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Carbon-14 released from nuclear facilities has been assessed to contribute significantly to the radiation dose that people are exposed to through the food chain. However, the current dose coefficient for members of public, which is the ratio of the 50-year committed effective dose to ingested 1 Bq 14C, recommended by the International Commission on Radiological Protection (ICRP) is not based on experimental human metabolic data for 14C in nutrients and diet. Therefore, to validate the coefficient, we administered 13C-labelled nutrients consisting of four amino acids, three fatty acids, and one monosaccharide to volunteers as substitutes for 14C labelled nutrients and measured the 13C concentration in various excreta samples. Although metabolic models were constructed from the excretion data, a significant fraction of administered 13C was not recovered from some nutrients. The dose coefficients of 14C in uniformly labelled Japanese diet, which were estimated under several assumptions about the unrecoverable fraction, varied from (6.2 ± 0.9) × 10–11 to (8.9 ± 4.4) × 10–10 Sv Bq−1 and were approximately comparable to the current value of 5.8 × 10–10 Sv Bq−1 recommended by the ICRP. Further studies are necessary to elucidate the metabolism of 14C in various nutrients in the unrecoverable fraction.

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