Solid-liquid separation method governs the in vitro bioaccessibility of metals in contaminated soil-like test materials

Changes in oral bioaccessibility of heavy metals in non-digestive sucking habits due to the formation of complexes between digestive fluid components and metals/metalloids

Humans, especially infants, are exposed to harmful substances through various means, including non-nutritive sucking behaviors. Here, we compared the "one-compartment model" and the "three-compartment model" within the "suck model" to assess the oral bioaccessibility of heavy metals in various products and evaluated whether these models can be employed to assess 12 heavy metals present in consumer products. Several certified reference materials, including plastic, paint, glass, and metals, were employed to ensure sample homogeneity. By comparing the two models, we validated that a considerable amount of complexes were formed between saliva components and the extracted heavy metals and that some of these complexes dissociated during reactions with the gastric/intestinal fluids. Furthermore, we observed that in the cases of Cu and Pb, additional complexes were formed as a result of reactions with gastric/intestinal fluids. We measured the total concentrations of the extracted heavy metals using artificial saliva through acid digestion and found that up to 99.7% of the heavy metals participated in the formation of complexes, depending on the characteristics of the sample (e.g., composition) and the target element. This result indicates that the current suck model may notably underestimate the oral bioaccessibility of heavy metals in products associated with sucking behaviors. Therefore, we propose a more conservative and simpler test method for assessing oral bioaccessibility of heavy metals that involves measuring the total concentrations of heavy metals extracted from consumer products using artificial saliva. By doing so, we can account for potential variations in the digestive milieu (e.g., due to ingested food) and the inconsistency in complex formation-dissociation characteristics.

Understanding how methodological aspects affect the release of trace metal(loid)s from urban dust in inhalation bioaccessibility tests

The bioaccessibility of metal(loid)s in ambient particulate matter (PM) has been recently used to represent the risk of inhalation exposure. Nevertheless, different methodological factors affect the bioaccessibility values; among these, the type and composition of surrogate biological fluids and the liquid to solid ratio have been revealed to be the most important. To better understand how these methodological aspects affect the bioaccessibility, a reference material corresponding to urban dust (SRM1648a) was contacted with synthetic biological fluids commonly used in the literature representing surrogate fluids that may interact with fine (Gamble's solutions, artificial lysosomal fluid (ALF)) and coarse particles (gastric fluid), for liquid to solid (L/S) ratios ranging from 500 to 20,000. Visual MINTEQ 3.1. was used to enhance the discussion on how the solubility of metals in the leaching solution depends on the composition of the simulated fluids and the speciation of metals. The results obtained indicate that a small change in the composition of Gamble's solution (the presence of glycine) may increase significantly the bioaccessibility at a L/S ratio of 5,000. The highest bioaccessibility of most of the studied metal(loid)s at a L/S ratio of 5,000 was found for ALF fluid. The study of the effect of the L/S ratio showed that metal(loid)s bioaccessibility in Gamble's fluid increased logarithmically with increasing L/S ratio, while it remained practically constant in ALF and gastric fluid. This different behavior is explained assuming that the leaching of metal(loid)s in Gamble's solution is solubility-controlled, while in ALF and gastric fluid is availability-controlled.

Analysis of the pesticide behavior in Chaenomelis speciosa and the role of digestive enzyme in vitro oral bioaccessibility

Problems with pesticide residues in medicinal and edible plant have received great attention. The dietary exposure risk induced by presence of pesticide residues depends on its release from the food matrix, i.e., its bioaccessibility. The bioaccessibility of pesticide residues in human food is poorly understood and thus, we used in vitro digestive method to measure the bioaccessibility of six pesticides in Chaenomelis speciosa. Results showed that the lower and upper boundary bioaccessibility values of the six pesticides in C. speciosa was 4.26 and 86.52%, and the bioaccessibility varied for the pesticide types and digestion phase. The α-amylase and pancreatin play an important role in vitro bioaccessibility. Our findings suggest that risk assessment studies should be taken into account the pesticide metabolism, and that previous studies may have underestimated pesticide bioaccessibility.

Investigating Lead Species and Bioavailability in Contaminated Soils: Coupling DGT Technique with Artificial Gastrointestinal Extraction and in Vivo Bioassay

Although strong in vivo-in vitro correlations (IVIVCs) between relative bioavailability (RBA) and bioaccessibility of soil Pb were well reported, knowledge on the fractions of bioaccessible Pb in simulated gastrointestinal (GI) fluids that are available for absorption into the systemic circulation is limited. Here, Pb-RBA in 14 Pb-contaminated soils were assessed using an in vivo mouse bioassay and compared to Pb bioaccessibility by the gastrointestinal phase of the UBM (Unified Bioaccessibility research group of Europe (BARGE) Method) in vitro assay with and without 0.45 μm filtration of GI fluid. Results showed good IVIVC between Pb-RBA and Pb bioaccessibility without filtration ( r ² = 0.62), while Pb bioaccessibility with filtration provided a poor correlation with Pb-RBA ( r ² = 0.16). This suggested that besides dissolved Pb ions, Pb-complexes formed in the UBM gastrointestinal fluid might also contribute to bioavailable Pb. To ascertain this, DGT (diffusive gradients in thin-films) devices which can measure both Pb²⁺ ions and labile inorganic and organic Pb-complexes were introduced to the UBM fluids to measure Pb DGT-bioaccessibility, which showed strong correlation to Pb-RBA ( r ² = 0.71). With increasing diffusive gel thickness which could enhance release of Pb ions from Pb-complexes, Pb DGT-bioaccessibility increased by 3.4-5.7 times, while inclusion of dialysis membrane within DGT devices significantly decreased Pb DGT-bioaccessibility by inhibiting diffusion of Pb complexes to binding gel. These results confirmed the contribution of Pb-complexes to Pb bioavailability, providing new insights to Pb bioavailability.

Effects of chemical speciation on the bioaccessibility of zinc in spiked and smelter-affected soils

Previous studies have suggested that understanding soil metal speciation, rather than relying solely on total metal content, can improve the accuracy and utility of contaminated site risk assessments. Because soil properties and reaction time can alter metal speciation, speciation should influence metal bioaccessibility. For example, under gastrointestinal conditions, it is expected that metal species will differ in bioaccessibility depending on their stability in acidic pH environments. We studied the links between metal speciation and bioaccessibility. A combination of synchrotron-based X-ray diffraction and X-ray absorption near edge structure (XANES) was used to identify the zinc (Zn) speciation in spiked and smelter-affected soils. After conducting in vitro digestion tests on the soil samples, XANES and linear combination fitting were carried out on the residual pellets to identify the species of Zn that remained after digesting the soils in the simulated gastric and duodenal fluids. The metal species that were not present in the residual pellets were inferred to have been dissolved and, thus, more bioaccessible. Sphalerite (ZnS), ZnO, and outer-sphere Zn contributed more to Zn bioaccessibility than franklinite (ZnFe₂ O₄ ) and Zn incorporated into a hydroxy interlayer mineral (Zn-HIM). The bioaccessibility of Zn-aluminum layered double hydroxides (Zn-Al-LDH) was found to be inversely proportional to its residence time in soil. It was also observed that the relatively high pH of the duodenum favors metal reprecipitation and readsorption, leading to a reduction in bioaccessible metal concentration. These results imply that metal speciation mainly controls metal bioaccessibility. Environ Toxicol Chem 2019;38:448-459. © 2018 SETAC.

In vivo and in vitro methods for evaluating soil arsenic bioavailability: relevant to human health risk assessment

Arsenic (As) is the most frequently occurring contaminant on the priority list of hazardous substances, which lists substances of greatest public health concern to people living at or near U.S. National Priorities List site. Accurate assessment of human health risks from exposure to As-contaminated soils depends on estimating its bioavailability, defined as the fraction of ingested As absorbed across the gastrointestinal barrier and available for systemic distribution and metabolism. Arsenic bioavailability varies among soils and is influenced by site-specific soil physical and chemical characteristics and internal biological factors. This review describes the state-of-the science that supports our understanding of oral bioavailability of soil As, the methods that are currently being explored for estimating soil As relative bioavailability (RBA), and future research areas that could improve our prediction of the oral RBA of soil As in humans. The following topics are addressed: (1) As soil geochemistry; (2) As toxicology; (3) in vivo models for estimating As RBA; (4) in vitro bioaccessibility methods; and (5) conclusions and research needs.

Factors Affecting the Bioaccessibility and Intestinal Transport of Difenoconazole, Hexaconazole, and Spirodiclofen in Human Caco-2 Cells Following in Vitro Digestion

This study examined how gastrointestinal conditions affect pesticide bioaccessibility and intestinal transepithelial transport of pesticides (difenoconazole, hexaconazole, and spirodiclofen) in humans. We used an in vitro model combining human gastric and intestinal digestion, followed with Caco-2 cell model for human intestinal absorption. Bioaccessibility of three tested pesticides ranged from 25.2 to 76.3% and 10.6 to 79.63% in the gastric and intestinal phases, respectively. A marked trend similar to the normal distribution was observed between bioaccessibility and pH, with highest values observed at pH 2.12 in gastric juice. No significant differences were observed with increasing digestion time; however, a significant negative correlation was observed with the solid-liquid (S/L) ratio, following a logarithmic equation. R² ranged from 0.9198 to 0.9848 and 0.9526 to 0.9951 in the simulated gastric and intestinal juices, respectively, suggesting that the S/L ratio is also a major factor affecting bioaccessibility. Moreover, significant dose- and time-response effects were subsequently observed for intestinal membrane permeability of difenoconazole, but not for hexaconazole or spirodiclofen. This is the first study to demonstrate the uptake of pesticides by human intestinal cells, aiding quantification of the likely effects on human health and highlighting the importance of considering bioaccessibility in studies of dietary exposure to pesticide residues.

A Review of Mercury Bioavailability in Humans and Fish

To estimate human exposure to methylmercury (MeHg), risk assessors often assume 95%–100% bioavailability in their models. However, recent research suggests that assuming all, or most, of the ingested mercury (Hg) is absorbed into systemic circulation may be erroneous. The objective of this paper is to review and discuss the available state of knowledge concerning the assimilation or bioavailability of Hg in fish and humans. In fish, this meant reviewing studies on assimilation efficiency, that is the difference between ingested and excreted Hg over a given period of time. In humans, this meant reviewing studies that mostly investigated bioaccessibility (digestive processes) rather than bioavailability (cumulative digestive + absorptive processes), although studies incorporating absorption for a fuller picture of bioavailability were also included where possible. The outcome of this review shows that in a variety of organisms and experimental models that Hg bioavailability and assimilation is less than 100%. Specifically, 25 studies on fish were reviewed, and assimilation efficiencies ranged from 10% to 100% for MeHg and from 2% to 51% for Hg(II). For humans, 20 studies were reviewed with bioaccessibility estimates ranging from 2% to 100% for MeHg and 0.2% to 94% for Hg(II). The overall absorption estimates ranged from 12% to 79% for MeHg and 49% to 69% for Hg(II), and were consistently less than 100%. For both fish and humans, a number of cases are discussed in which factors (e.g., Hg source, cooking methods, nutrients) are shown to affect Hg bioavailability. The summaries presented here challenge a widely-held assumption in the Hg risk assessment field, and the paper discusses possible ways forward for the field.