Annual Report

48. Contributions of 18 Food Categories to Intakes of 232Th and 238U in Japanese

Kunio Shiraishi, Keiko Tagami, Yasuyuki Muramastu and Masayoshi Yamamoto* (* Kanazawa Univ.)

Keywords: dietary intakes, 232Th, 238U, Japanese, market basket study


Dietary intake studies have been conducted to estimate the representative values of radioactive and non-radioactive nuclides from the viewpoints of radiation protection for Japanese. Assessments of radiation dose from 232Th and 238U are important because both nuclides are the parent nuclides in decay series. Dietary nuclide intakes are estimated by several methods. Market basket studies and duplicate portion studies are two representative ways. Although duplicate portion studies offer the greatest degree of reality compared to other methods, market basket studies are advantageous regarding identification of what kinds of food are critical foods for radionuclide intakes. In the report, a market basket study using 18 food categories was conducted to clarify the food pathways of 232Th and 238U in Japanese subjects. A total of 336 foodstuffs equivalent to a total wet-weight of 120 kg was used.

During 1994-1995, foodstuffs were bought from markets in the vicinity of Mito, Ibaraki Prefecture. Statistical consumption data of 1989-1991 (MHW 1993) was used for collection of the food samples. Food categories, daily intakes, and the number of foodstuffs used in the present study are shown in Table 6. The purchased foodstuffs, were divided into the18 categories a follows: 1) rice; 2) cereals, excluding rice; 3) nuts and seeds; 4) potatoes; 5) sugars and confectionaries; 6) fats and oils; 7) bean products; 8) fruits; 9) green vegetables; 10) other vegetables; 11) mushrooms; 12) seaweeds; 13) seasonings and beverages; 14) fishes and shellfishes; 15) meats; 16) eggs; 17) milk and milk products; and 18) cooked meals. Each food group was homogenized and was freeze-dried. An aliquot of sample was taken and completely decomposed with a mixture of nitric acid, perchloric acid, and hydrofluoric acid. The sample solution was measured with inductively coupled plasma mass spectrometry.

For 238U and 232Th, concentrations (Bq per g-fresh) and daily intakes (mBq per person) of the 18 food categories are shown in Table 6. Food categories having higher 238U contents were found to be as follows: seaweeds 1140Bq; fishes and shellfishes 37Bq; nuts and seeds 11Bq; bean products 8.6Bq; and cooked meals 7.3Bq. Big contributors to the daily 238U intake in Japanese were as follows: seaweeds (50%); fishes and shellfishes (26%); and bean products (4.3%). For 232Th, higher contents were found as follows: seaweeds 28Bq; fishes and shellfishes 13Bq; nuts and seeds 8.2Bq; green vegetables 3.9Bq; cooked meals 3.5Bq; and bean products 2.9Bq. Big contributors to the daily 232Th intake were as follows: fishes and shellfishes (44%); green vegetables (11 %); bean products (7.4%); and seaweeds (6.0%). For both nuclides, marine food products were big contributors, while minor contributors were oil and fats, eggs, and cooked meals. Daily intakes of 232Th and 238U in Japanese were estimated to be 2.7 mBq and 14 mBq per person from the intakes of the 18 categories, respectively. Annual effective doses (Sv/y) were estimated from these experimental results with dose coefficients (Sv/Bq) of the International Commission on Radiological Protection Publication 61 ( ICRP 1994) as 2.2 x 10-7 for 232Th and 2.2 x 10-7 for 238U in Japanese.

In conclusion, big contributors to Japanese dietary 232Th and 238U intakes were found to be marine products. Dietary intake studies using eighteen or more food categories should be effective for constructing the critical pathway. Furthermore, foodstuffs in the big contributor groups and foodstuffs having high contents of the nuclides should be analyzed in detail to find individual critical foods in the food chain.

Publication:
Shiraishi, K., Tagami, K., Muramatsu, Y. and Yamamoto, M.: Health Phys., 78, 28-36, 2000.

Table 6. Actitity concentrations and daily intakes of 238U and 232Th in 18 food categories for Jpanese.

Table 6

a Daily food intakes of fresh weight per person (MHW1993).
b Mean and SD for three analysee (Bq per g-fresh weight).
c Millibecquerels per person per day


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