During this period, iron metabolism research framed a unifying view that digestive milieu, dietary iron bioavailability, and trace mineral interactions shape iron status. Studies showed that gastric and duodenal acidity modulates iron solubility and absorption, while iron bioavailability varies across dietary sources for hemoglobin formation. Copper was identified as a central regulator, required as a cofactor for soluble iron in erythropoiesis, linking copper nutrition to iron metabolism and metal interactions.