Lucas Cassulatti Dos Santos, Fernando Barbosa
Amazonian riverine communities experience chronic mercury (Hg) exposure alongside generally elevated selenium (Se) status. We evaluated seasonal variation in whole-blood Hg and Se and blood Se:Hg molar ratios in 13 Brazilian Amazon riverine communities during the dry (n = 1,084) and rainy (n = 1,118) seasons using a repeated cross-sectional design. Whole-blood Hg and Se were determined by inductively coupled plasma mass spectrometry (ICP-MS). Dietary Hg and Se intakes from fish and Brazil nuts, dietary Se:Hg molar ratios, and concentration-based Selenium Health Benefit Values (HBVSe) from fish-tissue data were estimated, and multivariable models evaluated factors associated with blood Hg and Se. Median blood Hg was approximately 24% higher during the dry than rainy season (21 vs. 17 μg/L), whereas Se was 4% higher (166 vs. 160 μg/L). The median blood Se:Hg molar ratio was higher during the rainy season (26 vs. 21; p < 0.001). Blood Hg was positively associated with Se during both dry (β = 0.111; 95% CI: 0.083-0.139) and rainy seasons (β = 0.257; 95% CI: 0.219-0.296), while the predicted Se:Hg molar ratio declined with increasing Hg. HBVSe ranged from -13.97 to 3.83 μmol/kg, with negative values for tucunaré and pescada. Brazil nut consumption was associated with 11.0% (95% CI: 3.8-18.6%) and 21.8% (95% CI: 10.2-34.7%) higher blood Se during the dry and rainy seasons, respectively. Including Brazil nuts increased estimated dietary Se:Hg molar ratios from 1.5 to 10.3 and from 1.3 to 1.9, respectively. Elevated blood Se therefore does not necessarily correspond to a more favorable Se balance under high Hg exposure. HBVSe and estimated dietary Se:Hg molar ratios should be interpreted as exploratory indicators of dietary balance rather than established clinical thresholds. Integrating Hg and Se biomarkers with fish trophic level, seasonal dietary patterns, and complementary Se sources may improve nutritional risk assessment.