Vitamin B6 deficiency anemia resembles iron-deficiency anemia but does not activate intestinal HIF2⍺.

Microcytic hypochromic anemias, including iron-deficiency anemia (IDA) and vitamin B6-deficiency anemia (B6DA), share similar hematologic features but differ fundamentally in etiology and therapeutic responsiveness. IDA results from insufficient dietary iron, activating adaptive intestinal iron absorption via hypoxia-inducible factor 2⍺ (HIF2⍺), whereas B6DA impairs heme biosynthesis without systemic iron depletion, rendering it refractory to iron therapy. Here, we investigated whether intestinal HIF2⍺ activation distinguishes these anemias. Using mouse models, we established that severe dietary iron restriction (<5 ppm for 2 weeks) robustly induced intestinal Hif2⍺ targets Dmt1, Dcytb, Fpn1, and Ncoa4, while suppressing hepatic hepcidin. In contrast, B6-deficient mice developed hypochromic microcytic anemia over 60 days without changes in intestinal Hif2⍺ signaling, duodenal ferritin, or hepcidin. Parenteral B6 supplementation rapidly restored hemoglobin but did not alter intestinal iron gene expression, confirming that low iron, not anemia, drives Hif2⍺-mediated intestinal adaptation. These findings reveal a mechanistic distinction: IDA engages a co-ordinated systemic and intestinal iron response, whereas B6DA produces anemia through heme biosynthesis defects independent of iron sensing. Importantly, assessment of intestinal HIF2⍺ activity or downstream targets provides a functional biomarker to differentiate iron-responsive from non-iron-responsive microcytic anemias. This approach could prevent misdiagnosis and guide more precise therapeutic strategies, highlighting the value of integrating molecular iron sensing into clinical evaluation of nutritional anemias.