**Hiroshi Terada 1,\*, Ikuyo Iijima 2, Sadaaki Miyake 3, Kimio Isomura <sup>4</sup> and Hideo Sugiyama <sup>1</sup>**


Received: 10 October 2020; Accepted: 31 October 2020; Published: 3 November 2020

**Abstract:** We conducted a total diet study (TDS) of 137Cs, 134Cs, and 40K to assess their average dietary exposure levels in a Japanese adult population before and after the Fukushima Daiichi nuclear power plant (FDNPP) accident. Nineteen market baskets were evaluated in 2006–2011. In each basket, a TDS sample comprising tap water and 160–170 food items, which were combined into 13 groups, were collected for analysis by gamma-ray spectrometry. From 2006 to 2010, the 137Cs activity concentration in the "fish and shellfish" group was 0.099 Bq/kg, representing the highest value obtained, whereas the total committed effective dose (CED) of radiocesium isotopes (137Cs + 134Cs) was 0.69 μSv. In 2011, "milk and dairy products" from Sendai City had a Cs activity concentration of 12 Bq/kg, representing the highest values among all food groups studied. However, the annual CED of radioactive Cs in Fukushima City was 17 μSv after the FDNPP accident, which is 60-fold lower than the maximum permissible dose of 1 mSv/year. The mean CED obtained for 40K was 180 μSv, which is comparable to the global average. Our results reveal the average dietary exposure of 137Cs, 134Cs, and 40K, which can aid in estimating the radiological safety of foods.

**Keywords:** total diet study; radioactive cesium; potassium-40; dietary intake; dose assessment; Fukushima accident

## **1. Introduction**

To ensure food safety, it is essential to assess the exposure levels to toxic substances in food. Currently, there are three approaches for estimating the dietary intake of such substances: total diet study (TDS), duplicate portion study (DPS), and selective study of individual foodstuffs. A TDS, also referred to as a market basket study, has an advantage over the two alternatives in terms of its accuracy. Furthermore, TDS takes into account the effect of kitchen preparation on the levels of toxic substances in food, and provides information on which food groups are the main sources of contamination [1]. Therefore, TDSs have been supported and endorsed by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) since the 1970s. According to a survey carried out in 2015 by Health Canada, in cooperation with the WHO, approximately 53 countries around the world perform TDS analyses [2].

The first TDS was conducted in response to public concerns about a radioactive fallout from atmospheric nuclear testing [3,4]. Fallouts contain hundreds of different radionuclides. Among these, 137Cs and 90Sr are the most significant sources of internal exposure to radiation, owing to their long half-lives of 30.17 years and 28.8 years, respectively, as well as to their chemical similarities to essential elements (137Cs resembles potassium and 90Sr resembles calcium). Therefore, the U.S. Food and Drug Administration (FDA) has been carrying out TDSs focusing on these two radionuclides since 1961 [5].

In Japan, TDSs have been performed annually by the Ministry of Health, Labour and Welfare (MHLW) since 1977 [6,7]. Initially, pesticides and their metabolites, seven metals (Pb, Cd, total Hg, total As, Cu, Mn, and Zn), and total polychlorinated biphenyl (PCB) were analyzed in these studies [6,8]. Similarly, radioactivity monitoring has been conducted by the Nuclear Regulation Authority (NRA) since the 1950s. Radionuclide levels in airborne dust, rainwater, river water, seawater, soil, and food have been analyzed in this monitoring [9]. Until 2008, the NRA also conducted DPSs to evaluate the daily dietary intake of radionuclides [10–12]. Subsequently, TDSs of radionuclides began in 2003 by the MHLW to assess total exposure levels of radionuclides in the average Japanese diet, as well as the contribution of each food group to this total. Between 2003 and 2011, we carried out a TDS, which was supported by a Health and Labour Sciences Research Grant (HLSRG) from the MHLW [13–19]. From 2003 to 2005, the dietary exposure to man-made radionuclides (137Cs, 134Cs, and 90Sr) and natural radionuclides (40K, 214Pb, 214Bi, 228Ac, 212Pb, 208Tl, and U) were determined. Sugiyama et al. [13] revealed that only trace amounts of 137Cs were found in TDS samples, and that "fish and shellfish" contained 0.145 Bq/kg, representing the highest activity concentration measured. They also found that the daily dietary intake and the committed effective dose (CED) of 137Cs were, at the very most, 0.080 Bq/person/day and 0.38 μSv, respectively, with the main sources of the exposure being from the "fish and shellfish", "meat and eggs", and "other vegetables, mushrooms and seaweeds" food groups.

The present study aimed to assess trends in dietary exposure levels to γ-emitting radionuclides, namely 137Cs, 134Cs, and 40K, from typical Japanese diets via TDS. The concentrations, dietary intake, and CEDs of these radionuclides from 2006 to 2011 have been presented. The results from before and after the Fukushima Daiichi nuclear power plant (FDNPP) accident that occurred in March 2011 have been compared. We also attempted to summarize the results of the TDS performed between 2012 and 2019 by other institutions in Japan, because almost 10 years have passed since the FDNPP accident.

#### **2. Materials and Methods**

#### *2.1. Sampling and Preparation*

Each year, TDS samples have been collected from three or four cities in different regional blocks of Japan, except for the year 2010. Each sample collection is referred to as a market basket (MB). We collected as many locally produced food items as possible. The most preferred production areas in descending order were same prefecture, same regional block, and other regional blocks in Japan. As shown in Figure 1, 19 MBs were collected from all regional blocks of Japan, other than Minami-Kyushu, from 2006 to 2011. In each MB, 160 to 170 food items were purchased from local retailers. These items were classified into 13 food groups as follows: rice and rice products (Group I); cereals, seeds, nuts, and potatoes (Group II); sugars and confectionaries (Group III); fats and oils (Group IV); pulses and their products (Group V); fruits (Group VI); green and yellow vegetables (Group VII); other vegetables, mushrooms, and seaweeds (Group VIII); beverages (Group IX); fish and shellfish (Group X); meat and eggs (Group XI); milk and dairy products (Group XII); and seasonings (Group XIII). The relative proportion of each food item within a group was based on average regional food consumption data for individuals 20 years and over, which were obtained from the National Health and Nutrition Survey (NHNS) performed by the MHLW between 2002 and 2004. Individual food items were cooked, if necessary, and then combined for analysis into 13 food groups. In addition, tap water was collected in each MB for drinking water (Group XIV). Thus, in total 19 × 14 = 266 TDS samples were obtained. The sample weights of individual food groups were approximately 5 kg, with the exception of 12 kg for "rice and rice products" and 100 kg for "drinking water".

**Figure 1.** Sampling sites of included in the total diet study conducted from 2006 to 2011. Sampling year of each site is denoted in parenthesis.

TDS samples, other than "fats and oils" and "drinking water", were freeze-dried or heat-dried and then incinerated at 450 ◦C for 24 h into ash. The ash samples were placed in separate plastic containers with a capacity of 100 mL. The "drinking water" samples were condensed by heating and evaporated to dryness, and the residues were stored in the plastic containers, as described above. The "fats and oil" samples were stored in 1 L Marinelli beakers in their raw state.
