Effect of Oral Magnesium on Vascular Calcification in Chronic Kidney Disease

Inclusion Criteria: * Estimated glomerular filtration rate between 45 and 15 mL/min for \> 3 months (i.e. CKD stage 3b-4). * Serum total magnesium \< 0,82 mmol/L and serum phosphate \> 1,15 mmol/L on average of previous measurements. or Serum total magnesium \< 0,92 mmol/L and serum phosphate \> 1,30 mmol/L on average of previous measurements. * Life expectancy \> 1 year. * Expected time until initiation of dialysis or transplantation \> 1 year. * Women of childbearing age must be actively using contraceptive therapy (p-pills, estrogen depots or intrauterine device) as well as have a negative pregnancy test. * Written informed consent. Exclusion Criteria: * Current hemodialysis or peritoneal dialysis treatment. * Kidney donor recipient. * Previous coronary artery bypass graft (CABG). * Parathyroid hormone \> 600 ρmol/L. * Previous parathyroidectomy. * Current treatment with magnesium containing medication or supplements. * Any condition impairing magnesium absorption from the gastrointestinal tract (e.g. short bowel syndrome, chronic pancreatitis). * Active malignancy (basal or squamous cell skin carcinoma, localized prostate cancer and cancer with no signs of reoccurrence after 5 years are exempt from this). * Other diseases or conditions, which, in the opinion of the site investigator, would prevent participation in or completion of trial. * Pregnancy or breastfeeding. * Allergy towards contents of interventional medication. * Participation in other interventional trials.