Department of Internal Medicine, Division of Nephrology, Groningen Institute for Drug Studies (GIDS)*, University Hospital, Groningen, The Netherlands
Correspondence and offprint requests to: W. D. Kloppenburg, Department of Internal Medicine, Division of Nephrology, PO Box 30.001, 9700 RB Groningen, The Netherlands.
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Abstract |
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Methods. We calculated five commonly used variants of normalized PNA and related them to indices of nutritional status in 57 stable chronic haemodialysis patients, 57±15 (mean±SD) years of age. PNA, determined by direct dialysate quantification, was normalized to actual post-dialysis dry body weight (DBW), normal body weight (DBWnormal), lean body mass (LBM), normal lean body mass (LBMnormal), and `normalized' body weight (N). Nutritional status was assessed using an index of nutrition composed of anthropometry derived parameters and plasma albumin concentration.
Results. PNADBW (0.85±0.14 g/kg/d) tended to be higher than PNADBWnormal (0.81±0.14 g/kg/d). PNALBM (1.17±0.19 g/kg/d) did not differ from PNALBMnormal (1.19±0.21 g/kg/d). PNAN (1.06± 0.14 g/kg/d) was significantly higher than PNADBW and PNADBWnormal, but lower than PNALBM and PNALBMnormal. Actual PNA (61±13 g/d) correlated significantly with DBW (r=0.52) and LBM (r=0.63) indicating that large patients eat more protein. Interestingly, actual PNA correlated with plasma albumin (r=0.33) and with the overall index of nutrition (r=0.27) as well. PNADBW correlated negatively with relative DBW (r=-0.32), expressed as a percentage of normal values, indicating that PNADBW is relatively high in underweight patients. In contrast, PNADBWnormal correlated positively with all nutritional parameters as well as with the overall index of nutrition (r=0.33). PNAN and PNALBM did not correlate with the nutritional status, but PNALBMnormal correlated positively with relative DBW (r=0.50) and with overall nutritional status (r=0.34). PNADBWnormal and PNALBMnormal in well-nourished patients showed overlap with the values in patients with evident malnutrition, despite the positive correlation of the normalized PNA values with nutritional status.
Conclusions. Normalizing PNA by DBWnormal and LBMnormal appeared to be the most appropriate method to standardize protein intake in haemodialysis patients. Since actual PNA is the purest estimate of protein intake that correlated with nutritional status, we recommend to evaluate actual PNA as well in studies that relate protein intake to patient outcome.
Keywords: dietary protein intake; haemodialysis; nutritional status; protein intake assessment; protein equivalent of total nitrogen appearance; urea kinetic modeling
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Introduction |
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Dietary protein intake can be assessed directly by means of a dietary diary, but this method depends heavily on the prolonged cooperation of the patient and is time consuming [7]. More easily, protein intake can be estimated indirectly from the protein equivalent of total nitrogen appearance (PNA) determined by urea kinetic modelling. PNA is assumed to be a reliable estimate of dietary protein intake, if the patient is in steady state with regard to his protein metabolism [5,8]. The nutritional status can be assessed by comparing measures of nutrition obtained by anthropometry and biochemical tests to reference values [9].
Since nutritional intake and requirements are related to body size and composition, protein intake is often factored by various terms related to the patients body weight, including dry body weight, normal body weight or lean body mass [8,10]. These normalized protein intake values are used to assess protein intake and to evaluate the relationship between protein intake, nutritional status, and clinical outcome. However, no consensus has been reached about which patient factor is most appropriate for normalizing protein intake in haemodialysis patients. In patients on peritoneal dialysis it has been shown that the relationship between protein intake and nutritional status is greatly influenced by different methods of normalization [11]. The use of various normalization methods may be responsible for the conflicting results found in studies that tried to relate protein intake to nutritional status, morbidity, and mortality in dialysis patients [46, 1016].
Aim of the present study was to evaluate which normalization method is appropriate to assess protein intake in stable chronic haemodialysis patients by relating different normalized PNA variants to commonly used indices of the nutritional status.
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Patients and methods |
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Fifty-seven patients (40 males and 17 females), 57±15 years of age, were included in this multicentre study. None of the included patients had overt oedema. Primary causes of their renal disease were: glomerulonephritis (n=8), interstitial nephritis (n=5), cystic kidney diseases (n=11), congenital kidney diseases (n=3), renal vascular diseases (n=14), unknown (n=10) and other (n=6).
The patients were dialysed three times weekly for 34.5 h per dialysis session and blood flow was set individually at a constant rate of 200300 ml/min. The patients were dialysed using a single-pass or dialysate recirculation dialysis machines with bicarbonate-based dialysate at a flow of 500 ml/min and low-flux (ultrafiltration coefficient <10 ml/mmHgh) dialysers with low complement activation. Delivered Kt/Veq was calculated using the equation by Daugirdas [17].
A diet containing 0.91.0 g/kg ideal body weight/day of protein was prescribed to the patients. Patients were encouraged not to change their usual dietary protein intake.
Sampling and laboratory analysis of blood, urine and dialysate
Blood samples were drawn in heparinized tubes from the fistula immediately before starting the dialysis, at 15 min after termination of the dialysis and before the next dialysis session.
Urine production in 10 of the 57 patients was more than 200 ml per 24 h. These patients collected 24-h urine on the day before the modelled dialysis session. Renal urea and creatinine clearance were calculated from the 24-h urinary output measurements and the time-averaged-concentrations. The residual renal clearance in these 10 patients, defined as the mean of the urea and creatinine clearances, was 1.5±0.8 ml/min.
Dialysate was collected by continuous partial dialysate sampling [18]. For the single pass dialysis machines, total dialysate volume was calculated by multiplying the dialysate flow by duration of the dialysate collection. The dialysate recirculating machines were adapted for dialysate collection as previously described [19]. Total dialysate volume was calculated by multiplying the number of transfers by the transfer volume. The transfer volume was determined before the study and was not changed during the study.
Urea and creatinine concentrations in plasma and urine were determined using routine laboratory methods. Plasma albumin concentration was determined using the cresol-green method. The dialysate urea concentration was determined together with series of standard dialysate samples containing known amounts of urea (3.08.0 mmol/l) in fresh dialysate. The measured urea concentration in the patient dialysate samples was corrected for measurement error using the regression equation determined from the urea concentration values in the standard dialysate samples.
Protein equivalent of total nitrogen appearance measurements
PNA was determined from the rise in plasma urea concentration during the interdialytic interval and an estimate of the patients urea distribution volume (UDV) during three mid-week haemodialysis sessions 4 weeks apart. UDV was calculated from the total urea output in the dialysate corrected for intradialytic urea appearance and ultrafiltration, and the decrease in urea concentration during the modelled dialysis sessions [18]. The averaged value of the three available UDV measurements was used in the PNA equations. PNA (g/d) was calculated from urea nitrogen appearance (UNA) and corrected for unmeasured nitrogen losses (45 mg protein per kg actual body weight per day) [5,8] according to the formulae:
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Methods of normalization
Five normalization methods were used to standardize PNA to body size. First, the PNA values were factored by the patients actual post-dialysis dry body weight (DBW). Secondly, PNA was factored by the patients normal dry body weight (DBWnormal). Normal dry body weight was defined as the median body weight of normal Americans adjusted for sex, frame size, height and age, described in the National Health and Nutrition Examination Surveys (NHANES) I and II [20]. Thirdly, PNA was factored by lean body mass (LBM). LBM was calculated from the percentage body fat as estimated from skinfold thickness measurements and post-dialysis body weight [21]. Fourthly, PNA was factored by normal lean body mass (LBMnormal). Normal LBM was calculated from median body weight and median triceps skinfold thickness of the NHANES reference population [20,21]. Fifthly, PNA was factored by a `normalized' body weight (N). Normalized body weight was calculated from the patient's UDV by dividing UDV by 0.58 for males and 0.55 for females [8].
Nutritional status assessment
In all patients the anthropometric measurements were performed after the dialysis session by a single observer (WDK). One male patient refused anthropometry. DBW was determined as a measure of overall nutrition. Skinfold thickness was measured using a Harpender skinfold caliber (British Indicators Ltd) at four sites. Biceps and triceps skinfold thickness was measured on the opposite arm of the A-V fistula. Subscapular and suprailiac skinfold thickness was measured on both sides and the values were averaged. Skinfolds were measured thrice to the nearest 0.50 mm and the averaged value was recorded. The sum of the four skinfolds thickness measurements were used in the body fat and LBM calculations [21]. Bone-free arm muscle area (AMA) on the opposite arm of the A-V fistula was calculated from the mid-arm circumference and triceps skinfold thickness (TSF) [20]. AMA was determined as a measure of somatic protein supplies. In order to assess the reproducibility of the anthropometric parameters the measurements were performed twice 4 weeks apart. The pre-dialysis plasma albumin concentration was used as a measure of the visceral protein status.
In order to assess the condition of the patient's nutritional status the absolute values of DBW, TSF, and AMA were expressed as percentages of the NHANES reference values [20]. Relative dry body weight (%DBW=DBW/NDBWx100%) was calculated as a measure of overall nutritional status. Relative TSF (%TSF) was calculated as a measure of body fat stores. Relative AMA (%AMA) was calculated as a measure of the somatic protein status.
In addition, the degree of nutrition was determined by a modified version of the nutritional index described by Harty et al. [11]. The index was derived from four subscores based on the values of DBW, TSF, AMA and the albumin concentration. A value of 3, 2, 1 or 0 was obtained for each anthropometric parameter (15th, 10th15th, 5th10th or <5th percentile of the reference population, respectively) and for the albumin concentration (
40 g/l, 3540 g/l, 3035 g/l or <30 g/l, respectively). Summation of the four subscores resulted in an index of nutrition ranging from a maximum of 12 to a minimum of 0. Evident malnutrition was defined as a score
1 in at least two of the four subscores or a score
2 in at least three of the subscores.
Statistical analysis
The mean of the three PNA measurements available for each patient were used in the statistical analyses. Results are presented as mean±SD. Differences in PNA or nutritional status variables within patients were tested using paired Student's t-tests and between patient groups using unpaired Student's t-tests. Correlation was tested with Spearman's correlation analysis. Reproducibility of the anthropometric parameters was evaluated by calculating the repeatability coefficient (RC) and the variation coefficient (VC) as recommended by Bland and Altman [22]. RC was defined as twice the SD of the differences between paired measurements, and VC as the SD of the differences divided by their average expressed as a percentage. A two-sided P-value of <0.05 was considered statistically significant.
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Results |
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The values of the various measures of body mass that were used to normalize PNA are shown in Figure 1A. In the total group, actual DBW (72±11 kg) tended to be lower than normal DBW, derived from the NHANES reference population (75±8 kg) (P=0.05). Actual LBM (52±8 kg) did not differ from normal LBM (52±8 kg). As expected, mean values of actual DBW, LBM and N in males (75±10 kg, 57±5 kg and 61±9 kg, respectively) were higher than that in females (66±9 kg, 43±5 kg and 50±8 kg, respectively) (P<0.001).
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Values of the normalized PNA variables are shown in Figure 1B, which not surprisingly is more or less the mirror image of Figure 1A
. In the total group, PNADBW (0.85±0.14 g/kg/d) tended to be higher than that PNADBWnormal (0.81±0.14 g/kg/d) (P=0.05). PNALBM (1.17±0.19 g/kg/d) did not differ significantly from PNALBMnormal (1.19±0.21 g/kg/d). PNAN (1.06±0.14 g/kg/d) was higher than PNADBW and lower than PNALBMnormal. Interestingly, PNALBMnormal in males (1.14±0.22 g/kg/d) was significantly lower than in females (1.30±0.15 g/kg/d) (P<0.01). PNADBW, PNADBWnormal, PNALBM, and PNAN did not differ between male and female patients.
Reproducibility of the anthropometric parameters was satisfactory (Table 1). No significant differences were observed for any anthropometric measurements performed 4 weeks apart. Measurement of LBM using anthropometry was very reproducible (VC, 1.4%).
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Discussion |
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In our study the nutritional status of the haemodialysis patients was assessed using anthropometry and comparing the anthropometric measures to the NHANES reference values. As anthropometric measurements are prone to large interobserver variation, anthropometry was performed by a single observer in all patients [9]. Reproducibility of these anthropometric measurements was satisfactory. Using the sum of all four skinfold measurements in the LBM calculation resulted in very reproducible LBM values. Comparing different measures of nutrition to normal reference values is an accepted method to assess the nutritional status in haemodialysis patients [8,9]. We are aware that the NHANES data set comprises American reference values and that these values may not necessarily compare to normal values in Dutch patients. However, there is no data available about normal anthropometrical values in The Netherlands or Europe. We also used the plasma albumin concentration in the assessment of the nutritional status. Besides a marker of visceral protein nutrition, plasma albumin also is a negative acute-phase protein. Thus, both protein malnutrition and inflammation can reduce synthesis and hence plasma concentration [2]. By studying only stable haemodialysis patients without inflammatory diseases we tried to reduce the influence of an acute-phase response as much as possible. To divide the patients into groups with different degrees of nutrition, we used a modified version of the nutritional index, described by Harty et al. [11]. To prevent bias of one of the nutritional indices, we used measurements of overall body mass, body fat, somatic protein and visceral stores only once in our index of nutrition. We realize that dividing patients into different degrees of nutrition according to the nutritional index is arbitrary. Despite the shortcomings of the applied index of nutrition we think that it is a valid method to assess nutrition, since it represents measurements of overall body mass, body fat and somatic as well as visceral protein stores.
Not surprisingly, actual PNA correlated with the various measures of body mass, indicating that large patients eat more protein. PNA correlated strongly with LBM and AMA. As LBM and AMA reflect the somatic protein pool, the strong correlation of PNA with LBM and AMA probably emphasizes the mutual relationship between these protein parameters. This observation confirms the correlation between total urea nitrogen appearance and mid-arm muscle circumference in haemodialysis patients participating in the NCDS study [12]. In CAPD patients, actual PNA correlated with LBM (r=0.53) and AMA (r=0.51) as well [11]. A new finding for haemodialysis patients was the positive correlation between actual PNA and plasma albumin (r=0.33), suggesting that actual protein intake is also associated with visceral protein stores. Our study confirms the correlation between actual PNA and plasma albumin (r=0.29) in CAPD patients [11]. The strong correlation between PNA and N is probably mainly due to the fact that PNA and N are not independent variables, as both are calculated from UDV.
Normalized PNA can only be considered a relevant measure of protein intake, if the normalized PNA values are positively related to markers of nutritional status. The negative correlation between PNADBW and %DBW and the index of nutrition, indicate that PNADBW is relatively high in underweight and malnourished patients and relatively low in patients with a normal nutritional status. Thus, actual dry body weight is not an appropriate measure of body mass for normalizing protein intake. Interestingly, PNADBW did significantly correlate with albumin confirming the results obtained by Kaysen et al. [2]. Positive correlations were observed between PNADBWnormal and all of the parameters of nutrition. These results are consistent with the observations made by Harty et al. [11] in CAPD patients. PNADBW was significantly higher in malnourished than in well-nourished CAPD patients. In addition, actual PNA and PNADBWnormal correlated positively with albumin and these were highest in well-nourished CAPD patients. Thus, DBWnormal is a more appropriate measure of body mass to normalize PNA than actual DBW. PNAN correlated with none of the nutritional parameters, as was the case in CAPD patients [11]. In the calculations of normalized body weight (N) it is assumed that UDV is a fixed percentage of post-dialysis body weight that amounts to 58% in males and 55% in females. In our patients these fixed percentages overestimated UDV calculated from urea output in dialysate. UDV determined by urea kinetics was about 48% of post-dialysis body weight in males and 42% in females and ranged from 32% to 60%. These UDV values correspond to values observed in other studies [18,23,24]. Therefore, a normalized body weight calculated from fixed percentages derived from healthy subjects should not be used to normalize PNA, because of the invalidity of this body mass measure due to inter-individual variation and systematic overestimation. Using LBMnormal to normalize PNA appeared to be more appropriate than using actual LBM. PNALBMnormal showed the strongest correlation with %DBW, %AMA and the index of nutrition, whereas PNALBM did not correlate with most of the nutritional parameters.
The use of PNADBW or PNAN in previous studies could be responsible for the fact that in these studies no association was observed between protein intake and nutritional status or mortality in dialysis patients. In the NCDS study no association between PNADBW and the nutritional status or patient outcome was found [13]. In contrast actual urea nitrogen appearance not standardized to body weight was positively associated with the nutritional status and patient outcome. Enia et al. [14] studying a group of patients treated by haemodialysis or CAPD for at least 4 months found no difference in PNADBW between well-nourished and malnourished patients classified using subjective global assessment. Morgenstern et al. [15] did not find any correlation between PNADBW and anthropometric parameters in 23 stable non-diabetic haemodialysis patients. In addition, PNADBW had no predictive value in two longitudinal studies on morbidity and mortality in haemodialysis patients [4,16]. Hospitalization nor survival differed in dialysis patients with a low or a high PNADBW and PNADBW was not an independent factor affecting mortality [4]. In a group with elderly haemodialysis patients PNAN did not predict mortality over 3 years follow-up [15]. We agree with the conclusion of Harty et al. [11] that normalizing PNA may flaw the nutritional value of PNA, particularly when actual or `normalized' body weight is used.
Canaud et al. [10] made a plea for normalizing PNA by LBM that mainly consists of muscle mass. In our study, however, PNALBM did not correlate with most of the nutritional parameters. The authors suggested that the LBM to DBW ratio can be used as an index of nutritional status of haemodialysis patients [10]. They argued that a ratio equal or above 0.70 indicates preserved nutritional status, while a ratio below 0.70 is indicative for wasting. We do not agree with them. The LBMnormal to DBWnormal ratio in our patient group, ranged from 0.58 to 0.83 with a mean of 0.69. In addition, the nutritional index was higher in patients with a LBM/DBW ratio <0.7 (11.2±1.0) than in patients with a ratio 0.7 (9.8±2.2) (P<0.05). These results indicate that the LBM/DBW ratio does not give any information about the nutritional status of individual patients, but only gives information about the relative amount of fat free mass. We therefore think that a simple classification using the LBM/DBW ratio violates the true complexity of the nutritional status.
According to the nutritional index, the nutritional status was normal in 43% of the patients, whereas there was evident malnutrition in 12%. In male patients particularly muscle mass was reduced, reflected by the reduced AMA values. In female patients particularly fat mass was below normal, reflected by the low TSF values. The discrepancy in reductions of muscle mass and fat mass according to sex has been observed in other studies as well [12]. LBM mainly consists of muscle mass that corresponds to the major somatic protein status and endogenous nitrogen store. Interestingly, PNA normalized by LBMnormal was significantly lower in males than in females. This low protein intake and consequently low nitrogen intake in males relatively to their LBMnormal, may be responsible for the susceptibility of male haemodialysis patients for developing muscle mass depletion.
It should be emphasized that in our patient group the relationship between the nutritional status and PNADBWnormal or PNALBMnormal was not very strong and normalized PNA values varied considerably in patients without malnutrition. Besides, normalized PNA values in well-nourished patients showed overlap with the values in malnourished patients. Consequently, normalized PNA was a poor indicator of nutrition. This might partly be due to the relatively small number of patients with evident malnutrition. Day-to-day variation in protein intake was probably not an important factor, since we determined PNA thrice over an 8 weeks period and used only averaged PNA values in the analyses. Differences in dialysis dose, age or gender were not responsible for the variation in PNA values. Possibly, variation in energy intake may partly explain the variation in PNA values in the well-nourished patients, as nitrogen balance is greatly dependent on energy intake [1]. Finally, it cannot be excluded that the nutritional status of well-nourished patients with a PNA in the lower range will deteriorate on the long-term.
Based on the results of our study, we feel that normalizing PNA by the normal value of the patient's DBW or LBM is the most appropriate normalization method. Probably the LBMnormal is preferable above DBWnormal in view of the mutual relationship between protein intake and somatic protein stores. We agree with Harty et al. [11] that the positive association between PNADBWnormal or PNALBMnormal and the nutritional indices observed in our study may reflect the change in the denominator and not the PNA in itself. We therefore think that both normalized PNA values and actual PNA should be considered in studies on the relationship between protein intake and nutritional status. At present no studies are available that analyse the relationship between PNA normalized by DBWnormal or LBMnormal and patient outcome. Future studies are needed to reveal the relevance of normalizing PNA by DBWnormal or LBMnormal for predicting changes in nutritional status, morbidity and mortality. Some optimism regarding to the expected results appears to be justified in view of the data presented in our study.
In conclusion, normalization of PNA using normal values of dry body weight or lean body mass is the most appropriate method to adjust protein intake to body size in haemodialysis patients. In addition, we strongly recommend to evaluate actual PNA as well in studies that relate protein intake to patient outcome, because actual PNA is the purest estimate of protein intake in haemodialysis patients that is positively related to nutritional status.
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Acknowledgments |
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Notes |
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References |
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