title: Evidence in favor of a severely impaired net intestinal calcium absorption in patients with (early-stage) chronic kidney disease.
abstract: BACKGROUND
  Calcium and phosphorus are essential to many vital physiological processes. Little is known about the net and fractional intestinal absorption of calcium and phosphorus in patients with chronic kidney disease (CKD) and their clinical and hormonal determinants.
  
  
  METHODS
  Blood and 24-hour urine samples were collected in 20 healthy volunteers (HV) and 72 stable CKD stage 1-4 patients and analyzed for parameters of mineral metabolism including calcidiol, calcitriol, and parathyroid hormone (PTH). Dietary intake was assessed by dietary history.
  
  
  RESULTS
  The 24-hour urinary calcium excretion, as opposed to the phosphorus excretion, showed a stepwise decrease across CKD stages (median of 219, 84, 40, and 22 mg/day in HV and patients with CKD stages 1-2, 3 and 4, respectively). Younger age, high serum calcitriol, and high estimated GFR were associated with a high 24-hour urinary calcium excretion. High serum calcitriol levels and dietary phosphorus intake were associated with a high 24-hour urinary phosphorus excretion. The fractional intestinal calcium absorption, as estimated by the urinary-to-ingested calcium ratio, decreased across CKD stages.
  
  
  CONCLUSIONS
  The 24-hour urinary excretion of calcium, as opposed to phosphorus, is markedly decreased in CKD, even in early-stage disease. This is partly explained by low calcitriol levels and older age. Assuming a neutral calcium balance at the time of urine collection, we infer that net intestinal calcium absorption may be severely impaired in CKD.
authors.affiliation: Division of Nephrology, Dialysis and Renal Transplantation, Department of Medicine, University Hospital Leuven, Leuven, Belgium.
authors.forename: L
authors.initials: L
authors.lastname: Viaene
authors.forename: B K I
authors.initials: BK
authors.lastname: Meijers
authors.forename: Y
authors.initials: Y
authors.lastname: Vanrenterghem
authors.forename: P
authors.initials: P
authors.lastname: Evenepoel
mesh_terms: D000328:Adult; D000368:Aged; D001530:Belgium; D002112:Calcifediol / Q000097:blood* / Q000652:urine; D002117:Calcitriol / Q000097:blood* / Q000652:urine; D002136:Calcium, Dietary / Q000378:metabolism*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007408:Intestinal Absorption / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood*; D010758:Phosphorus / Q000097:blood* / Q000652:urine; D051436:Renal Insufficiency, Chronic / Q000097:blood* / Q000652:urine
chemical_list: D002136:Calcium, Dietary; D010281:Parathyroid Hormone; D010758:Phosphorus; D002117:Calcitriol; D002112:Calcifediol