Metabolism of Radiocarbon and Tritium in the Human Body
Tsuyoshi MASUDA, Kensaku MATSUSHITA, Yasuhiro TAKO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
In the radiation safety review for nuclear facilities including the first commercial spent nuclear fuelreprocessing plant in Rokkasho, Japan, the internal doses of the pubic due to radiocarbon (14C) and 3H havebeen estimated using the dose conversion factors based on the ICRP metabolic models of carbon and those ofhydrogen in the human body, respectively. The models are simple and are believed to be conservative leading
to overestimation of radiation dose. The carbon model has one compartment of total carbon with biologicalhalf-life of 40 d, and the hydrogen model comprises a free water tritium (FWT) compartment and anorganically bound tritium (OBT) compartment having biological half-lives of 10 and 40 d, respectively.
Although the biological half-life of tritium water (HTO) in the human body was examined in many casessuch as accidental intakes or experimental administrations, actual data on the metabolism of organic 14C andOBT are quite limited. The objectives of this research program are to establish experimentally the metabolismsof organic 14C and OBT in the human body for more realistic dose estimations. The metabolism model of 14C
is also to be utilized for OBT, because a significant part of the hydrogen atoms in OBT covalently bonds tocarbon and is hard to remove from the carbon excluding decomposition to inorganic forms of the elements:
CO2 and H2O.
We have tried to establish a more realistic metabolic model of ingested 14C through the metabolism
experiments of volunteers who ingested one or two 13C-labeled chemicals each of which were representativeof carbohydrates, proteins or lipids. The metabolic models constructed by the results of experiments wellsimulated the metabolism of 13C -labeled rice, but failed in the case of 13C-labeled bean. This failure suggeststhat each nutrition group cannot be represented by only one or two chemicals, and the metabolism of morechemicals representing each nutrition group is necessary for developing the realistic metabolic model of 14C
and 3H in diet. We have launched a further project to study the metabolism of additional 13C-labeled compounds
during FY 2015-2019.
All processes of the experiment were approved by the IES Review Board for Human Subject
Experiments, and written informed consents were obtained from all volunteers. After oral administration of
13

C-labeled compounds, breath samples are collected at pre-determined times as representative of inorganiccarbon excretion. Hair samples are also collected on the final day of the experiment as representative of organiccarbon excretion such as via hair, nail, skin cells, mucus, and other secretions. For estimating growth rate ofeach hair sample, a small dose of 13C-labeled compounds was administered 7-10 days before collecting thesample. In FY 2015, 13C-labeled linoleic acid and glutamic acid with dose rates of 14 and 21 mg kg-1 d-1 13C,
respectively, were administered to volunteers as part of lunches consumed on four successive days. Breath
samples were collected until 154-170 d after the first administration and hair samples were collected at the endof the experimental period. Concentrations of 13C in breath samples were measured in FY2015, while hairsamples are to be analyzed in FY2016.
A five-compartment metabolic model was developed for organic carbon ingestion; it consisted of adigestive tract CDT, fast and slow compartments excreting carbon as an inorganic form (CBf and CBs) and fastand slow compartments excreting carbon as an organic form (COf and COs),. Some parameter values in themodel were estimated by a least square fitting method using the measured data for breath, while all parameters
are to be estimated in FY2016 by using the additional data for hair. Mean distribution factor to carbon excretion
via an inorganic form of linoleic acid (0.62±0.14) was lower than that for the palmitic acid group (0.97±0.05)
studied previously, suggesting that there is less utilization of linoleic acid for energy production than palmitic
acid. The mean distribution factor of the glutamic acid group (0.84±0.08) was larger than that of the leucinegroup (0.69±0.14) though the differences were not statistically significant.

CDT
kDT

dBdBf

CBf

1‐dBf

dOf

CBs

COs

COf
kBs

1‐dOf

kOf

kOs
13

kBf

dO

呼気Inorganic
13
C excretion

C concentration (%)

13C ingestion
経口摂取Organic
毛髪等
13

C excretion

Time (d)Time course of the elevation of 13C concentration

Fig. 1 Structure of the metabolic model for ingested

Fig. 2

13C. CDT is a digestive tract compartment. CBf and CBs,arefrom the mean pre-control concentration in breath

fast and slow compartment excreted as an inorganic

samples during the 13C labeled fatty acid administration

form, and COf and COs are those excreted as an organic

period of four days. Solid circles, linoleic acid. Open

form. d and k denote distribution factor and elimination

circles, palmitic acid.

rate constant, respectively.

C concentration (%)

13Time (d)Fig. 3Time course of the elevation of 13C concentrationfrom the mean pre-control concentration in breath samples
during the 13C labeled amino acid administration period offour days. Solid circles, glutamic acid. Open circles,leucine.

Table 1 Estimated parameters for 13C in fatty acids*.
Parameters

Linoleic acid(d-1)

26±19

dB

0.62

dBfkDT

Palmitic acid**
2.7

0.1

0.14

0.97

0.05

0.50

0.16

0.54

0.19

kBf (d-1)

3.6

3.7

2.7

0.1

kBs (d-1)

0.061

0.019

0.073

0.069

* Mean value with a standard deviation for 3 volunteers.
** Institute for Environmental Sciences, 2010.

Table 2Parameters

Estimated parameters for 13C in amino acids*
Glutamic acid

Leucine**StandardMeanStandard
Meandeviation

deviation

kDT

3228

196

dB

0.84

0.08

0.69

0.14

dBf

0.74

0.04

0.50

0.11

kBf

26

24

5.4

1.1

kBs

0.19

0.07

0.061

0.029

* Mean value with a standard deviation for 3 volunteers.
** Institute for Environmental Sciences, 2009.