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Original Research Communication |
1 From the Division of Nephrology, San Francisco Veterans Affairs Medical Center (KLJ and AMH); Moffitt-Long Hospitals and Mt Zion Medical Center (GMC), University of California, San Francisco; the Department of Medicine (KLJ, GMC, BSY, and MdS), University of California, San Francisco; and the Division of Nephrology, University of California, Davis (GAK).
2 Conducted in the General Clinical Research Center at San Francisco General Hospital (RR-00083). The results were presented in abstract form at the American Society of Nephrology/International Society of Nephrology World Congress of Nephrology meeting, October 2001, San Francisco.
3 Supported by the National Kidney Foundation of Northern California. AMH received fellowship support from the National Kidney Foundation.
4 Address reprint requests to KL Johansen, San Francisco VA Medical Center, Division of Nephrology, 4150 Clement Street, 111J, San Francisco, CA 94121. E-mail: johanse{at}itsa.ucsf.edu.
See corresponding editorial on page 760.
| ABSTRACT |
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Objective: The aim was to determine the extent, pace, determinants, and optimal methods of assessing wasting in patients undergoing hemodialysis.
Design: Laboratory variables, body composition, and physical activity, function, and performance were tested 4 times over 1 y in 54 hemodialysis patients. Changes in repeated measures were evaluated, with adjustment for baseline differences by age, sex, race, diabetes status, and dialysis vintage (ie, time since initiation of dialysis).
Results: No significant changes in body weight, fat mass, lean body mass, or laboratory variables were observed. Phase angle, a bioelectrical impedance analysisderived variable related to body cell mass, decreased significantly (linear estimate: -0.043°/mo, or
0.5 °/y; P = 0.001). Physical activity measured by accelerometry declined 3.4%/mo (P = 0.01). The Maximum Activity Score of the Human Activity Profile (HAP) also declined significantly (linear estimate: -0.50/mo, or
6 points/y; P = 0.025). Higher interleukin 1ß (IL-1ß) concentrations were associated with a narrower phase angle (P = 0.004) and with a more rapid decline in phase angle with time (time x IL-1ß interaction, P = 0.01); similar effects of IL-1ß on physical activity were observed. Dietary protein and energy intakes were associated with changes in the HAP.
Conclusions: Evidence of adverse changes in body composition and physical activity, function, and performance and of a modest influence of inflammation and dietary intake on these changes was observed in this cohort. Tools such as bioelectrical impedance analysis, accelerometry, and the HAP may be required to identify subtle changes.
Key Words: End-stage renal disease hemodialysis nutritional status physical activity physical performance physical function inflammation longitudinal study
| INTRODUCTION |
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A recent cross-sectional analysis of the association between dialysis vintage (ie, time since initiation of dialysis) and nutritional status included information derived from bioelectrical impedance analysis as well as from traditional biochemical indicators and provided some informative data (7). Generally, body-composition indexes were more closely linked with vintage than were biochemical measures. Body weight, estimated total body water, estimated body cell mass, and phase angle were all lower among patients with longer vintage, with the largest relative differences observed for phase angle. However, it is difficult to draw conclusions about the change in body composition with time from a cross-sectional study because of confounding and selection and lead-time bias.
The goal of the current study was to make prospective longitudinal measurements of nutritional status (body composition and biochemical indexes) and of physical activity, function, and performance to determine whether nutritional and functional status decline over time in patients receiving dialysis. In addition, food intake and markers of inflammation were measured to determine whether any observed changes could be related to inadequate protein or energy intake or to ongoing inflammation.
| SUBJECTS AND METHODS |
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Measurements
Study measurements were performed at baseline and 4, 8, and 12 mo from enrollment during a visit to the General Clinical Research Center at San Francisco General Hospital on a midweek day between dialysis treatments. Specifically, body-composition measures included body weight, phase angle derived from bioelectrical impedance analysis (RJL Systems, Detroit), and fat and fat-free mass by dual-energy X-ray absorptiometry (Lunar, Madison, WI). Phase angle is calculated as the arctangent of reactance over resistance and is related to body cell mass and to the distribution of fluid between intracellular and extracellular compartments (11). Biochemical indexes of nutritional status included serum albumin, creatinine, and total cholesterol concentrations. Physical functioning was assessed by 2 questionnaires [the Human Activity Profile (HAP) and the Physical Functioning and Physical Component Scores on the 36-item, short-form questionnaire of the Medical Outcomes Study] and 3 physical-performance tests (gait speed, stair-climbing time, and chair-rising time) as described elsewhere (9, 10). The HAP consists of a list of 94 activities ranked in ascending order of the level of energy required to perform each activity (12). Subjects are asked to assign each activity to 1 of 3 categories: 1) "still doing this activity," 2) "have stopped doing this activity," or 3) "never did this activity." The Maximum Activity Score is the numeral identifying the activity with the highest oxygen-consumption requirement that the subject still performs. The Adjusted Activity Score is the difference between the Maximum Activity Score and the number of activities that the subject has stopped performing, which gives a better estimate of the range of activities performed and of the presence of impairment.
Blood was drawn at each study visit and stored for analysis of inflammatory markers. Enzyme-linked immunoassay was used to measure the following inflammatory markers at the end of the study: C-reactive protein (CRP; detection range: 150 µg/mL; Hemagen Diagnostics, Columbia, MD), interleukin 1ß (IL-1ß; detection range: 3.9250 pg/mL; R & D Systems, Minneapolis), IL-1-receptor antagonist (detection range: 46.93000 pg/mL; R & D Systems, Minneapolis), and tumor necrosis factor
(detection range: 15.61000 pg/mL; Biosource International, Camarillo, CA).
Nutritional intake was assessed by using the BlockNational Cancer Institute 110-item food frequency questionnaire (13). Subjects were asked to report their usual food intake during the previous year. The questionnaire was described in detail previously, as were its validity and reproducibility (1315). With the use of software developed for the survey instrument, the frequency of consumption of each food was multiplied by the nutrient content of the reported portion sizes to generate average daily intakes of protein and energy. Food-frequency information was collected only at the initial study visit.
Statistical analysis
Continuous variables were described as means ± SDs or medians and interquartile ranges; categorical variables were described as proportions. Inflammatory markers were log transformed to attenuate the influence of very high values on inference testing. Repeated-measures analysis of variance was performed by using the MIXED model with an unstructured covariance matrix (16). The MIXED model was chosen for its flexibility, because it allowed us to incorporate all available data. We applied an unstructured variance-covariance matrix to obviate assumptions of particular data structure that might otherwise have been required and to ensure that P values were conservatively estimated. Model fit was assessed by using Aikaikes information criteria. Only patients with
2 study visits were included in the analyses.
Time was the main independent variable of interest; in other words, we focused on the association between time and all dependent variables to evaluate for the presence of wasting. Base models were adjusted for age, sex, race, diabetes status, and dialysis vintage, because these factors were correlated with many of the dependent variables tested in the analyses. To evaluate whether (and to what degree) inflammation influenced the trends in dependent variables, we included in each base model terms for individual inflammatory markers and the time x inflammatory-marker interaction. The latter terms tested whether the trend observed was dependent on the concentration of the inflammatory markers. In other words, if the serum albumin concentration tended to decrease over time, the time x CRP interaction term would evaluate whether the downward trend in albumin was dependent on the CRP concentration. Finally, estimates of dietary protein (g/d) and energy (kcal/d) intakes over the year before study enrollment were assessed individually by adding them to the base model. All analyses were conducted by using SAS 8.0 (SAS Institute, Cary, NC).
| RESULTS |
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0.5°/y; P = 0.001). Karnofsky scores and scores on the 36-item short form of the Medical Outcomes Study did not change appreciably over the course of the study. The Maximum Activity Score of the HAP declined by an average of 0.50 points/mo (P = 0.025). Physical activity measured by accelerometry also declined over time [3390 arbitrary units (3.4% of baseline)/mo, P = 0.012].
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The associations of protein and energy intakes with changes in outcome variables were also modeled. Dietary protein and energy intakes were not significantly associated with changes in any measure of body composition or in any laboratory marker of nutritional status. However, dietary protein and energy intakes were associated with changes in measures of physical function, especially the HAP. Although the Maximum Activity Scores tended to decline overall, the rate of decline was attenuated with increased dietary protein (time x protein interaction, P < 0.0001) and energy intake (time x energy interaction, P = 0.009). The Adjusted Activity Score and other tests of physical activity, function, and performance showed qualitatively similar associations with dietary intake (data not shown), albeit with lesser degrees of statistical significance.
| DISCUSSION |
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Markers of inflammation were associated with some measures of nutritional status or body composition and in some cases with the changes in these variables over time. However, inflammatory markers were not associated with measures of, or changes in, physical function. It is possible that alteration of function could be a late consequence of malnutrition and, thus, is less likely related to rapidly changing concentrations of inflammatory markers. Although the principal cytokines measured here were IL-1ß and IL-1-receptor antagonist, these are released by the same processes that initiate the release of tumor necrosis factor
(20). The half-lives of these cytokines are short and their serum concentrations may underestimate the biologic events accompanying their release because they act locally.
The change in phase angle and its modulation by IL-1ß may reflect a reduction in muscle mass influenced by inflammation in this population. Reactance reflects the stored charge on cell membranes, and may diminish with a reduction in viable cell number or size. Inflammation causes wasting of muscle mass through a ubiquitin-mediated process (21, 22). Although the recovery of visceral proteins (specifically albumin) after inflammatory events occurs fairly quickly, regeneration of lost somatic proteins, specifically muscle, is less well assured. Although inflammation occurs episodically in hemodialysis patients (23), patients who have evidence of inflammation at one point in time are more likely to experience inflammation later.
The small size of the study did not allow assessment of more complex (eg, 3-way) interactions and may have resulted in the failure to detect more subtle changes over time or the influence of inflammatory markers on changes in other measures of body composition or physical function and performance. However, the confirmation of known associations, such as those between serum albumin and CRP (17, 18) and between body fat and serum leptin (24, 25), suggests that the analytic approach was valid and the novel findings described here are likely to be correct.
Protein and energy intakes were not associated with measures of body composition or nutritional status but were associated with several measures of physical function. The lack of a direct relation between intake and nutritional status could reflect the modulating influence of inflammation. The associations of serum albumin and creatinine concentrations with CRP concentrations lend some support to this construct. The association between intake and physical function could be mediated by the energy requirements of physical activity. For example, greater energy expenditure through physical activity is associated with better physical functioning and with greater energy intake to preserve energy balance.
The study has several important limitations. First, the sample size was relatively small, and as noted above, we may have lacked the sensitivity to detect subtle changes in the natural history of disease or subtle influences of inflammation or intake on these changes. Second, the sample was somewhat healthier and more fit than was the general population with end-stage renal disease because of the rigors of testing; studying a less able-bodied sample might have identified more abnormalities. Third, there was limited follow-up. It is possible that certain elements of nutritional status, including body composition, might not change appreciably within 12 mo but would do so over 24 or 36 mo. This delay in identification of wasting may be more pronounced given the general good health of the cohort. We previously showed that the relation between body composition and dialysis vintage was rather subtle, except among patients dialyzed for > 5 y (7). Finally, because we only studied hemodialysis patients, these results may not be generalizable to patients receiving peritoneal dialysis or to those who have undergone kidney transplantation. The body composition (and physical function) of these populations may be greatly affected by excess calories derived from peritoneal dialysate and the use of glucocorticoids, respectively.
In summary, we showed that wasting and physical decline occur in hemodialysis patients and that some measures of body composition and physical activity and function appear to be more sensitive to change than others. Bioelectrical impedance analysis and HAP testing are easily performed and may be particularly useful for evaluating interventions to improve or prevent the decline of body composition and physical function in hemodialysis patients. Larger cohort studies and intervention trials will be required to better understand the natural history of body-composition changes and impaired physical function in hemodialysis patients and methods to maintain and improve body composition and physical function.
| ACKNOWLEDGMENTS |
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