As iron deficiency develops gradually without anemia, symptoms can appear that can include pallor and problems of behavior, cognition, learning and attention span. These can particularly manifest themselves in children. Adults may witness problems related to work performance and productivity.
The effects may be related to impairment of the neurotransmitter gamma-aminobutyric acid (GABA) used to inhibit dopamine production in the brain and/or immune system impairment leading to more susceptibility to infection and inability to keep a normal body temperature.
The first stages of iron deficiency result in diminished liver, spleen and bone marrow iron stores and decreased plasma ferritin. Second stages begin when stores are low and iron transport decreases. Transferrin saturation decreases, which increases total iron-binding capacity, and there is limited iron for function in hemoglobin. This leads to a rise in protoporphyrin, the precursor to heme in hemoglobin. Finally, the anemia occurs.
Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009, pp. 485-87.