Abbreviations: ID = iron deficiency; IDA = iron-deficiency anemia; TSAT = transferrin saturation; Hb = hemoglobin; Ret-He/CHr = reticulocyte hemoglobin content; sTfR = soluble transferrin receptor; FBC-A = co-processed ferrous bisglycinate with alginate; GI = gastrointestinal; IBD = inflammatory bowel disease; IV = intravenous; EGD-/RC-scopy = esophagogastroduodeno-/rectocolon-scopy; CE = capsule endoscopy. Here, GSTM1 is linked to nutritional disorder.