

Parameters for carbon in amino acids

Parameter

GlycineGlutamic acid*

Leucine*kDT

14±325±87.5±1.1dB

0.75±0.02

0.84±0.03

0.57±0.05

dBf

0.37±0.03

0.73±0.02

0.52±0.05

kBf

28±636±1214±3

kBs

0.18±0.01

0.21±0.03

0.13±0.06

dO0.23±0.01

0.061±0.014

0.37±0.04

dOf

0.48±0.02

0.39±0.08

0.33±0.02

kOf

0.22±0.02

0.39±0.05

0.29±0.01

kOs

0.011±0.001

0.017±0.008

0.013±0.000

Cumulative body burden (Bq d)Mean value±standard deviation. *, Determined using data from previous research.

Fig.2

Cumulative body burden for 50 years after an ingestion of 1 Bq of 14C in nutrients.
*, Determined using data from previous research.