Dietary Exposure Assessment and Contaminants Biomonitoring in the Dehcho Region, Northwest Territories: Exploring the Relationship Between Mercury Exposure, Omega-3 Fatty Acid Status, and Fish Consumption
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Background: Fish are often rich in essential micronutrients including omega-3 fatty acids (FA), and are a cultural and dietary staple in traditional food systems of First Nations communities in the Canadian subarctic. Country foods including fish contribute to improved food security, and promote the cultural sovereignty of First Nations communities. However, these foods are often a primary route of exposure to methylmercury, an environmental contaminant that can pose significant adverse health risks. Objectives: The objectives of this study are to: 1) Determine the concentration of total mercury (Hg) and long chain omega-3 polyunsaturated fatty acids (omega-3 PUFAs) in the muscle tissue of various wild-caught freshwater fish species harvested from eight lakes in the Dehcho region, Northwest Territories (NWT); 2) Construct a probabilistic, population-based retrospective dose reconstruction model to assess dietary omega-3 PUFA intake and Hg exposure across several Dehcho First Nations communities; 3) Characterize and quantify sources of mercury and omega-3 PUFA exposure from country food consumption, and identify key contributors from the diet; and 4) Assess the utility and accuracy of the probabilistic exposure model at estimating population-level profiles of risk and cardioprotective benefit using biomarkers for omega-3 PUFAs in blood plasma and Hg in hair. Methods: Samples from eight freshwater fish species [Burbot (Lota lota; also known as Loche or Mariah), Cisco (Coregonus artedi; also known as Herring), Lake Trout (Salvelinus namaycush), Lake Whitefish (Coregonus clupeaformis), Longnose Sucker (Catostomus catostomus), Northern Pike (Esox lucius; known locally as Jackfish), Walleye (Sander vitreus; also known as Pickerel) and White Sucker (Catastomus commersoni)] were harvested from eight lakes of the Dehcho (including Ekali, Trout, Sanguez, Tathlina, McGill, Gargan, Mustard, and Kakisa Lakes) in August of 2013, 2014, and 2015. Omega-3 PUFA levels in fish tissue were determined by a lipid extraction on pulverized, homogenized fish tissue and quantified using a gas chromatograph with a flame ionization detector, and freeze-dried, homogenized fish muscle tissue samples were analyzed and quantified for total Hg using a Direct Mercury Analyzer. Fish mercury and fatty acid profiles were paired with primary species-specific country food consumption data collected during the Contaminants Biomonitoring Study in the Northwest Territories Mackenzie Valley using a food frequency questionnaire (FFQ). A retrospective probabilistic dose reconstruction model was developed using Oracle Crystal Ball™ advanced risk modeling software, to simultaneously characterize intake of methylmercury (MeHg) and n-3 PUFAs through country food consumption, and to estimate both the population proportion at risk of exceeding the tolerable daily intake for MeHg, and those not meeting adequate intakes for PUFAs. A two-dimensional Monte Carlo analysis was conducted and profiles of exposure to MeHg and n-3 PUFAs were generated. Results from the model output were compared to toxicological and nutritional data from the results of the Contaminant Biomonitoring Study in the Northwest Territories Mackenzie Valley. Results: Mean HgT concentrations within piscivorous fishes (e.g. Northern Pike, Walleye, Burbot and Lake Trout) were up to 7.3 times higher than observed in benthivorous and planktivorous fishes (e.g. Cisco, Lake Whitefish, and Sucker). Further, EPA+DHA concentrations in Lake Trout were up to 4.5 times higher than observed in other piscivorous fish species harvested, including Burbot, Northern Pike, and Walleye. Significant differences were noted for mercury and fatty acid profiles in fish between lakes. Negative correlations were observed between mercury and fatty acids for Burbot, Northern Pike and Walleye. Stratifying by species, mean DHA:HgT ratios for Lake Whitefish and Cisco were up to 8.7-fold higher than in piscivorous fish species including Northern Pike, Walleye and Burbot. As an exception, Lake Trout, demonstrating higher omega-3 PUFAs than other species, had accordingly higher fatty acid:mercury ratios. Based on fatty acid and mercury levels in fish species of the Dehcho, and results from the FFQ, estimates for mercury exposure from fish consumption among the Dehcho population indicated that up to 7% of trials exceeded the pTWI of 3.29 μg/kg/week (0.47 μg/kg/day). In contrast, only 0.5% of respondents within participating Dehcho communities exceeded Health Canada’s recommended guidance value of 6 mg/kg. Mean hair mercury was 0.74 mg/kg, with a geometric mean of 0.38 mg/kg. Generally, only a small proportion of trial values exceeded Dietary Reference Intakes for fatty acid subgroups DHA, EPA+DHA, and total omega-3 PUFAs. Similarly, the Omega-3 Index of participants indicated levels of EPA+DHA that fell within the category associated with very low cardioprotective benefits. Sensitivity analyses indicated that input variables corresponding to Lake Whitefish were strong drivers of fatty acid intake across all fatty acid subgroups, while the proportion of the population consuming Northern Pike and Walleye were primary drivers of exposure to methylmercury intake. Conclusion: Probabilistic models provide an important lens for characterizing the risks and benefits from country food consumption in First Nations communities of the Dehcho region. Future studies in probabilistic human health risk assessment will incorporate a component to the model that characterizes risk not only within the general population, but in demographics most vulnerable to the risks associated with mercury exposure, including young children, women of childbearing age, and pregnant women. Any consumption notices and advisories that outline recommendations to modify country food use must consider the multitude of sociocultural, nutritional and spiritual benefits of these foods in subarctic Indigenous populations.
Cite this version of the work
Matthew Laird (2017). Dietary Exposure Assessment and Contaminants Biomonitoring in the Dehcho Region, Northwest Territories: Exploring the Relationship Between Mercury Exposure, Omega-3 Fatty Acid Status, and Fish Consumption. UWSpace. http://hdl.handle.net/10012/12673