Protein Catabolic Rate May Underestimate Dietary Protein Intake in Hemodialysis Patients

Protein catabolic rate (PCR) or its normalized value, nPCR, is widely used to define nutritional status in dialysis patients [1]. This term PCR obviously is a misnomer and refers to the net PCR since the total PCR is as high as 300 g/day and does not necessarily bear a relationship with protein intake [2]. In the steady state, the net PCR has to be equal to the dietary protein intake as long as the method to calculate PCR includes all the body nitrogen output. The usual method to calculate PCR is from nitrogen appearance rate [3] based on a formula developed from a detailed study of 5 dialysis patients with very low residual renal function who were judged to be in nitrogen balance [4]. In this study, total nitrogen output from stool, urine and dialysate and urea nitrogen appearance rate were measured independently and the following regression equation, which included both terms, was developed: urea nitrogen generation = 0.154PCR – 1.7. The authors recognized that in dialysate, nitrogen contained in urea, creatinine and uric acid accounted for only about 94% of total nitrogen measured by the Kjeldahl method, and that the remainder 6%, or about 1.4 g of nitrogen, could be in the form of amino acids. These losses of about 8.75 g of amino acids (1.4 ! 6.25) are very similar to the losses of about 8 g of amino acids per treatment that were documented in the early 80s by a different group of investigators in hemodialysis patients using conventional cellulosic membranes and glucose-free dialysate [5]. Moreover, this regression equation agreed very closely with the plot derived from data in nondialyzed patients with stable chronic renal insufficiency gathered independently by another group of investigators [6]. This latter equation, urea generation rate = 0.149PCR – 1.2, differed from the above formula only by the magnitude of the intercept on the Y-axis; of interest, this difference of about 0.5 g of N per day is numerically almost identical with the losses of about 8 g of amino acids per dialysis (0.5 g/day ! 7 days divided by 3, the number of dialysis per week = 1.16 g of N ! 6.25 = 7.3 g of amino acid per treatment). Recently, several authors have documented a significant increase in the losses of amino acids and protein into the dialysate as the result of repetitive processing of dialyzers [7–10]. Thus, with the current use of high-flux dialyzers and with at least 80% of the dialysis units in this country practicing dialyzer reuse, this formula may not measure accurately PCR since a large amount of nitrogen loss, either in the form of amino acids or protein, is not included in this calculation. The purpose of this presentation is to demonstrate that the current method used to calculate PCR in hemodialysis patients may indeed underestimate this parameter and hence dietary protein intake [3].