Stoichiometric and probabilistic characterization of Se:Hg interactions in yellowfin tuna for Iberian consumers

Mercury in tuna is routinely assessed against regulatory thresholds in isolation, an approach that disregards the biological antagonism between methylmercury and selenium central to Hg toxicity. We characterized Se and Hg concentrations in tuna (Thunnus albacares) integrating stoichiometric and probabilistic analyses to reframe Hg risk. [Se:Hg] and HBVSe values were uniformly protective across all samples, and Se and Hg accumulation were shown to be independent. A 2×2 classification matrix, combining the EU regulatory Hg threshold (1.0 μg/g ww) with the [Se:Hg] stoichiometric threshold of 1, was applied to simultaneously assess regulatory compliance and selenoprotective status. All samples were in the protective quadrant, with Hg below the regulatory ceiling and Se in stoichiometric excess in every case. Monte Carlo simulations (n = 100,000) further demonstrated that tuna consumption contributes meaningfully to dietary Se adequacy for both Portuguese and Spanish adults. After correction for Hg-mediated Se sequestration (reducing theoretically bioavailable Se by a mean of 18.6%) median theoretically bioavailable Se intake remained at 25.0% and 14.4% of the Dietary Reference Value for Portugal and Spain, respectively. At typical Iberian consumption levels, tuna contributes meaningfully to dietary Se adequacy, with Se present in stoichiometric excess of Hg across all samples.