Inclusion Criteria | Exclusion Criteria |
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• Age ≥ 18 years • Non-metastatic CRC patients (UICC stage I-III) • Within 9 months after surgical removal of tumor • At least 3 weeks in-patient rehabilitation in a cooperating clinic is planned • Sufficient knowledge of the German language and mental capabilities to be able to give written informed consent and comply with the study requirements | • No season adapted vitamin D insufficiency or deficiency (see Table 2 for definition) • BMI > 40 kg/m2 • Severe anemia (Hemoglobin < 8.0 g/dl) • Severe renal impairment (eGFR < 30 ml/min/1,73 m2 calculated with the CKD-EPI equation [26]) • Hypercalcemia (Albumin-corrected serum calcium > 2.65 mmol/L) • Hypercalciuria (Random urine calcium ≥0.60 mmol/mmol creatinine) • High-dose vitamin D3 therapy (≥ 2000 IU daily or similar dosage which corresponds to ≥2000 IU daily) • Therapy with vitamin D analogs • Topical therapy with vitamin D3 or vitamin D analogs • Therapy with high-dose calcium supplements (> 1000 mg calcium daily) • Therapy with cardiac glycosides • Hypersensitivity towards ingredients in Dekristol® 20,000/1000 I.E.: peanuts, soy, gelatin, lactose, maize starch or sucrose or ingredients in the placebo capsules (mannitol, silicon dioxide) • Nephrolithiasis with symptoms in the last 12 months • Pseudohyperparathyreodism • Sarcoidosis • Participation in another interventional trial • Pregnancy, planned pregnancy in the next 12 weeks, or lactation • No use of adequate contraceptive measures in women with childbearing potential |