Evidence in Favor of a Severely Impaired Net Intestinal Calcium Absorption in Patients with (Early-Stage) Chronic Kidney Disease

Abstract

Introduction: 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.

Stone Composition as a Function of Age and Sex

“…CKD is often associated with type 4 renal tubular acidosis, which in turn, is characterized by decreased renal ammoniagenesis and more acidic urine (21) that favors UA supersaturation (22). Furthermore, urinary calcium excretion is reduced among those with declining GFR, perhaps making calcium stones less likely (23). Obesity, insulin resistance, and diabetes, which are also more common with aging, are associated with lower urinary pH and UA stones (24,25).…”

Large database study of urinary stone composition in South Korea: Korean Society of Endourology and Robotics (KSER) research series

“…In contrast, the percentage of patients with UA stones increased with age. The UA increase could be associated with decreasing urine pH and increasing UA excretion [ 24 ] related to obesity, insulin resistance, and diabetes that are common with aging [ 10 24 25 ].…”

Calcium Balance in Chronic Kidney Disease

“…It has been previously suggested that calcium absorption may be severely impaired in early CKD based on very low observed urinary calcium excretion in these patients [21•]. However, evidence from calcium balance and kinetic studies illustrate that, despite very low urine calcium levels, patients with moderate (stage 3/4) CKD have fractional calcium absorption rates similar to healthy adults when dietary calcium is adequate [7•].…”