Study on Carbon Metabolism in the Human Body
Tsuyoshi MASUDA, Kensaku MATSUSHITA, Yasuhiro TAKO, Yuji NAKAMURA
Department of Environmental Simulation
Abstract
In the safety assessment around the spent nuclear fuel reprocessing plant at Rokkasho, Aomori, 14C is
expected to be the most attributable to radiation dose received by the neighboring population, amongradioactive nuclides released from the plant. The radiation dose due to 14C reaches around one third of thetotal annual radiation dose estimated as 22 μSv. However, the estimate of 14C dose is thought to be rather
conservative, because of possible overestimation of 14C dose conversion factor. This might be largely
attributed to excessive simplification of the metabolic model of 14C in the human body. The objective of thestudy is to clarify experimentally carbon metabolism, especially the biological half-time, of 14C in the humanbody.
Using the data from previous experiments on carbon metabolism through ingestion of protein, fat andsugar, a Human Body Carbon Metabolism Model was developed. To confirm the validity of the the model,changes in 13C isotopic ratio after oral administration of 13C labeled foods (rice and soybean) were examined
during 16 weeks in breath air, urine, feces, hair and serum in three adult males. The estimate of the change in13

C concentration by the model was compared to experimental data. Finally, retention of 14C in the humanbody after oral intake of 14C through ingestion of a typical Japanese diet was estimated by the model. The
estimate of 14C retention was lower than that predicted by the ICRP model on which is based the dosecoefficient of organic 14C intake for the general public.

1

0.9
0.8
0.7

AB

0.1

0.01

P1

f1

Gut

L1

C1

13C concentration (atom% excess)

0.6

C2

L2

1-f1

L3
L5

L4

C3

Ex1
Ex2Fig. 1 Human Body Carbon Metabolism Model.
C1: low molecular weight carbon pool, C2:high molecular weight carbon pool, Gut:
intestine, P1: intakes of carbon, f1: fraction

0.001

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0.00010.3

0.00001

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0.000001

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0.0000001

00

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0.00010.0000100

0.2

0.40.60.8

50100

Time (day)Fig. 3

between administration and time when thehighest

C concentration in breath wasbreath.
A, B: rice, C, D: soybean, A, C: time 0 to

0.9

0.9

0.9

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B

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00

0

-0.1

-0.1

-0.1

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124 hr, B, D: day 1 to day 112. Solid line

CCarbon Metabolism Model.

0.1

00.5

1

00.5

1

10.1

DEF0.010.01

0.01

0.001

0.001

0.001

0.00010.00010.0001

1

show the predictions by Human Boby

11

0.1

0.1

Retentonraterate
Retention

A0R

0.00001

0.00001

0.00001
51

Comparison between the expected 13C
concentration and experimental data in

13

observed.
13C concentration (atom% excess)01

Absorption of carbon in Gut wasassumed to continue during the short period

101

050

100

151

101

Time (day)

0.1

0.01
0Fig. 21001

excretion rate, Ex1: feces, Ex2: breath and150

DC

0.5

0.1

absorbed, L1, L3: transfer rate, L2, L4, L5:urine.

01204060

80100120

13C concentration in breath after oral
Time (day)administration of amino acid, fat, and sugar.
A, D: amino acid, B, E: fat, C, F: sugar, A-C:

Fig. 4Comparison between the model and thetime 0 to 24 hr, D-F: day 1 to day112. SolidICRP estimates of 14C retention in thelines show approximation by the Human Body

human body.

Carbon Metabolism Model. Data were adopted

Black line: estimate by the developed

from a series of experiments preceding thismodel, Gray line: estimate by ICRP

study.

model.