Association Between Natural Lithium Exposure and Suicide Rate: An Ecological and Biomonitoring Study in Portugal

Background/Objectives: Higher lithium (Li) levels in drinking water have been linked to lower suicide rates in the general population in several ecological studies, though this relationship is not always consistent. The main limitation of such studies is the assumption that Li content in drinking water is directly correlated with total Li intake, which may not always be the case for several reasons. Methods: In this context, we conducted a biomonitoring study to compare urinary Li levels-assumed as a reliable indicator of total Li intake-among individuals from three regions with different suicide relative risks (RRs): Porto Metropolitan Area (PMA; low RR), Central region (CT; intermediate RR), and Trás-os-Montes region (TM; high RR). Each participant provided a urine sample (first morning) and two water samples (drinking water and environmental water). Li concentrations were determined using ICP-MS. Results: A total of 311 individuals participated in this study. The median (P25-P75) urinary Li concentration was 21.9 (15.1-46.0) in PMA, 19.0 (12.6-30.4) in CT, and 24.2 (14.6-38.7) µg/L in TM, with no statistically significant differences between regions (Kruskal-Wallis test with Bonferroni correction). Urinary Li was weakly correlated with Li in drinking water (ρ = 0.174; p = 0.002) but not with Li in environmental water (ρ = -0.036; p = 0.694). Conclusions: These findings do not support a protective role of natural Li exposure in suicide risk at the low levels found in drinking (P75 = 3.75 µg/L) and environmental (P75 = 6.87 µg/L) water. More robust and comprehensive biomonitoring studies are needed to clarify the potential impact of natural Li exposure on suicide rates.