Quantifying Human Capital Loss: Biological Pathways and Occupational Performance Impacts of Micronutrient Deficiencies in Adults

Abstract To synthesize the biological pathways through which iron, vitamin D, and calcium deficiency may influence physical, cognitive, and psychosocial performance in working-age adults, and to identify the strength and limitations of occupationally relevant evidence. This Systematic Literature Review and Conceptual Synthesis follows a documented PRISMA 2020-informed protocol. PubMed, Scopus, and Google Scholar were searched for peer-reviewed human studies, systematic reviews, meta-analyses, and mechanistic reviews addressing biomarkers, deficiency, fatigue, cognition, muscle function, sleep, mood, work capacity, absenteeism, or presenteeism. The synthesis reports source-specific estimates and does not fabricate a primary sample or pooled prevalence. Iron deficiency is linked to reduced oxygen utilization, mitochondrial cytochrome activity, fatigue, and altered dopaminergic signaling; non-anemic iron deficiency is also associated with lower executive-function performance. In 18 trials enrolling 1,170 participants, iron supplementation reduced self-reported fatigue (SMD −0.38; 95% CI −0.52 to −0.23; 4 trials; n=714), but did not improve maximal oxygen consumption (SMD 0.11; 95% CI −0.15 to 0.37; 9 trials; n=235). Vitamin D acts through the VDR axis in skeletal muscle, with deficiency associated with muscle-fiber atrophy and functional weakness. Calcium participates in action-potential-mediated sarcoplasmic-reticulum calcium release and excitation–contraction coupling; however, direct occupational productivity metrics for calcium deficiency remain scarce. The evidence supports a biologically plausible and partly empirically demonstrated pathway from micronutrient deficiency to occupational performance impairment, strongest for iron-related fatigue and cognition. Science-faculty research should prioritize biomarker-confirmed, longitudinal, multi-deficiency studies with objective physiological and work-performance endpoints.