Allan Rofe

In Vivo Assessment of Arsenic Bioavailability in Rice and Its Significance for Human Health Risk Assessment

Background Millions of people worldwide consume arsenic-contaminated rice; however, little is known about the uptake and bioavailability of arsenic species after arsenic-contaminated rice ingestion. Objectives In this study, we assessed arsenic speciation in greenhouse-grown and supermarket-bought rice, and determined arsenic bioavailability in cooked rice using an in vivo swine model. Results In supermarket-bought rice, arsenic was present entirely in the inorganic form compared to greenhouse-grown rice (using irrigation water contaminated with sodium arsenate), where most (~ 86%) arsenic was present as dimethylarsinic acid (organic arsenic). Because of the low absolute bioavailability of dimethylarsinic acid and the high proportion of dimethylarsinic acid in greenhouse-grown rice, only 33 ± 3% (mean ± SD) of the total rice-bound arsenic was bioavailable. Conversely, in supermarket-bought rice cooked in water contaminated with sodium arsenate, arsenic was present entirely in the inorganic form, and bioavailability was high (89 ± 9%). Conclusions These results indicate that arsenic bioavailability in rice is highly dependent on arsenic speciation, which in turn can vary depending on rice cultivar, arsenic in irrigation water, and the presence and nature of arsenic speciation in cooking water. Arsenic speciation and bioavailability are therefore critical parameters for reducing uncertainties when estimating exposure from the consumption of rice grown and cooked using arsenic-contaminated water.

Assessment of Four Commonly Employed in Vitro Arsenic Bioaccessibility Assays for Predicting in Vivo Relative Arsenic Bioavailability in Contaminated Soils

Currently, a number of in vitro methods are in use worldwide to assess arsenic (As) bioaccessibility in soils. However, a dearth of research has been undertaken to compare the efficacy of the in vitro methods for estimating in vivo relative As bioavailability. In this study, As bioaccessibility in contaminated soils (n = 12) was assessed using four in vitro assays (SBRC, IVG, PBET, DIN). In vitro results were compared to in vivo relative As bioavailability data (swine assay) to ascertain which methodologies best correlate with in vivo data. Arsenic bioaccessibility in contaminated soils varied depending on the in vitro method employed. For the SBRC and IVG methods, As bioaccessibility generally decreased when gastric-phase values were compared to the intestinal phase. In contrast, extending the PBET and DIN assays from the gastric to the intestinal phase resulted in an increase in As bioaccessibility for some soils tested. Comparison of in vitro and in vivo results demonstrated that the in vitro assay encompassing the SBRC gastric phase provided the best prediction of in vivo relative As bioavailability (R(2) = 0.75, Pearson correlation = 0.87). However, relative As bioavailability could also be predicted using gastric or intestinal phases of IVG, PBET, and DIN assays but with varying degrees of confidence (R(2) = 0.53-0.67, Pearson correlation = 0.73-0.82).

In vivo-in vitro and XANES spectroscopy assessments of lead bioavailability in contaminated periurban soils.

Lead (Pb) bioaccessibility was assessed using 2 in vitro methods in 12 Pb-contaminated soils and compared to relative Pb bioavailability using an in vivo mouse model. In vitro Pb bioaccessibility, determined using the intestinal phase of the Solubility Bioaccessibility Research Consortium (SBRC) assay, strongly correlated with in vivo relative Pb bioavailability (R(2) = 0.88) following adjustment of Pb dissolution in the intestinal phase with the solubility of Pb acetate at pH 6.5 (i.e., relative Pb bioaccessibility). A strong correlation (R(2) = 0.78) was also observed for the relative bioaccessibility leaching procedure (RBALP), although the method overpredicted in vivo relative Pb bioavailability for soils where values were <40%. Statistical analysis of fit results from X-ray absorption near-edge structure (XANES) data for selected soils (n = 3) showed that Pb was strongly associated with Fe oxyhydroxide minerals or the soil organic fraction prior to in vitro analysis. XANES analysis of Pb speciation during the in vitro procedure demonstrated that Pb associated with Fe minerals and the organic fraction was predominantly solubilized in the gastric phase. However, during the intestinal phase of the in vitro procedure, Pb was strongly associated with formation of ferrihydrite which precipitated due to the pH (6.5) of the SBRC intestinal phase. Soils where Fe dissolution was limited had markedly higher concentrations of Pb in solution and hence exhibited greater relative bioavailability in the mouse model. This data suggests that coexistence of Fe in the intestinal phase plays an important role in reducing Pb bioaccessibility and relative bioavailability.

Determination of cadmium relative bioavailability in contaminated soils and its prediction using in vitro methodologies

In this study, cadmium (Cd) relative bioavailability in contaminated (n = 5) and spiked (n = 2) soils was assessed using an in vivo mouse model following administration of feed containing soil or Cd acetate (reference material) over a 15 day exposure period. Cadmium relative bioavailability varied depending on whether the accumulation of Cd in the kidneys, liver, or kidney plus liver was used for relative bioavailability calculations. When kidney plus liver Cd concentrations were used, Cd relative bioavailability ranged from 10.1 to 92.1%. Cadmium relative bioavailability was higher (14.4-115.2%) when kidney Cd concentrations were used, whereas lower values (7.2-76.5%) were derived when liver Cd concentrations were employed in calculations. Following in vivo studies, four in vitro methodologies (SBRC, IVG, PBET, and DIN), encompassing both gastric and intestinal phases, were assessed for their ability to predict Cd relative bioavailability. Pearson correlations demonstrated a strong linear relationship between Cd relative bioavailability and Cd bioaccessibility (0.62-0.91), however, stronger in vivo-in vitro relationships were observed when Cd relative bioavailability was calculated using kidney plus liver Cd concentrations. Whereas all in vitro assays could predict Cd relative bioavailability with varying degrees of confidence (r(2) = 0.348-0.835), large y intercepts were calculated for a number of in vitro assays which is undesirable for in vivo-in vitro predictive models. However, determination of Cd bioaccessibility using the intestinal phase of the PBET assay resulted in a small y intercept (5.14; slope =1.091) and the best estimate of in vivo Cd relative bioavailability (r(2) = 0.835).

Effect of soil ageing on in vivo arsenic bioavailability in two dissimilar soils

Arsenic (As) bioavailability in spiked soils aged for up to 12 months was assessed using in vitro and in vivo methodologies. Ageing (natural attenuation) of spiked soils resulted in a decline in in vivo As bioavailability (swine assay) of over 75% in soil A (Red Ferrosol) but had no significant effect on in vivo As bioavailability even after 12 months of ageing in soil B (Brown Chromosol). Sequential fractionation, however, indicated that there was repartitioning of As within the soil fractions extracted during the time course investigated. In soil A, the As fraction associated with the more weakly bound soil fractions decreased while the residual fraction increased from 12% to 35%. In contrast, little repartitioning of As was observed in soil B indicating that natural attenuation may be only applicable for As in soils containing specific mineralogical properties.

Principles and application of an in vivo swine assay for the determination of arsenic bioavailability in contaminated matrices

The assessment of arsenic (As) bioavailability from contaminated matrices is a crucial parameter for reducing the uncertainty when estimating exposure for human health risk assessment. In vivo assessment of As utilising swine is considered an appropriate model for human health risk assessment applications as swine are remarkably similar to humans in terms of physiology and As metabolism. While limited in vivo As bioavailability data is available in the literature, few details have been provided regarding technical considerations for performing in vivo assays. This paper describes, with examples, surgical, experimental design and analytical issues associated with performing chronic and acute in vivo swine assays to determine As bioavailability in contaminated soil and food.

In vivo measurement, in vitro estimation and fugacity prediction of PAH bioavailability in post-remediated creosote-contaminated soil

In this study, PAH bioavailability was assessed in creosote-contaminated soil following bioremediation in order to determine potential human health exposure to residual PAHs from incidental soil ingestion. Following 1,000 days of enhanced natural attenuation (ENA), a residual PAH concentration of 871 ± 8 mg kg(-1) (∑16 USEPA priority PAHs in the <250 μm soil particle size fraction) was present in the soil. However, when bioavailability was assessed to elucidate potential human exposure using an in vivo mouse model, the upper-bound estimates of PAH absolute bioavailability were in excess of 65% irrespective of the molecular weight of the PAH. These results indicate that a significant proportion of the residual PAH fraction following ENA may be available for absorption following soil ingestion. In contrast, when PAH bioavailability was estimated/predicted using an in vitro surrogate assay (FOREhST assay) and fugacity modelling, PAH bioavailability was up to 2000 times lower compared to measured in vivo values depending on the methodology used.

Application of an in vivo swine model for the determination of arsenic bioavailability in contaminated vegetables

Considerable information is available in the literature regarding the uptake of arsenic (As) from contaminated soil and irrigation water by vegetables. However, few studies have investigated As speciation in these crops while a dearth of information is available on As bioavailability following their consumption. In this study, the concentration and speciation of As in chard, radish, lettuce and mung beans was determined following hydroponic growth of the vegetables using As-contaminated water. In addition, As bioavailability was assessed using an in vivo swine feeding assay. While As concentrations ranged from 3.0 to 84.2mg As kg(-1) (dry weight), only inorganic As (arsenite and arsenate) was detected in the edible portions of the vegetables. When As bioavailability was assessed through monitoring blood plasma As concentrations following swine consumption of As-contaminated vegetables, between 50% and 100% of the administered As dose was absorbed and entered systemic circulation. Arsenic bioavailability decreased in the order mung beans>radish>lettuce=chard.

Predicting lead relative bioavailability in peri-urban contaminated soils using in vitro bioaccessibility assays

In this study, lead (Pb) bioaccessibility was assessed in peri-urban contaminated soils using a variety of established in vitro assays. Bioaccessibility data was then used to predict Pb relative bioavailability (RBA) using published in vivo-in vitro regression models in order to compare calculated estimates and measured values. Lead bioaccessibility varied depending on the in vitro methodology employed with the relative bioavailability leaching procedure (RBALP) and in vitro gastrointestinal (IVG) assays providing more conservative Pb bioaccessibility values compared to those determined using PBET, UBM and Rel-SBRC-I assays. When Pb RBA was calculated, predicted values using PBET-G and UBM-G data were similar to measured Pb RBA values. However, Pb RBA was over-estimated by 1.6–5.5- and 2.6–6.6-fold when data and regression models from RBALP and IVG-G assays were employed.

Assessment of DDT Relative Bioavailability and Bioaccessibility in Historically Contaminated Soils Using an in Vivo Mouse Model and Fed and Unfed Batch in Vitro Assays

In this study, DDTr (DDTr = DDT + DDD + DDE) relative bioavailability in historically contaminated soils (n = 7) was assessed using an in vivo mouse model. Soils or reference materials were administered to mice daily over a 7 day exposure period with bioavailability determined using DDTr accumulation in adipose, kidney, or liver tissues. Depending on the target tissue used for its calculation, some variability in DDTr relative bioavailability was observed; however, it did not exceed 25% (range 2-25%). When DDTr bioaccessibility was determined using organic physiologically based extraction test (Org-PBET), unified BARGE method (UBM), and fed organic estimation human simulation test (FOREhST) in vitro assays, bioaccessibility was less than 4% irrespective of the assay utilized and the concentration of DDTr in the contaminated soil. Pearson correlations demonstrate a poor relationship between DDTr relative bioavailability and DDTr bioaccessibility (0.47, 0.38, and 0.28, respectively), illustrating the limitations of the static in vitro methods for predicting the dynamic processes of the mammalian digestive system for this hydrophobic organic contaminant.