Study On The Bioaccessibility And In Vivo And Vitro Bioavailability Of Cadmium In Rice

Backgrounds:Heavy metal pollution in soil is one of the most important environmental safety problems in the world.About 19.4%of cultivated land is polluted by heavy metals in China,of which cadmium is the most polluted.Cadmium can enter the food chain through crop cultivation,which seriously threatens food safety and human health.Cadmium from food sources accounts for 90%of the total exposure of non-smokers in China,especially the average contribution rate of rice reaches 58.6%.Influencing by the"Hunan Cadmium-contaminated Rice Event",consumers have talked about cadmium scarily,and the public has paid unprecedented attention to the health risks of cadmium-contaminated food.At present,the total amount of cadmium in food is usually used as the basis for assessment of cadmium exposure.However,considering the influence of bioaccessibility and bioavailability,the current assessment methods can not truly reflect the health risks of cadmium exposure.Therefore,this subject intends to study the bioaccessibility and in vivo and vitro bioavailability of cadmium in three different levels of cadmium contaminated rice,so as to accurately assess the health risk of cadmium contamination in rice,and provide reference value for revising the existing standards of cadmium limitation in grain,which is conducive to the rational and full utilization of grain resources in China.Method:1.In vitro digestion model-RIVM method was used to study the bioaccessibility of cadmium in low cadmium-contaminated level of rice（Rice-L）,medium cadmium-contaminated level of rice（Rice-M）and high cadmium-contaminated level of rice group（Rice-H）at different digestive parts（mouth,stomach,small intestine）,different digestive conditions（digestive juice pH,digestive fluid volume,digestive time）and different food intake in order to determine the best digestive parameters in vitro.2.A single-layer model of Caco-2 cells was established,and the tightness,polarity and permeability of the cell monolayer was verified by measuring TEER value,AKP enzyme activity ratio at AP side and BL side,and Papp value of sodium fluorescein.Cadmium-contaminated rice at different levels were digested under the optimum conditions of digestion model in vitro.Using Caco-2 cell model to absorb and transport cadmium,in vitro bioavailability of cadmium was determined.3.Thirty-six SPF grade female ICR mice aged 4-5 weeks were fed in metabolic cages.After 7 days of adaptive feeding,they were randomly divided into six groups:blank control group（Control group）,cadmium chloride group（CdCl₂ group）,physical mixture of cadmium chloride and rice（CdCl₂+Rice-N group）and experience group（Rice-L group,Rice-M group,Rice-H group）.The experimental period was 21 days.After feeding,cadmium accumulation in organs,blood and urine of mice was measured to study the absorption and distribution of cadmium in mice and in vivo bioavailability of cadmium.Results:1.Bioaccessibility of cadmium in RiceThe optimum conditions for simulated digestion in vitro of cooking Rice-L（0.111mg/kg）,Rice-M（0.400 mg/kg）and Rice-H（0.655 mg/kg）were as follows:salivary pH were both 6.2,salivary volume were both 8 mL,oral digestion time were both 6 min;gastric juice pH were both 1,gastric juice volume were 11 mL,11 mL and 12 mL respectively,gastric juice digestion time were 1.5 h,2 h and 2 h respectively;intestinal juice pH were both 6.9,and intestinal juice（duodenal juice+bile）volume were 14 mL+7mL,12 mL+6 mL and 12 mL+6 mL respectively,intestinal digestion time were 2 h,2 h and 2.5 h respectively;food intake were both 0.6 g.Rice-L,Rice-M,Rice-H,CdCl₂,CdCl₂+Rice-N（physical mixture of cadmium chloride and rice）were digested in vitro under the optimal conditions（The cadmium concentration and digestion condition of CdCl₂ and CdCl₂+Rice-N is the same as that of Rice-H）.The bioaccessibility of Rice-L,Rice-M and Rice-H were 94.73%,91.11%and90.04%respectively,and there was no significant difference among the three groups（P>0.05）.The bioaccessibility of CdCl₂ and CdCl₂+Rice-N were 99.29%and 92.57%respectively.CdCl₂ was significantly higher than CdCl₂+Rice-N and Rice-H（P<0.05）,but CdCl₂+Rice-N and Rice-H had no significant difference（P>0.05）.The results showed that with the increase of cadmium concentration in rice,the bioaccessibility of cadmium decreased gradually,but there was no significant difference among the three groups;rice matrix had significant effect on the bioaccessibility of cadmium.2.In vitro bioavailability of cadmium in RiceThe Caco-2 cell model established in the experiment accorded with the basic requirements of the transport experiment.In the Caco-2 cell model,the transport and uptake rates of cadmium in Rice-L,Rice-M and Rice-H were 38.49%,29.52%and 24.33%respectively,and there were significant differences among three groups（P<0.05）;the transport and uptake rates of cadmium in CdCl₂ and CdCl₂+Rice-N were 27.70%and22.45%respectively,there were significant differences between the three groups compared with Rice-H（P>0.05）.The results showed that there was a negative correlation between transport and uptake rate of cadmium in rice and cadmium concentration,rice matrix had no significant effect on the transport and uptake rate of cadmium.In vitro bioavailability of cadmium in Rice-L,Rice-M and Rice-H were 36.46%,26.90%and 21.90%respectively,and there were significant differences among the three groups（P<0.05）.In vitro bioavailability of cadmium in CdCl₂ and CdCl₂+Rice-N were27.50%and 20.78%respectively,CdCl₂ was significantly higher than CdCl₂+Rice-N and Rice-H（P<0.05）,but there were no significant differences between CdCl₂+Rice-N and Rice-H（P>0.05）.The results showed that in vitro bioavailability of cadmium in rice was negatively correlated with cadmium concentration,and rice matrix had a significant effect on bioavailability of cadmium in vitro.Meanwhile,the results of health risk assessment of three kinds of rice showed that the THQ（target hazard quotient）value about intake of three kinds of rice without considering in vitro bioavailability was 2.7-4.6 times higher than that with considering in vitro bioavailability.3.In vivo bioavailability of cadmium in RiceAfter 21 days of cadmium exposure,there were no significant differences among body weight of mice in six groups（P>0.05）.In vivo absorption and distribution of cadmium showed that most cadmium accumulated in liver and kidney,but less cadmium accumulated in heart,spleen,lung and blood.Compared with Rice-L group,Rice-M group and Rice-H group,the cadmium content in liver,lung and kidney increased significantly with the increase of cadmium concentration（P<0.01）.Compared with Rice-H group,CdCl₂ group and CdCl₂+Rice-N group,the cadmium content in lung of rice-H group was significantly higher that in CdCl₂ group and CdCl₂+Rice-N group（P<0.05）,while cadmium content in liver of rice-H group was significantly lower than that in CdCl₂ group and CdCl₂+Rice-N group（P<0.05）.The results showed that the distribution of different forms of cadmium in tissues was different.Cadmium accumulation rates in vivo of Rice-L group,Rice-M group and Rice-H group were 0.45%,0.32%and 0.21%respectively,and there were significant differences among the three groups（P<0.05）.Cadmium accumulation rates of CdCl₂ group and CdCl₂+Rice-N group were 0.30%and 0.27%respectively,CdCl₂ group was significantly higher than Rice-H group and CdCl₂+Rice-N group（P<0.05）,but there was no significant difference between Rice-H group and CdCl₂+Rice-N group（P>0.05）.The results showed that there was a negative correlation between cadmium accumulation rate and cadmium concentration,rice matrix had a significant effect on cadmium accumulation rate,a small numble of cadmium with different forms accumulated in vivo after absorption.In vivo bioavailability of Rice-L group,Rice-M group and Rice-H group were 21.36%,16.93%and 11.85%respectively,and there was significant difference between Rice-L group and Rice-H group（P<0.05）.In vivo bioavailability of CdCl₂ group and CdCl₂+Rice-N group were 13.60%and 12.82%respectively,and the Rice-H group was lower than CdCl₂ group and CdCl₂+Rice-N group,but there were no significant differences among the three groups（P>0.05）.The results showed that in vivo bioavailability of cadmium in rice was negatively correlated with cadmium concentration,and rice matrix had no significant effect on bioavailability of cadmium in vivo.At the same time,the results of health risk assessment of three kinds of rice showed that the THQ value about intake of three kinds of rice without considering in vivo bioavailability was 4.6-8.3 times higher than that with considering in vivo bioavailability.Conclusion:There were no significant differences in bioaccessibility of cadmium in low,medium and high contaminated levels of rice under the optimal conditions of digestion model in vitro.Rice matrix had a significant effect on the bioaccessibility of cadmium.The bioavailability of cadmium in rice was negatively correlated with cadmium concentration in vitro and in vivo.Rice matrix had a significant effect on the bioavailability of cadmium in vitro,but it had no significant effect on the bioavailability of cadmium in vivo.The target hazard quotient about intake of three kinds of rice without considering the bioavailability was significantly higher than that with considering bioavailability.