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 objective of this research program is to establish experimentally the metabolic models oforganic 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 13C to label organic molecules inoral administration experiments as a substitute for 14C.
Until FY 2017, 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 FY2018, hair samples from volunteers administered with 13C-labeled phenylalanine or glucose weremeasured, and 13C-labeled peanut was administered to volunteers, followed by collecting breath and hairsamples. The breath samples were measured for 13C, and the hair samples will be analyzed in FY 2019. Allprocesses of the experiment were approved by the IES Review Board for Human Subject Experiments, andwritten informed consents were obtained from all volunteers.
A metabolic model was constructed based on our result for each nutrient. Data for 13C inventories inthe breath and hair samples were treated as representatives of inorganic and organic excretion. When thesum of 13C inventories via both excretions after correction of the digestive tract absorption ratio was lower
than the administered inventory, the differences between them was assumed as an undetected fraction, which
had the residence time of C in adipose tissue. The 50-year cumulative body burden for 14C was estimated byeach model for eight nutrients, and showed that the burden of monounsaturated and polyunsaturated fatty
acids were distinctly large among the nutrients. Contribution of undetected fraction to the burden was
dominant for unsaturated fatty acids. We coupled eight metabolic models in series, which could estimate theretention of 14C ingested in various nutritional compositions, and named it the IES model. The cumulative
body burden for 50 y was estimated by the IES model for 1 Bq of 14C intake through uniformly
contaminated foods having average nutritional composition according to the National Health and Nutrition
Survey by the Ministry of Health, Labour and Welfare, Japan. A large proportion of the result estimated by
the IES model was contributed by the undetected fractions. Although the result by the IES model had a large
error, it roughly agreed with the value obtained by the ICRP model.

Structure of the metabolic model for ingested 13C.

Fig.1

Compartments of 13C: CDT, digestive tract; CBf and CBs, fast and slow compartments for inorganic excretion,respectively; COf and COs, fast and slow compartments for organic excretion; CND, compartment for undetectedfraction. f1, absorption ratio; dB, dBf, dO, dOf, distribution factors; kBf, kBs, kOf, kOs, kND, elimination rate constants.

350
Cumulative body burden(Bq d)

300
250
20015010050
Monosaccharide

Aromatic amino acid

Collagen material amino acid

Branched chain amino acid

Major amino acid

Polyunsaturated fatty acid

Monounsaturated fatty acid

Saturated fatty acid0Fig. 3100500IES

ICRP

Cumulative body burden for 50 years after aningestion of 1 Bq of 14C intake throughuniformly contaminated foods having average
nutritional composition according to theNational Health and Nutrition Survey by
Ministry of Health, Labour and Welfare, Japan.

Open, filled, and hatched areas in the IES bar show theFig. 2

Cumulative body burden for 50 years after an14

ingestion of 1 Bq of C in nutrients estimated by ametabolic model for each nutrient.
Open, filled, and hatched areas show the burden frominorganic excretion, organic excretion and undetected
fraction, respectively.

burden from inorganic excretion, organic excretion andundetected fraction, respectively.