Impact/Purpose:

The goal is to develop an extraction protocol that mimics the human digestive tract and then to use it to assess the bioavailable fraction of arsenic from complex dietary mixtures such as a daily composite -- to move current methods toward a better human physiologically-based exposure estimate method which approximates the "true" bioavailability of arsenic within an environmental or dietary matrix.

Description:

People are exposed to arsenic (As) principally through the water they drink and the food they eat. But the chemical form of As (arsenicals) in food or water strongly influences its toxicity. The toxicity of arsenicals in foods are considerably more difficult to assess analytically (relative to water) because the arsenicals can be chemically bound to proteins, fats and starches. In addition, one must consider whether or not these complex forms of As are bioavailable. The solubilized ("free") anions in drinking water at the point of ingestion are almost 100% bioavailable while the arsenicals associated with foods at the point of ingestion are likely far less bioavailable. Most analytical extractions utilize non-physiologically based extraction techniques to solubilize the arsenicals and do not mimic bioavailability or simulate potential biotransformations of the ingested species. Recent data have also demonstrated that some arsenicals may undergo a biotransformation induced by the stomach's harsh acid conditions. For example, arsenosugars found in seafoods have been shown to partially convert to dimethylarsinic acid (DMA) in the stomach/small intestine; thus, the bioavailable fraction has been converted to a more toxic species. This change in toxicity would not be predicted by conventional extraction procedures.

The amount of As that people are exposed to through their diet is very important, because assumptions of total (aggregate) exposure from food and water (which has both high variability and uncertainty) are used in interpreting and designing epidemiology (EPI) studies, leading to more accurate cancer dose-response estimates and risk characterizations. Aggregate exposure assumptions for the US population are also needed to produce more accurate health risk reduction benefit analyses supporting future rulemakings (6 year review, CCA reanalyses and herbicides re-registrations). Depending upon the assumptions about the amount of water ingested and quantity and type of foods people eat, the estimate for As intake from food can range from a large fraction to several times more than the amount from drinking water. (See NAS reports on As in 1999 and 2001, and the EPA Arsenic Rule.) In setting the 2001 Arsenic Rule, EPA assumed that the average adult in the U.S. got 10 Fg/day of inorganic As from food (Note: this considers only inorganic As, and excludes the various organic-bound forms), compared to an average intake from drinking water (at the Maximum Contaminant Level) of 10 - 12 Fg/day. Since the implementation of the Arsenic Rule is estimated to cost ~ $177 million/yr by 2006 (http://www.epa.gov/safewater/ars/ars_rule_techfactsheet.html ), the choice of MCL has significant ramifications, both in terms of public health and in terms of cost to ensure safe drinking water.

The current state of the science in dietary As speciation is to extract single component foods. This requires considerable time devising extraction procedures capable of extracting arsenicals from each food type (e.g., starches, meat, fruits and vegetables) while providing little guidance on the truly "bioavailable" As within that food type. Therefore, this task will investigate a more physiologically based approach to the extraction of arsenicals from complex dietary samples. The goal is to develop a protocol which mimics the human digestive tract, use it to assess the bioavailable fraction of As from complex dietary mixtures, and study transformations of arsenicals under these extraction conditions. This approach may eliminate the need for "food specific extraction" procedures while providing some insight on what is bioavailable. This protocol would be ideal for assessing As exposures in future EPI and exposure assessment studies in which composite diets are collected. Estimating the dietary exposure to As in future EPI studies has been identified as a critical need by the NRC in order to minimize uncertainties associated with cumulative exposure.

Record Details:

Record Type:PROJECT

Start Date:10/01/2000

Projected Completion Date:09/01/2007

OMB Category:Other

Record ID: 18325

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Project Information:

Progress : Some target dietary samples (Oysters, seaweeds, etc.) were identified for study based on low extraction efficiencies (oysters 53%, seaweeds 25-75%) utilizing accelerated solvent extraction (ASE) and would be logical choices for sample types used in the preliminary evaluation of the gastric juice extraction. The extraction efficiencies for the target foods were increased by 15-20% by using the gastric juice. However, this added extraction efficiency was achieved at the expense of generating arsenosugar degradation products. The major degradation product was arsenosugar (254) which is produced in acidic media (pH~2) from each of the primary sugars. These research findings were published in The Analyst in 2002. One of the major findings was that a significant percent (~30%) of the arsenic extracted was unchromatographable. This reduces the percentage of the arsenic which could be speciated. Size exclusion experiments have indicated that the unchromatographable fraction was arsenosugar (482) bound to a substrate which could be liberated with a mild acid hydrolysis. In FY04, the unchromatographable material was identified as a sulfur analog of the arsenosugar As(482). These results were published in September 2004. These findings are significant given that sulfur bound arsenosugars had previously been unreported and that the sulfur based arsenosugars can represent up to 30% of the total arsenic in shellfish.

In FY02, Gamble et al presented data on the base hydrolysis of arsenosugars at the 2002 Winter Conference. These data indicated that a tetramethylammonium hydroxide (TMAOH) extraction should not produce measurable arsenosugar degradation products. Therefore, the extraction utilizing the TMAOH extraction solvent should liberate the native arsenicals found in the seafood sample without producing degradation products. These research findings were accepted for publication in The Analyst on 9/30/03. These data in combination with the results published in FY01 provide guidance in terms of extraction conditions which will not produce degradation products from the arsenosugars. The shortcoming with some of this research is that the extraction procedure does not mimic the actual human digestive conditions.

In FY04, research collaboration with Dave Thomas (NHEERL) involved evaluating the chemical stability of arsenosugar (392) in the gastric / intestinal fluid of a mouse. The degradation products formed during this study will be compared to a synthetic stomach / intestinal procedure. This synthetic procedure has been used by the USDA to assess the bioavailability of iron from foods. This research is of growing interest because the arsenosugars produce DMA in the body and DMA has been cited as a potential carcinogen. The results from treating arsenosugar (392) with the gastric / intestinal fluid of a mouse indicate arsenosugar (392) is converted to the sulfur analog and does not undergo any additional degradation after a 48hr incubation at 37 degrees C. These results indicate that the arsenosugars do not degrade into DMA in the small intestine but rather the sugar breaks down (conversion to DMA) after digestion.

Relevance :Relevance: Currently, drinking water regulations assume arsenic from the diet is 100% bioavailable which may overestimate the risk. Estimating the true bioavailable fraction from an exposure is the goal or ideal situation in an exposure assessment. Specifically, the need to estimate the bioavailability of arsenic is called out in Exposure Issue 8 in the Office of Research and Development's Arsenic Research Plan (ARP). Since 1997, NERL/MCEARD has produced the analytical capability to speciate arsenicals in dietary and biological matrices. These advancements will allow for a more physiological based estimate of the dietary exposure to arsenic. Therefore, this task is relevant to the Agency's mission in that it addresses scientific shortcomings identified in the ARP needed to improve exposure source estimates which ultimately will improve drinking water regulations. Specifically, this research will lead to improved design and interpretation of EPI studies and US population dietary exposure estimates which, in turn, will lead to a more accurate cancer dose-response estimates and more accurate risk characterizations, as well as more accurate health risk reduction benefit analyses supporting future rulemakings (i.e., Arsenic Rule 6 year review, CCA reanalyses and herbicides re-registrations). Significance: While arsenic in drinking water is assumed to be 100% bioavailable (i.e., it is a free ion in solution), the type of arsenic (i.e., organic vs. inorganic) and the availability of arsenicals within dietary samples (i.e., bound to proteins, fats and starches) is a relative unknown. As a result, all toxic arsenicals present in a dietary sample are assumed to be 100% bioavailable when calculating the exposure source estimate (water vs. diet) for arsenic within drinking water regulations. Therefore, improved data are necessary to minimize the error associated with using this default assumption. This will aid in assessing the true exposure source estimate (based on bioavailability). Secondly, the dietary component of epidemiology (EPI) studies have been largely overlooked because of a lack of analytical capability to estimate the exposure to arsenic from dietary ingestion. Estimating the dietary exposure in future EPI studies will provide estimates for dietary components where previously there had been default assumptions. Impact: The National Research Council report identifies dietary exposure estimates in EPI studies as a source of uncertainty in estimating cancer risks from EPI studies. Therefore, research aimed at improving dietary estimates of exposure are required. The research in this task will aid in estimating the dietary exposure to arsenic and, in turn, improve future arsenic EPI studies and rulemaking. Collaboration and Customer Interaction: Communication is maintained with the Office of Ground Water and Drinking Water (OGWDW) in order to determine research needs and provide information which could impact future arsenic drinking water regulations. This exchange is accomplished through teleconferences, through communication with NERL's Assistant Laboratory Director for Drinking Water or through exchange of research results. Through recent communications, OGWDW reconfirmed the importance of research to improve the estimate for bioavailability of arsenicals in dietary and other non-aqueous environmental exposure sources.

Clients :Office of Ground Water and Drinking Water (Irene Dooley, Hiba Shukairy, Valerie Blank, Kesha Forrest)

Research Component :ARSENIC

Risk Paradigm :EXPOSURE

Project IDs:

ID Code :none

Project type :ORD-DW Plan

ID Code :5880

Project type :OMIS