Improving mercury exposure estimates: a meta-analysis of methylmercury-to-total mercury ratios in Canadian traditional foods

BACKGROUND: Methylmercury (MeHg) exposure poses health risks to Indigenous Peoples in Canada, who often rely on fish and wildlife as part of their traditional diet. Risk assessments tend to be overly conservative because most available data report total mercury (tHg) rather than MeHg, leading to uncertainty and potential overestimation of exposure, particularly for foods in which the proportion of tHg present as MeHg is lower or highly variable. OBJECTIVE: To conduct a meta-analysis of MeHg and tHg levels in traditional foods, develop a comprehensive database, and generate empirically derived conversion factors to improve the estimation of dietary MeHg exposure. METHODS: We critically reviewed paired MeHg and tHg data from 31 published studies and two unpublished datasets. Species- and tissue-specific MeHg:tHg ratios were estimated using random-effects meta-analysis, and conservative conversion factors were derived from upper confidence limits with sensitivity analyses. Conversion factors were evaluated using dietary exposure data from the Inuit Health Survey and the First Nations Food, Nutrition, and Environment Study. Variability in ratios was explored using meta-regression and within-species analyses. RESULTS: MeHg:tHg ratios were quantified for 65 species-tissue combinations, revealing strong and systematic tissue- and species-specific patterns. Conversion factors were derived for 38 species-tissue combinations, with most values below 100%, particularly for liver and other internal organs, indicating that assuming tHg equals MeHg frequently overestimates dietary MeHg exposure. Application of these factors substantially reduced overestimation of MeHg intake in most regions, while providing limited benefit in regions dominated by low-concentration foods. Exploratory analyses indicated variability by tissue, species, and location, with limited evidence of additional effects of age or size. SIGNIFICANCE: This study provides a systematic database and validated conversion factors for MeHg in traditional foods, improving dietary risk assessment while underscoring the need for continued species and site-specific monitoring. IMPACT STATEMENT: This study advances mercury exposure science by providing empirically derived conversion factors to estimate methylmercury (MeHg) in traditional foods when only total mercury is measured. By reducing overestimation of MeHg exposure while retaining a precautionary framework, the findings support more accurate dietary risk assessment, improve hazard characterization, and inform culturally relevant food safety guidance for Indigenous Peoples. The results also highlight biological and environmental drivers of variability that can guide future monitoring and exposure assessment efforts.