In Advanced Renal Failure, Dietary Phosphorus Restriction Reverses Hyperparathyroidism Independent of Changes in the Levels of Calcitriol

Dr. M. Aparicio, Service de Néphrologie, Hôpital Pellegrin, F-33076 Bordeaux (France) Dear Sir, In patients with chronic renal failure (CRF), the restriction of dietary phosphorus results in serum phosphate reduction and improvement of secondary hyperparathyroidism. These data are usually ascribed to a concomitant increase in the production of calcitriol [1, 2]. However, it has been reported in patients with advanced renal failure [3] and in severe experimental renal failure [4, 5] that phosphorus restriction reversed hyperparathyroidism without significant change in plasma calcitriol levels. The aim of the present investigation was to examine the effects of phosphorus restriction on calcium phosphate metabolism and more especially on plasma calcitriol levels. Our study comprised 23 patients with advanced CRF (GFR: 11.63 ± 3.9 ml/min per 1.73 m2) receiving a diet providing daily, per kilogram body weight, 0.4 g protein of vegetable origin, 3-5 mg of inorganic phosphorus and 35 kcal. The diet was supplemented with ketoanalogues (CSW 20/4, Clintec, France) and with calcium carbonate to keep unchanged their habitual calcium intake: 758 ± 124 mg/day at the start of the study. No other phosphate binder was prescribed. The patients additionally received a multivitamin preparation providing 1,000 IU of vitamin D2 per day. Serum calcium, serum phosphorus, serum bicarbonate, PTH activity (intact hormone), calcitriol, alkaline phosphatase and osteocalcin were measured at the start of the study and 3 months later. The evolution of renal function was assessed by 5lCr-EDTA clearances. Results were expressed as mean ± standard deviation (SD). Student’s t test was used for paired data. Multiple regression analysis was applied to analyze the relationship between the concentrations of PTH and calcitriol and the different variables. At the start of the study, PTH concentration was directly correlated with GFR (r = 0.46; p = 0.04), with inorganic phosphorus (r = 0.59; p = 0.01) and with osteocalcin (r = 0.5; p = 0.02). On the other hand, PTH did not correlate with either calcitriol or calcium concentrations, no correlation was found between calcitriol and the other biochemical variables.