Manifestations of Lithium Deficiency in the Human Body: What is Known to Date (A Narrative Review)

Lithium is a pharmacological agent with a well-established role in psychopharmacology. Interest in its "trace" doses as a potential nutrient has increased in the context of geochemical variability of lithium concentrations in drinking water and reports suggesting its possible influence on cognitive status. Within the current regulatory framework, lithium is not recognized as an essential element for humans; no diagnosable "deficiency syndrome" exists, nor are there validated biomarkers of lithium status under background exposure. A key methodological premise of this review is to distinguish between two "dose domains": therapeutic concentrations (mmol/L) and background intake (µg/day, µg/L). This targeted narrative review critically summarizes evidence on human lithium exposure, candidate status markers, and reported deficiency-related hypotheses in the context of the current absence of a clinically defined lithium-deficiency syndrome, validated biomarkers, and binding nutritional reference standards. A targeted narrative search was conducted in PubMed/MEDLINE, Scopus, Web of Science Core Collection, and eLIBRARY.RU. Regulatory and programmatic documents were retrieved from WHO IRIS, EFSA Publications, the U.S. Environmental Protection Agency (EPA; including the Fifth Unregulated Contaminant Monitoring Rule [UCMR-5] and related materials), the U.S. Geological Survey (USGS), and national regulatory agencies. Selected grey literature was used only to contextualize monitoring practices, exposure assessment, and water-treatment technologies. Because the evidence base is heterogeneous, sources were internally stratified into green, yellow, and red evidence zones. This stratification was used as a transparency tool in a narrative review and was not intended to represent a formal systematic-review risk-of-bias assessment or meta-analytic grading scheme. The combination of "green" evidence (lack of recognized essentiality and reference norms, regulatory monitoring without standards, and engineering challenges of lithium removal) and "yellow" signals (ecological and cohort associations, preclinical data) does not form a clinically reproducible syndrome of "lithium deficiency" in humans. Overall, lithium may be regarded as a candidate "beneficial trace element," for which validated biomarkers and prospective studies are required.