ID: Iron deficiency (determined by serum ferritin levels in most studies); Anemia: Determined by hemoglobin levels in most studies; IDA: Iron deficiency anemia (determined by all or some of these parameters in most studies: Hemoglobin, transferrin saturation, MCV, ferritin);  EIA: Enzyme Immunoassay;  UBT: Urea breath test; RCT: Randomized controlled trial; MCV: Mean corpuscular volume; MCH: Mean corpuscular hemoglobin;  ELISA: Enzyme-linked immunosorbent assay;  IgG: Immunoglobin G;  sTfR: Serum transferrin receptor; H. This evidence concerns the gene TFRC and anemia.