Dietary protein, urea nitrogen appearance and total nitrogen appearance in chronic renal failure and CAPD patients

Abstract

This study was carried out to examine the mathematical relationships between the urea nitrogen appearance (UNA), total nitrogen appearance (TNA) and dietary nitrogen intake (DNI) in patients with chronic renal failure. Studies were conducted in 20 nondialyzed patients with advanced chronic renal failure (CRF) who were fed 27 constant protein diets for 24.8 +/- 9.5 days (SD) and eight patients undergoing continuous ambulatory peritoneal dialysis (CAPD), who ingested 13 constant protein diets for 20.3 +/- 4.9 days. All patients lived in a hospital research ward throughout the study and underwent full nitrogen balance measurements. Data were analyzed after patients attained equilibrium or near equilibrium with each dietary protein intake. In the CRF patients, using the mean values obtained during the equilibrium phase, there was a direct and precise correlation between the TNA and UNA, where TNA g/day = 1.19 UNA g/day + 1.27 g/day, r = 0.948. The correlation between DNI and UNA was also highly significant, but less precise, with somewhat greater 95% confidence intervals: DNI g/day = 1.20 UNA g/day + 1.74 g/day, r = 0.865. The relationship between DNI and TNA was not much more precise: DNI g/day = 0.97 TNA g/day + 0.65 g/day, r = 0.880. With the CAPD patients, the relationships were as follows: TNA g/day = 0.94 UNA g/day + 5.54 g/day, r = 0.956; DNI g/day = 0.97 UNA g/day + 6.80 g/day, r = 0.705; DNI g/day = 1.07 TNA g/day + 0.63 g/day, r = 0.760. For the CAPD patients, the lowest 95% confidence intervals were also found for the correlation between TNA and UNA. Thus, in both CRF and CAPD patients, the TNA is highly and precisely correlated with the UNA. The DNI is also significantly correlated with UNA and TNA, but the relationship is less precise. In both of these two groups of patients, the difference between the regression equations for TNA versus UNA and DNI versus UNA was, to a substantial degree, accounted for by the intercept.

Low Protein Nitrogen Appearance as a Surrogate of Low Dietary Protein Intake Is Associated with Higher All-Cause Mortality in Maintenance Hemodialysis Patients1–3

“…Another potential limitation of nPNA is that the metabolic status of any given patient cannot be estimated by it; patients may be in negative or positive nitrogen balance for a variety of reasons (47). However, MHD patients are rarely in negative or, particularly, in positive nitrogen balance (>1.0 to 1.5 g/d) for more than a very few days (26,47). Thus, except for those exceptional times when an MHD patient is in strongly negative or positive nitrogen balance, the nPNA should rather closely reflect the dietary protein intake (e.g., within 610-15%).…”

“…All multivariate models in our study were adjusted for Kt/V; hence, it is unlikely that the mathematical or clinical association between these 2 measures had a bearing on the multivariate-adjusted association between the nPNA and mortality. Another potential limitation of nPNA is that the metabolic status of any given patient cannot be estimated by it; patients may be in negative or positive nitrogen balance for a variety of reasons (47). However, MHD patients are rarely in negative or, particularly, in positive nitrogen balance (>1.0 to 1.5 g/d) for more than a very few days (26,47).…”

Is controlling phosphorus by decreasing dietary protein intake beneficial or harmful in persons with chronic kidney disease?

“…(21) The nPNA, also known as nPCR, is a urea kinetic-based estimate of dietary protein intake in MHD patients assuming minimal or no residual renal function. (38, 39) However, the nPNA is collinear with Kt/V, since both nPNA and Kt/V are calculated using the same urea nitrogen levels. (38, 39) Higher protein intakes may indeed lead to lower mortality by virtue of improving nutritional status; this is why we did not adjust for additional nutritional markers in models presented in Figures 2.…”

“…(38, 39) However, the nPNA is collinear with Kt/V, since both nPNA and Kt/V are calculated using the same urea nitrogen levels. (38, 39) Higher protein intakes may indeed lead to lower mortality by virtue of improving nutritional status; this is why we did not adjust for additional nutritional markers in models presented in Figures 2. While a moderate rise in nPNA was associated with reduced mortality, more drastic rise in nPNA exhibited a paradoxical trend towards higher mortality, as shown in the left lower panel of Figure 2.…”

Assessment of Protein and Energy Nutritional Status

“…The following equations can be used to measure nitrogen appearance [160]. The following equations can be used to measure nitrogen appearance [160].…”