Inclusion Criteria:  Planned for curative intended elective colon or rectum cancer surgery UICC stage I-III (at diagnosis) Hemoglobin > 7.0 mmol/L Serum ferritin <101 microgram/L or transferrin saturation <21%  Exclusion Criteria:  Chronic kidney failure with need for dialysis Metachronous diagnosed cancer Unable to speak or understand Danish Cognitive impairment e.g. moderate to severe dementia Concurrent severe active bacterial infection Known allergy for Iron(III)isomaltoside Contraindications for intravenous iron (Pofryria, livercirrosis, active hepatitis, transaminases three times the upper limit, hemosiderosis and hemochromatosis) Withdrawal of informed consent Neoadjuvant chemo or radiation therapy Pregnancy