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Assessment of macro, trace and toxic element intake from rice: differences between cultivars, pigmented and non-pigmented rice

The analytical data for elements in the 20 rice varieties from different areas have been summarised as Tables 1,2. The concentrations of macro and trace elements in the samples studied occurred respectively in the following orders: P > K > Mg > Ca > Na and Mn > Zn > Fe > Cu > Se (Table 1), and the order of concentrations of toxic metals was Cr > As > Cd (Table 2).

Table 1 Contents of macro and trace element in rice samples.
Table 2 Contents of toxic elements in rice samples (μg/kg).

Total trace element concentrations

The average concentration of total Cu in rice grains was 1.03 ± 0.53 mg kg-1. The FAO permissible limit for Cu in food is 40 mg kg−1. Therefore, the Cu concentration in all rice grain samples was lower than the permissible limit. The highest Cu concentration was red glutinous rice (2.50 mg kg−1) from Honghe, followed by soft rice 88 (1.51 mg kg−1) and purple rice (1.26 mg kg−1) from Dehong, while Cu content was undetectable in red and white rice from Lvchun, purple rice from Banna, Yunda 107 and Chujing 40 from Chuxiong, and white rice from Kunming (Table S1). The recommended nutrient intake (RNI) and tolerable upper intake level (UL) of Cu in foods are 0.8 mg d-1 and 8.0 mg d-1 for Chinese adults, respectively.

The average Fe concentration in rice was 4.80 ± 3.34 mg kg-1 (Table 1), which is lower than the allowable limit in rice (15 mg kg-1) and in food (5 mg kg−1, WHO /FAO 2011). The highest Fe content was found in Bianhua red soft rice from Dehong (11.49 ± 0.58 mg kg−1), while both Yunda 107 (0.56 mg kg−1) and Yunjing 37 (1.07 mg kg−1) from Chuxiong had lower Fe concentration. According to the Chinese Dietary Reference Intakes (NHCPRC 2017 ), the RNI for Fe in food for adults male are 12 mg d−1 and 20 mg d−1 for mg kg−1 body weight, respectively. UL for Fe in food for adults is 42 mg d−1 for mg kg−1 body weight.

The average Mn concentration of rice was 14.31 ± 21.66 mg kg−1. Chujing 39(105.70 ± 5.31 mg kg−1) was the highest followed by Mojiang purple rice (20.47 ± 0.85 mg kg−1) from Pu’er. Moreover, the lowest was detected in Yunda 107 (4.45 mg kg−1) from Chuxiong, followed by Kunming white rice (4.76 mg kg−1) (Table 1).

The mean total Zn concentration in rice samples was 13.48 ± 3.26 mg kg−1, which was below the allowable limit for cereals of United States Department Of Agriculture (50 μg g−1, USDA 2003) and in conventional foods (60 μg g−1, WHO /FAO 2001). The highest value of 18.82 ± 1.01 mg kg−1 was found in red glutinous rice1 from Honghe, while the lowest Zn content was found in rice Yunda 107 (7.64 ± 0.36 mg kg−1) from Chuxiong (Table 1). The RNI and UL for dietary Zn for adults in China are 12.5 mg d−1 and 40 mg d−1, respectively.

The average Se content in rice was measured to be 29.27 ± 26.14 μg kg−1. The highest content was found in Mojiang purple rice, followed by Bianhua red soft rice and Zhefang rice from Dehong, while Yunda 107 from Chuxiong had the lowest Se content (Table 1). Chinese standard GB/T 22499–2008 specifies that rice whose selenium content is less than 40 μg kg−1 is considered non-selenium-enriched rice; if it is above 0.3 mg kg−1, the selenium content exceeds the standard. Therefore, Mojiang purple rice, Bianhua red soft rice and Zhefang rice can be designated as selenium-rich rice varieties. The RNI and UL values for dietary selenium for adults in China were 40 μg d−1 and 400 μg d−1, respectively.

Total macro element concentrations

The macro element analytical data of 20 rice varieties from different areas as shown in Supplementary Table 1. The average level of Ca in all the examined rice was measured at 69.62 ± 26.19 mg/kg. The greatest CA content was detected in Bianhua red soft rice (114.80 ± 1.97 mg kg−1) from Dehong, whereas Menghai fragrant rice from Xishuangbanna samples had the lowest Ca level (38.45 ± 0.79 mg kg−1). The adult estimated average requirement (EAR) of dietary Ca for Chinese residents is 650 mg d-1 for adults over 18 years old, and 800 mg d-1 for adults over 50 years old.

The average level of K in rice was measured at 1080.46 ± 603.31 mg kg−1. Bianhua red soft rice (2564.00 ± 65.05mg kg−1) and soft rice 88 (410.80 ± 13.75 mg kg−1) from Dehong have the greatest and lowest K level, respectively (Table 1). The adult adequate intake (AI) of dietary K for Chinese was 2000 mg d−1.

The average level of Mg in rice was measured at 491.60 ± 21.66 mg/kg. The greatest Mg content was detected in Bianhua red soft rice (1239.42 ± 20.95mg kg−1) from Dehong, whereasYunda 107 from Chuxiong had the lowest Mg level (110.39 ± 6.20 mg kg−1) (Table 1). The adult EAR and RNI of dietary Mg for Chinese was 280 mg d−1 and 330 mg d−1.

The average level of Na in rice was at 9.68 ± 5.73 mg kg−1. The greatest content was in Menghai fragrant rice (31.97 ± 2.72 mg kg−1) from Xishuangbanna, and the lowest level (6.06 ± 0.10 mg kg−1) was found in Banna purple rice also from Xishuangbanna (Table 1). The adult adequate intake (AI) of dietary Na for Chinese was 1500 mg d−1.

The average level of P in all rice samples was at 1635.29 ± 915.56 mg kg−1. The greatest P content was detected in Bianhua red soft rice (3341.38 ± 67.78mg/kg) from Dehong, whereas Yunda 107 from Chuxiong samples had the lowest level (697.45 ± 14.87 mg/kg) (Table 1). The adult RNI and UL of dietary P for Chinese residents is 720 mg d−1 and 3500 mg d−1.

Toxic metal concentrations

Table 2 shows the results of the toxic element analysis. The average concentrations of Cr, As and Cd in rice were 236.83, 117.60 and 47.42 μg kg1, respectively. Compared to the Chinese standard (GB 2762-2022) limit indicators for Cr, As and Cd concentrations, the average concentrations of the toxic metal were lower than the value of the Chinese standard value. However, Cd concentrations in Dehong purple rice and purple glutinous rice were above the Chinese standard limit.

Origins differences of elements in rice

The 20 rice varieties were classified into five categories based on the content of elements (Fig. 1a). Menghai fragrant rice was a separate category characterised by low Ca, high Na and Cr content, and Chujing 39 was also a separate category characterised by high Mn content. The three coloured rice varieties Dehong purple rice, red glutinous rice1 and Yuanyang red rice were grouped into a category characterised by high Cu, Zn and Cd content. Purple glutinous rice, red glutinous rice2, Ivchun red rice, Mojiang purple rice and Banna red soft rice were categorised as having high levels of Ca, Fe, K, Mg, P and Se. Other varieties were categorised as lower in Ca, Fe, K, Mg and high in As, Se, Cu, Zn and Cd.

Figure 1
figure 1

Heat map for rice samples. The blue to red colour indicates the high to low concentration. Figure 2a Heat map for rice varieties, Fig. 2b Heat map for rice geographical origins. Generated by Origin 2021 (Originlab, Massachusetts, USA).

There were large differences in the elements of rice from different origins, and can be divided into 4 categories (Fig. 1b). The lowest K (485.96 ± 11.13 mg kg-1), Mg (165.76 ± 10.26 mg kg−1), P (796.01 ± 34.06 mg kg−1), Mn (4.76 ± 0.09 mg kg−1), Zn (8.75 ± 0.25 mg kg−1), Fe (1.19 ± 0.05 mg kg−1), Se (6.10 ± 0.36 μg kg−1), Cr (170.00 ± 20.00 mg kg−1), Cd (ND) and the highest Na (16.74 ± 0.17 mg kg−1) were found in Kunming Yiliang white rice. This area should focus on the accumulation of micronutrients in rice between cultivation and processing. The second category was Lvchun and Yuanyang, where rice has high Cd content, as priority areas for heavy metal remediation. Honghe Yuanyang rice production had the highest Cu content (1.79 ± 0.78 mg kg-1), but Honghe Luchun rice samples were undetectable. The third category were Lufeng, Menghai, Shizong and Mangshi, lower levels of Ca, Fe, Mg, P, Zn, As and Se, also need to increase essential micronutrients. The highest Mn concentration was in Chuxiong Lufeng fong, while black rice from Qujing Shizong had the lowest levels of Ca (39.77 ± 1.54 mg kg−1), Na (6.09 ± 0.15 mg kg-1) and As (47.67 ± 5.51 μg kg−1). The fourth category was Mojiang and Lianghe with high contents of Ca, Fe, Mg, P, K, Zn and Mn. For example, rice from Dehong Lianghe had the highest contents of K (2564.00 ± 65.09 mg kg−1), Mg (1239.42 ± 20.95 mg kg−1), P (3341.38 ± 67.78 mg kg−1), Zn (17.00 ± 0.38 mg kg−1), Fe (11.49 ± 0.58 mg kg−1), Se (64.00 ± 83.14 μg kg−1) and Cr (293.33 ± 20.82 mg kg−1) (Fig. 2).

Figure 2
figure 2

Element content of rice from different geographical origins.

Pigmented and non-pigmented rice

The results of the analysis showed that the element contents were quite different between pigmented and non-pigmented rice. Compared with pigmented rice, Na, Mn and Cr were higher in non-pigmented rice, the differences were not significant. In addition, with significant differences in P, K, Mg, Ca, Fe (P < 0.01), Zn and As (P < 0.05) (Fig. 3). In general, pigmented rice has a higher content of minerals, but this may cause the content of toxic elements to exceed the limit. In the samples of Red Glutinous Rice 2 and Black Rice, the As content is up to 240 μg kg−1 , which is above the limit of the international standards. The Cd content in Purple Glutinous Rice and Dehong Purple Rice (206.67 μg kg-1 and 250 μg kg-1) exceeds the limit of the Chinese standard (Table 2).

Figure 3
figure 3

Comparison of element content between pigmented and non-pigmented rice. ***Indicates have significant difference at the 0.05 and 0.01 level, respectively; (A), (B) stand for pigmented rice and non-pigmented rice, respectively.

Estimation of dietary intakes of mocro and trace elements from rice

The daily intakes of minerals in rice were estimated and compared with the RNI and UL values of the Chinese reference intakes for adults aged 18–50 years (Table S1). Daily intakes of minerals were calculated as mean body weight multiplied by daily rice consumption (0.346 kg). The Cu intake of sample No. 3 (red glutinous rice1) exceeded the RNI limit. The Mg intake from samples 1, 4, 5, 8 and 10 exceeded the RNI limit. With the exception of samples No. 13 and No. 15, the Mn intake from the other samples exceeded the RNI limit. The P uptake from samples 1, 4, 5, 8 and 10 all exceeded the RNI limit. All rice varieties exceeding the RNI limit were pigmented rice. For the minerals Ca, Cu, Fe, Na and Zn, there is a risk of inadequate intake, especially for non-pigmented rice, where almost all elements were inadequately absorbed. Compared with the RNI and UL (Table S1), the daily intake of Cr for adults exceeds the safe limit according to the Chinese reference intake. It indicates that the long-term large consumption of rice will result in the high exposure of Cr in Yunnan.

Health risk of heavy metals

Health risk indicators were calculated for each rice variety using the equations. Supplementary Table S2 shows the values of the indicators estimated daily intake amount (EDI), hazard Quotient (HQ) and Hazard index (HI) for the population. The EDI of heavy metals was higher in females than in males. The EDI of Cr was the highest, and Menghai fragrant rice had the highest value in males (2.59 × 10−3 mg kg−1 d−1) and females (3.05 × 1−3 mg kg−1 d−1), respectively. Among the rice varieties, the samples of red glutinous rice 2 and Black rice had the highest EDI value of As, while the samples of Yuanyang red rice had the lowest EDI value. For all rice samples, the EDI value for Cd was highest in the samples of Dehong purple rice, while Yuanyang red rice and Kunming white rice had the lowest EDI value. The EDI value of Cd for a single rice ranged from 0 to 12.43 × 10−4 mgkg−1 d−1. Considering rice color, nonpigmented rice was found to have the higher average EDI value of Cr (1.24 × 10−3 mg kg−1 d−1 in males and 1.47 × 10−3 mg kg−1 d−1 in females, respectively), whereas pigmented rice had 1.11 × 10−3 mg kg−1 d−1 in males and 1.31 × 10−3 mg kg−1 d−1 in females. In contrast to pigmented rice with the highest average EDI of As and Cd, the EDI of As was 7.63 × 10 −4 mg kg−1 d−1 in males and 9.01 × 10−4 mg kg−1 d−1 in females, respectively. For non-pigmented rice, the value was 4.06 × 10−4 mg kg−1 d−1 in males and 4.79 × 10−4 mg kg−1 d−1 in females, respectively. As Cd, the EDI of pigmented rice was calculated as 3.28 × 10−4 mg kg−1 d−1 in males and 3.86 × 10−4 mg kg−1 d−1 in females, respectively, and of non-pigmented rice as 1.44 × 10−4 mg kg−1 d−1 in males and 1.70 × 10−4 mg kg−1 d−1 in females, respectively.

The THQ and HI assessed are summarized in Supplementary Table S2. The THQ for Cr ranged from 1.27 × 10−3 in males and 1.50 × 10−3 in females for Yuanyang red rice and Kunming white rice, and 3.88 × 10−3 in males and 4.57 × 10−3 in females for Menghai fragrant rice. Red glutinous rice 2 and black rice were the samples with the highest THQ for As, in contrast to Yuanyang red rice, where the THQ was the lowest, while the highest THQ for Cd ranged from 0 in Yuanyang red rice and Kunming white rice to 7.13 × 10−7 in males and 8.41 × 10−7 in females in the samples from Yunhui 290. The mean total THQ for Cr of in nonpigmented rice (1.87 × 10−3 in males and 2.20 × 10−3 in females) was higher than in pigmented rice (1.67 × 10−3 in males and 1.96 × 10−3 in females, respectively). In contrast to Cr, the mean THQs for As and Cd were higher in pigmented rice than in nonpigmented rice. The THQ of pigmented rice for As (2.29 × 10−7 in meals and 2.70 × 10−7 in femeals, respectively), whereas non-pigmented rice had the lower THQ (1.22 × 10−7 in meals and 1.44 × 10−7 in femeals, respectively). Analysis of the THQ for Cd in pigmented rice products, showed a THQ of 3.28 × 10−7 in meals and 3.86 × 10 -7 in female meals, respectively, while the THQ for non-pigmented rice was 1.44 × 10−7 in male meals and 1.70 × 10−7 in female meals.

The HI value was used to indicate the total exposure due to the intake of the toxic elements in rice. In our result, it was estimated to be 1.27 × 10−3 to 3.88 × 10−3 in mals and from 1.50 × 10−3 to 4.57 × 10−3 in females, respectively, doesn’t pose an increased health risk (the reference value was < 4). However, Menghai fragrant rice has the highest value of exposure to the toxic elements, while Yuanyang red rice and Kunming white rice have the lowest. Analyzed separately, HI proved to be lower in pigmented rice sample (1.67 × 10−3 in males and 1.97 × 10−3 in females, respectively) than non-pigmented rice sample (1.87 × 10−3 in males and 2.20 × 10−3 in females, respectively).

The CR and TCR for toxic elements with carcinogenic risk were estimated in Table 3. Our results show that the cancer risk from consumption of the studied rice products is all greater than 10−4. Overall, CR was was lower in males than in females, with Cr, As and Cd values of 5.9 × 10−4, 8.8 × 10−4, and 14.3 × 10−4 in males and 6.9 × 10−4, 10.3 × 10−4, and 17.0 × 10−4 in females, respectively. TheTCR value was 0.0029 and 0.0034 for males and females, respectively, indicating a high potential cancer risk from research rice. The toxic heavy metal elements CR value were as follows Cd > As > Cr.

Table 3 Carcinogenic risks estimated for studied rice.

Carcinogenic risks between pigmented and non-pigmented rice. According to our results, According to our results, both male and female CR values for Cr were higher in non-pigmented rice than in pigmented rice, but no significant difference was observed (Fig. 4a). In contrast, CR values of As and Cd were higher in males and females in pigmented rice and differed significantly from those in non-pigmented rice (p < 0.01) (Fig. 4b,c). In non-pigmented rice, the CR values for As and Cr were 6.1 × 10−4, 8.8 × 10−4 in males and 7.2 × 10−4, 10.4 × 10−4 in females, respectively. For pigmented rice, the values were 11.5 × 10−4, 20.0 × 10−4 for males and 13.5 × 10−4, 23.6 × 10−4 for females. The TCR value also showed the same trend: pigmented rice was significantly higher than non-pigmented rice (Fig. 4d). In non-pigmented rice, the TCR value was 21.1 × 10−4 and 24.9 × 10−4 for males and females, respectively. In contrast, the values for pigmented rice were 37.0 × 10−4 for males and 43.6 × 10−4 for females.

Figure 4
figure 4

Carcinogenic risk comparison between pigmented and non-pigmented rice. ##Indicates have significant difference at the 0.01 level.